# The 2022 high-energy outburst and radio disappearing act of the magnetar 1E 1547.0-5408

Marcus E Lower, George Younes, Paul Scholz, Fernando Camilo, Liam Dunn, Simon Johnston, Teruaki Enoto, John M Sarkissian, John E Reynolds, David M Palmer, et al.

► To cite this version:

Marcus E Lower, George Younes, Paul Scholz, Fernando Camilo, Liam Dunn, et al.. The 2022 high-energy outburst and radio disappearing act of the magnetar 1E 1547.0-5408. Astrophys.J., 2023, 945 (2), pp.153. 10.3847/1538-4357/acbc7c hal-04021922

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# The 2022 High-energy Outburst and Radio Disappearing Act of the Magnetar 1E 1547.0–5408

Marcus E. Lower<fnref n="1" />, George Younes<fnref n="2" />,<fnref n="3" />, Paul Scholz<fnref n="4" />, Fernando Camilo<fnref n="5" />, Liam Dunn<fnref n="6" />,<fnref n="7" />, Simon Johnston<fnref n="1" />, Teruaki Enoto<fnref n="8" />, John M. Sarkissian<fnref n="9" />, John E. Reynolds<fnref n="1" />, David M. Palmer<fnref n="10" />, Zaven Arzoumanian<fnref n="2" />, Matthew G. Baring<fnref n="11" />, Keith Gendreau<fnref n="2" />, Ersin Göğüş<fnref n="12" />, Sebastien Guillot<fnref n="13" />,<fnref n="14" />, Alexander J. van der Horst<fnref n="3" />, Chin-Ping Hu<fnref n="15" />, Chryssa Kouveliotou<fnref n="3" />, Lin Lin<fnref n="16" />, Christian Malacaria<fnref n="17" />, Rachael Stewart<fnref n="3" />, and Zorawar Wadiasingh<fnref n="2" />,<fnref n="18" />,<fnref n="19" />

*Received 2022 December 19; revised 2023 February 9; accepted 2023 February 14; published 2023 March 16*

## Abstract

We report the radio and high-energy properties of a new outburst from the radio-loud magnetar 1E 1547.0–5408. Following the detection of a short burst from the source with Swift-BAT on 2022 April 7, observations by NICER detected an increased flux peaking at $(6.0 \pm 0.4) \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$ in the soft X-ray band, falling to a baseline level of $1.7 \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$ over a 17 day period. Joint spectroscopic measurements by NICER and NuSTAR indicated no change in the hard nonthermal tail despite the prominent increase in soft X-rays. Observations at radio wavelengths with Murriyang, the 64 m Parkes radio telescope, revealed that the persistent radio emission from the magnetar disappeared at least 22 days prior to the initial Swift-BAT detection and was redetected two weeks later. Such behavior is unprecedented in a radio-loud magnetar, and may point to an unnoticed slow rise in the high-energy activity prior to the detected short bursts. Finally, our combined radio and X-ray timing revealed the outburst coincided with a spin-up glitch, where the spin frequency and spin-down rate increased by $0.2 \pm 0.1$ µHz and $(-2.4 \pm 0.1) \times 10^{-12}$ s$^{-2}$, respectively. A linear increase in the spin-down rate of $(-2.0 \pm 0.1) \times 10^{-19}$ s$^{-3}$ was also observed over 147 days of postoutburst timing. Our results suggest that the outburst may have been associated with a reconfiguration of the quasi-polar field lines, likely signaling a changing twist, accompanied by spatially broader heating of the surface and a brief quenching of the radio signal, yet without any measurable impact on the hard X-ray properties.

*Unified Astronomy Thesaurus concepts: Magnetars (992); Pulsars (1306); Radio pulsars (1353); High energy astrophysics (739); Neutron stars (1108)*

## 1. Introduction

Magnetars are slowly rotating neutron stars with the strongest known magnetic fields in the universe. Outbursts from these highly magnetized objects are typically manifested by a sudden enhancement of their persistent X-ray emission by up to three orders of magnitude, which is often accentuated by spectral hardening and increased levels of complexity in their X-ray pulse profiles (see Coti Zelati et al. 2018 for a review). This increase in high-energy activity is sometimes

accompanied by the emission of up to hundreds of short-duration, bright, hard X-ray bursts. Five out of the nearly 30 known magnetars have also been found to display pulsed radio emission during outburst states (Olausen & Kaspi 2014).<fnref n="20" /> These radio-loud episodes are typically preceded by several years of radio quiescence followed by the sudden activation of intense, pulsed radio emission. Strong temporal variability in the detected radio flux, pulse-profile shape, and polarization fraction are often observed (Levin et al. 2012; Camilo et al. 2018; Dai et al. 2019; Lower et al. 2021b). Moreover, rotational variations in the form of rapid increases in spin-down and sudden spin-up (glitches) or, less commonly,

<figure><img src="image_3.png" /></figure>

<footnote n="1">¹ Australia Telescope National Facility, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia; marcus.lower@csiro.au</footnote>

<footnote n="2">² Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, 20771, USA; george.a.younes@nasa.gov</footnote>

<footnote n="3">³ Department of Physics, The George Washington University, 725 21st Street NW, Washington, DC, 20052, USA</footnote>

<footnote n="4">⁴ Dunlap Institute for Astronomy & Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON, M5S 3H4, Canada</footnote>

<footnote n="5">⁵ South African Radio Astronomy Observatory, 2 Fir Street, Black River Park, Observatory 7925, South Africa</footnote>

<footnote n="6">⁶ School of Physics, University of Melbourne, Parkville, VIC 3010, Australia</footnote>

<footnote n="7">⁷ OzGrav: The ARC Centre of Excellence for Gravitational-wave Discovery, Parkville, VIC 3010, Australia</footnote>

<footnote n="8">⁸ RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan</footnote>

<footnote n="9">⁹ Australia Telescope National Facility, CSIRO, Space and Astronomy, Parkes Observatory, PO Box 276, Parkes NSW 2870, Australia</footnote>

<footnote n="10">¹⁰ Los Alamos National Laboratory, Los Alamos, NM, 87545, USA</footnote>

<footnote n="11">¹¹ Department of Physics and Astronomy—MS 108, Rice University, 6100 Main Street, Houston, TX, 77251-1892, USA</footnote>

<footnote n="12">¹² Sabancı University, Faculty of Engineering and Natural Sciences, İstanbul 34956 Turkey</footnote>

<footnote n="13">¹³ IRAP, CNRS, 9 avenue du Colonel Roche, BP 44346, F-31028 Toulouse Cedex 4, France</footnote>

<footnote n="14">¹⁴ Université de Toulouse, CNES, UPS-OMP, F-31028 Toulouse, France</footnote>

<footnote n="15">¹⁵ Department of Physics, National Changhua University of Education, Changhua 50007, Taiwan</footnote>

<footnote n="16">¹⁶ Department of Astronomy, Beijing Normal University, Beijing 100875, People's Republic of China</footnote>

<footnote n="17">¹⁷ International Space Science Institute (ISSI), Hallerstrasse 6, 3012 Bern, Switzerland</footnote>

<footnote n="18">¹⁸ Department of Astronomy, University of Maryland, College Park, MD, 20742, USA</footnote>

<footnote n="19">¹⁹ Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD, 20771, USA</footnote>

<footnote>Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.</footnote>

<footnote n="20">²⁰ https://www.physics.mcgill.ca/~pulsar/magnetar/main.html</footnote>
spin-down events (antiglitches) are also detected following outbursts (Archibald et al. 2013; Scholz et al. 2017; Hu et al. 2020; Younes et al. 2020; Caleb et al. 2022). These observational properties are likely a result of magnetic stresses from the decaying internal magnetic fields precipitating sudden crustal motion or fracturing, leading to an impulsive release of energy in the form of electromagnetic radiation and occasionally an alteration of the external magnetic field topology (Duncan & Thompson 1992; Harding et al. 1999; Thompson et al. 2002; Younes et al. 2022b)

With a spin period of ~2.1 s, 1E 1547.0–5408 (also known as PSR J1550–5418) is the second fastest rotating magnetar found to date. First discovered as a persistent X-ray source by the Einstein Observatory (Lamb & Markert 1981), it was not until several decades later that it was recognized as a potential magnetar candidate embedded within the putative supernova remnant G327.24–0.13, estimated to be ~4 kpc away (Gelfand & Gaensler 2007). Targeted pulsar-search observations revealed the presence of highly polarized radio pulses originating from the source (Camilo et al. 2007). Subsequent timing observations enabled measurements of both the aforementioned spin period and a spin-down rate of 2.138 × 10⁻¹¹ s s⁻¹, thereby confirming its magnetar status (with an equatorial field of B ≈ 2 × 10¹⁴ G; Camilo et al. 2007) and making it the second radio-loud magnetar to be discovered (Camilo et al. 2007, 2008). This serendipitous discovery appeared to follow an unnoticed outburst sometime in the mid-2000s (Camilo et al. 2008) (possibly during 2007; see Halpern et al. 2008), and was soon followed by two intense high-energy outbursts in 2008 and 2009 (Mereghetti et al. 2009; Israel et al. 2010; Bernardini et al. 2011; Scholz & Kaspi 2011; van der Horst et al. 2012). Detections of the radio pulses became highly intermittent following the outbursts, but the source stabilized to a persistent radio-loud state after several years (F. Camilo et al. 2023, in preparation).

Since its major outburst of 2009 January, 1E 1547.0–5408 has remained in an unusual high X-ray flux state, bearing a much stronger resemblance to a persistent magnetar as opposed to its initial “transient” classification (Coti Zelati et al. 2020; F. Camilo et al. 2023, in preparation). No additional X-ray outbursts above the baseline were observed during this time, barring the detection of two short bursts seen with the Neil Gehrels Swift Observatory (hereafter Swift) Burst Alert Telescope (BAT) in 2011 and 2013 (Holland et al. 2013). Neither of these two short bursts were followed up by targeted high-cadence monitoring.

In this work, we present the detection of a new outburst from 1E 1547.0–5408 that began between 2022 late March and early April. An overview of our radio and high-energy observations are detailed in the following section. In Section 3 we present our analysis of the timing and radiative properties of 1E 1547.0–5408 during the outburst. We conclude in Section 4 with a discussion on the interplay between changes in the high-energy, radio, and timing properties of 1E 1547.0–5408, and potential implications for the outburst mechanism.

## 2. Observations

### 2.1. Parkes Observatory

Regular monitoring of 1E 1547.0–5408 with an approximately 10 day cadence has been undertaken with Murriyang, the 64 m Parkes radio telescope, under the P885 project since its discovery as a radio-loud magnetar in 2007

(Camilo et al. 2007). Our observations from 2019 January have been performed using the Ultra-Wideband Low (UWL) receiver system (Hobbs et al. 2020), where two independent pulsar-search-mode data streams are recorded with the pulsar digital filterbank v4 (PDFB4) and GPU-based MEDUSA signal processors. Individual observations lasted between 2 and 15 minutes. For this work, we analysed a subset of the PDFB4 data spanning from 2021 June to 2022 August and MEDUSA data covering 2021 December to 2022 August. Data collected by PDFB4 were taken with 1 ms time sampling at a center frequency of 3100 MHz with 512 channels covering 1024 MHz of bandwidth. Full Stokes data collected with MEDUSA were recorded with a 128 µs time resolution with 3328 channels covering the full UWL band from 704 to 4032 MHz, and coherently dedispersed at a dispersion measure (DM) of 830 pc cm⁻³. Both data sets were subsequently folded at the rotation period of the magnetar using *dspsr* (van Straten & Bailes 2011), such that a single pulse period is covered by 1024 phase bins. Frequency channels affected by radio-frequency interference (RFI) were excised using the *paz* and *pazi* tools in *psrarchive* (Hotan et al. 2004; van Straten et al. 2012). The folded MEDUSA data were then flux and polarization calibrated following the procedures outlined in Lower et al. (2020).

### 2.2. NICER

The Neutron star Interior Composition ExploreR (NICER) is a nonimaging, soft X-ray telescope mounted on the International Space Station, with a ≈5' diameter field of view (FoV; Gendreau et al. 2016). NICER provides a large collective area peaking at 1900 cm² at 1.5 keV and a timing resolution <300 ns. We initiated a series of NICER target-of-opportunity observations of 1E 1547.0–5408 two days after the 2022 April 7 Swift-BAT detection of a magnetar-like burst coincident with 1E 1547.0–5408. The NICER monitoring consisted of about 1 ks of observations every few days, starting on 2022 April 9. We provide the observations log in Table 1.

We cleaned and calibrated all NICER data utilizing NICERDAS version v008c as part of HEASoft version 6.30.1. We applied the standard filtering criteria as described in the NICER Data Analysis Guide.<fnref n="21" /> Moreover, detectors 14 and 34 can show larger than normal background noise, hence, were excluded from all analyses. Finally, we compared the light curve (binned with a 5 s resolution) created in the energy range 1–6 keV (where the source is supposed to dominate the emission) to the one created in the 13–15 keV energy range. Almost all the counts in the latter band are due to the high-energy particle background. We eliminated any simultaneous flaring background intervals that appeared in both light curves. These were mostly present at the start and/or end of a good time interval (GTI) due to imprecise modeling of the South Atlantic Anomaly. We provide the total GTIs of each NICER observation in Table 1. We utilize Xselect as part of HEASoft version 6.30.1 to extract the source spectra, and estimated the background contribution for each observation utilizing the 3C50 model (Remillard et al. 2022).

### 2.3. NuSTAR

The Nuclear Spectroscopic Telescope ARray (NuSTAR; Harrison et al. 2013) is a focusing hard (3–79 keV) X-ray

<footnote n="21">²¹ NICER data analysis guide</footnote>
<table><caption>**Table 1**
List of X-Ray Observations Analysed in This Work</caption><thead><tr><th>Telescope</th><th>Observation ID</th><th>Date (UTC)</th><th>Total GTI (ks)</th></tr></thead><tbody><tr><td>NuSTAR</td><td>30101035002</td><td>2016-04-23</td><td>84</td></tr><tr><td>NuSTAR</td><td>30401008002</td><td>2019-02-15</td><td>90</td></tr><tr><td>Swift/XRT</td><td>00013176022</td><td>2022-03-5</td><td>2.6</td></tr><tr><td>Swift/XRT</td><td>00013176023</td><td>2022-03-6</td><td>1.8</td></tr><tr><td>NICER</td><td>5020300101</td><td>2022-04-9</td><td>1.9</td></tr><tr><td>NICER</td><td>5020300102</td><td>2022-04-12</td><td>0.1</td></tr><tr><td>NICER</td><td>5020300103</td><td>2022-04-13</td><td>0.2</td></tr><tr><td>NICER</td><td>5020300104</td><td>2022-04-14</td><td>0.3</td></tr><tr><td>NICER</td><td>5020300106</td><td>2022-04-16</td><td>1.2</td></tr><tr><td>NICER</td><td>5020300107</td><td>2022-04-17</td><td>0.6</td></tr><tr><td>NICER</td><td>5020300108</td><td>2022-04-18</td><td>0.4</td></tr><tr><td>NICER</td><td>5020300109</td><td>2022-04-21</td><td>1.5</td></tr><tr><td>NuSTAR</td><td>90801309002</td><td>2022-04-21</td><td>24</td></tr><tr><td>NICER</td><td>5020300110</td><td>2022-04-22</td><td>1.8</td></tr><tr><td>NICER</td><td>5020300111</td><td>2022-04-25</td><td>0.4</td></tr><tr><td>Swift/XRT</td><td>00013176025</td><td>2022-05-7</td><td>3.0</td></tr><tr><td>NICER</td><td>5020300113</td><td>2022-05-8</td><td>1.6</td></tr><tr><td>Swift/XRT</td><td>00013176026</td><td>2022-05-11</td><td>1.5</td></tr><tr><td>NICER</td><td>5020300114</td><td>2022-05-13</td><td>0.9</td></tr><tr><td>NICER</td><td>5020300115</td><td>2022-05-15</td><td>0.2</td></tr><tr><td>NICER</td><td>5020300116</td><td>2022-05-21</td><td>1.0</td></tr><tr><td>NICER</td><td>5020300117</td><td>2022-05-22</td><td>0.5</td></tr><tr><td>NICER</td><td>5020300118</td><td>2022-05-28</td><td>0.7</td></tr><tr><td>NICER</td><td>5020300119</td><td>2022-06-7</td><td>1.5</td></tr><tr><td>NICER</td><td>5020300120</td><td>2022-06-17</td><td>1.2</td></tr><tr><td>NICER</td><td>5020300121</td><td>2022-06-27</td><td>0.4</td></tr><tr><td>NICER</td><td>5020300123</td><td>2022-07-7</td><td>0.9</td></tr><tr><td>Swift/XRT</td><td>00013176027</td><td>2022-07-9</td><td>4.3</td></tr><tr><td>NICER</td><td>5020300124</td><td>2022-07-13</td><td>0.4</td></tr><tr><td>NICER</td><td>5020300125</td><td>2022-07-17</td><td>2.1</td></tr><tr><td>NICER</td><td>5020300126</td><td>2022-07-20</td><td>1.1</td></tr><tr><td>NICER</td><td>5020300127</td><td>2022-07-21</td><td>0.7</td></tr><tr><td>NICER</td><td>5020300128</td><td>2022-07-28</td><td>1.3</td></tr><tr><td>NICER</td><td>5020300129</td><td>2022-07-30</td><td>0.4</td></tr><tr><td>NICER</td><td>5020300130</td><td>2022-08-2</td><td>0.1</td></tr><tr><td>NICER</td><td>5020300131</td><td>2022-08-3</td><td>0.2</td></tr><tr><td>NICER</td><td>5020300132</td><td>2022-08-5</td><td>0.2</td></tr><tr><td>NICER</td><td>5020300133</td><td>2022-08-9</td><td>0.4</td></tr><tr><td>NICER</td><td>5020300134</td><td>2022-08-15</td><td>1.8</td></tr><tr><td>NICER</td><td>5020300135</td><td>2022-08-17</td><td>2.3</td></tr></tbody></table>

telescope, consisting of two identical focal plane modules (FPMs), FPMA and FPMB. Each module is formed by an array of 2 × 2 detectors providing a total of about 12′ × 12′ FoV. Prompted by the increase in the soft X-ray flux, we requested NuSTAR director discretionary time and 1E 1547.0–5408 was observed for 24 ks on 2022 April 21. We utilized NuSTARDAS software version 2.1.2 to clean and calibrate the event files. We employed the task `nuproducts` to extract source events, light curves, and spectra from a circular region with a 50′′ radius around the source central brightest pixel, which corresponds to 70% of the encircled-count fraction of the detectors point-spread function. This choice of extraction region is driven by the desire to maximize source counts, while limiting the stray-light background, which is present in both NuSTAR modules in proximity to the source. Similarly, we extracted the background files from a source-free region on the same detector as the source with the same radius. For comparison purposes, we analysed two archival NuSTAR observations taken on 2016 April 23 and 2019 February 15, following the

same procedures outlined above. Table 1 summarizes the NuSTAR observations.

## 2.4. Swift

The X-ray Telescope (XRT) on board Swift is an imaging charge-coupled device, sensitive to photons in the energy range of 0.2–10 keV (Burrows et al. 2005). XRT observed 1E 1547.0–5408 on several occasions in 2022 March, May, and July, bracketing the time of the outburst. Thus, these observations provide the source soft X-ray spectral state shortly before the outburst onset, and a confirmation of its properties as observed with NICER following the outburst. For these observations, XRT was operating in windowed-timing mode, affording a 1D image with a time resolution of 1.7 ms. We utilized XRTDAS version 3.7.0 to reduce and calibrate the data. We extracted source events from each GTI of a given observation separately. We used a circular region with a 20 pixel radius centered on the brightest pixel. We extracted background events from an annulus with inner and outer radii of 80 and 120 pixels, respectively, centered at the same position as the source. We created the ancillary files using `xrtmkarf`, and used the response matrices in CALDB version 20220803. We excluded all GTIs where the source landed within a three pixel distance from a bad column or the edge of the CCD due to an increase in the systematic uncertainties.<fnref n="22" /> For the remaining good GTIs, we add the source and background spectra, as well as the ancillary and response files using the HEASOFT tool `addspec`. We provide the XRT observation IDs, dates, and total GTIs in Table 1.

BAT, also on board Swift, is a coded aperture telescope which images a ~1.5 sr FoV in the energy range of 15–150 keV. Images are generated by the onboard software in response to count rate increases at timescales from 4 ms to 24 s or, in the absence of rate increases, on minute and longer timescales. Bursts from sources outside of the FoV can often be detected due to the leakage of photons through the instrument shielding, yielding a count rate increase without a corresponding image peak. These can be used to confirm the astrophysical nature of count rate increases seen by other instruments and constrain their direction of origin.

## 2.5. Fermi-GBM

The Gamma-ray Burst monitor (GBM; Meegan et al. 2009) is a nonimaging instrument on board the Fermi Gamma-ray Space Telescope that is sensitive to photons in the 8–1000 keV range. GBM is capable of automatic onboard triggers, with one of the software algorithms tailored to magnetar-like short bursts. Apart from the triggered data, GBM also provides continuous time-tagged events (CTTE), with a time resolution of 2 microseconds. We utilized the CTTE data to search for any untriggered bursts from the direction of 1E 1547.0–5408 from its last radio detection on 2022 March 16 until the confirmed BAT burst on 2022 April 7. This analysis provides an estimate of the start time of the 2022 outburst.

<footnote n="22">²² https://www.swift.ac.uk/user_objects/lc_docs.php</footnote>
# 3. Analysis and Results

## 3.1. Radio Disappearance and Reactivation

Unlike other radio-loud magnetars which fade into radio silence in the months to years after an outburst, 1E 1547.0–5408 has remained radio-loud for well over a decade since the 2009 outburst (F. Camilo et al. 2023, in preparation). Following the 2008 and 2009 outbursts, detections of radio pulses were intermittent (see, e.g., Israel et al. 2021), possibly reflecting the highly dynamic nature of its magnetosphere during that period. Intermittent detections continued up to 2013, after which radio pulses were consistently detected during every observation. 1E 1547.0–5408 remained in this persistently radio-loud state until our two observations on 2022 March 16 and April 9, where we failed to detect radio pulses from the magnetar. Radio pulses were redetected during the 2022 April 19 observation, ~12 days after the BAT-confirmed trigger, and have persisted up to at least our Parkes-UWL observation on 2022 August 3.

Using a high signal-to-noise ratio (S/N) observation taken on 2022 July 9, we measured a structure-optimized DM of $697 \pm 1$ pc cm$^{-3}$ with the DM_phase package (Seymour et al. 2019). The updated DM is a significant improvement on the previously reported value of $830 \pm 50$ pc cm$^{-3}$ (Camilo et al. 2007). This is likely due to differences in the methods applied to compute the DM (profile structure versus S/N optimization) in addition to the substantially larger instantaneous bandwidth afforded by the UWL. Using the NE2001 (Cordes & Lazio 2002) and YMW16 (Yao et al. 2017) electron density models, this updated DM corresponds to estimated distances of 8.3 kpc and 5.9 kpc, respectively. The latter distance is comparable to the commonly assumed distance of ~4.5 kpc inferred from dust-scattering halos detected after the 2009 outburst (Tiengo et al. 2010), when considering the large systematic uncertainties that are associated with any of these methods, which are on average 50%. Similarly, we obtained an updated rotation measure (RM) of $-1847.6 \pm 0.5$ rad m$^{-2}$ by directly fitting the Stokes Q and U spectra in the 2000–4032 MHz range (to avoid complications from scatter broadening) with RMNest (Lower et al. 2022). Our updated RM is consistent with the previously reported value of $-1860 \pm 20$ rad m$^{-2}$ (Camilo et al. 2008).

The shape of the average radio profile detected throughout 2022 January–August was highly variable, and can be broadly categorized into three general archetypes:

1. Profile A: strong leading component with one to three trailing components.

2. Profile B: complex, multicomponent profile, sometimes with a central peak.

3. Profile C: two dominant profiles joined by weaker subcomponents or a flat bridge of emission.

Examples of all three profile types are shown in Figure 1, all of which were dedispersed and de-Faraday rotated using our updated DM and RM values. Note, these categorizations are purely qualitative. For instance, the complex 2022 June 13 profile (bottom-middle panel in Figure 1) is largely double peaked, and could also be classified as Profile C. Transitions between profile types tend to be sudden and reminiscent of transient profile changes reported in earlier observations of this magnetar (e.g., Figure 5 of Halpern et al. 2008) and those seen in emission mode-changing/state-switching pulsars

(Wang et al. 2007). The radio profiles of 1E 1547.0–5408 in Figure 1 display clear epoch-to-epoch variations in flux density. We computed the frequency-averaged radio flux density as

$$F_f = \frac{1}{N_{\text{bin}}} \sum_{i}^{N_{\text{on}}} F_{f,i}, \qquad (1)$$

where $N_{\text{bin}} = 1024$ is the number of phase bins covering a full rotation of the magnetar, $N_{\text{on}}$ is the number of bins covering the on-pulse region, and $F_{f,i}$ is the flux of the ith phase bin. Uncertainties in the flux density were computed from the rms of the off-pulse region

$$\sigma_{F,f} = \frac{\sqrt{N_{\text{on}}}}{N_{\text{bin}}} \sqrt{\sum_{i}^{N_{\text{off}}} F_{f,i}^2}, \qquad (2)$$

where $N_{\text{off}}$ is the number of off-pulse phase bins. The measured flux densities from our observations are displayed in the bottom panel of Figure 2. We used Equation (2) to set sensitivity limits for our observations on 2022 March 26 and April 9 where we failed to detect radio pulses, obtaining limiting flux densities of $1.3 \pm 0.3$ mJy and $0.9 \pm 0.2$ mJy, respectively. Visually, there appears to be a shallow increase in the revived radio flux over time. However, a direct connection between this apparent secular evolution and the outburst cannot be confirmed without a comparison to the long-term flux history of the magnetar.

Also indicated in Figure 2 are the corresponding profile types seen during each observation. In terms of relative numbers, we observed the Profile B state in more than half (25/49) of our observations, while Profiles A and C were seen in 17 and seven, observations respectively. The appearance of any one mode appears to be largely stochastic, though there is a clear preference for Profile B after the outburst up until 2022 May 13 (MJD 59712) where the magnetar was in the Profile A state. No further systematic change in the profile or the mode-changing behavior is apparent after this date. It is possible that this preference may be related to changes in the emission region plasma following reactivation. However such profile variations persist among radio-loud magnetars for many months to years after an outburst (Levin et al. 2012; Lower et al. 2021b). Additionally, the swing of the linear polarization position angle (PA) across the profile remains largely flat and unchanged both before and after the outburst. This is consistent with previous observations of 1E 1547.0–5408 in 2007, where rotating vector model fits indicated that the magnetar is a near-aligned rotator (Camilo et al. 2008). Occasional departures from the smooth PA swing are detected in individual epochs, an example of which is shown in the top-right panel of Figure 1. The relatively smooth evolution of the PA swing persists at higher observing frequencies where the effects of interstellar scattering are negligible, indicating these features are due to either an unusually smooth transition between two orthogonal polarization modes or the result of birefringent propagation effects in the near-field environment (Dyks 2020). Birefringent propagation effects would also explain the unusually high circular polarization fraction (up to $|V|/I=0.54$) as well as changes in the handedness between
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Examples of the three polarization profile states (A, B, and C) of 1E 1547.0−5408 seen at different epochs. Note, the profiles from 2022 March 16 (top-left) and 2022 April 19 (middle-right) correspond to the last preoutburst and first postoutburst radio detections, respectively. Each observation was dedispersed using a DM of 697 pc cm⁻³ and corrected for a Faraday RM of −1847 rad m⁻² prior to averaging in time and frequency.</figcaption></figure>

observations. A more detailed investigation is left for future work.

## 3.2. Timing

The variable radio profile of 1E 1547.0−5408 is not conducive to pulsar timing methods that use a single reference template. Instead, we obtained pulse times of arrival (ToAs) from the Parkes/PDFB4 data by using the *get_toas* tool in PRESTO (Ransom 2011) to fit a single Gaussian template to individual observations via the fast Fourier transform based approach of Taylor (1992). We added an extra 0.05 s uncertainty in quadrature with the formal ToA uncertainties to account for excess epoch-to-epoch scatter in the ToAs that arise from the varying radio profile (often referred to as “jitter”).

For NICER, we merged all barycenter-corrected event files and only accepted counts in the 1–5 keV energy range, which maximized the source pulse signal according to the $Z^2$ power

(Buccheri et al. 1983). We then split the data into time intervals that contain ≈1000 counts each, without allowing individual intervals to exceed a 3 day integration. We employed a nonbinned maximum likelihood estimation (MLE) method to measure the X-ray pulse ToAs (Livingstone et al. 2009; Ray et al. 2011). First, we model a high S/N pulse profile with a Fourier series including only the fundamental harmonic; adding higher-order harmonics did not improve the quality of the fit. Second, we fit, using MLE, each unbinned data segment to the same model, only allowing for a phase shift $\Delta\phi$. The best-fit phase shift is the value that corresponds to the minimum of the negative log likelihood, $-\log(L)_{min}$. The $1\sigma$ uncertainty on the phase shift was established by stepping over the full parameter space $\Delta\phi \in [0, 2\pi)$ and noting a change in $-\log(L)_{min}$ by 0.5 (corresponding to the ±34% or $1\sigma$ confidence interval of a $\chi^2$ distribution with 1° of freedom).

We obtained an initial phase-connected solution by using the Hidden Markov Model (HMM)-based glitch detector of
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Average unabsorbed X-ray (0.5–10 keV; top) and radio (0.7–4 GHz; bottom) flux of 1E 1547.0–5408 from 2022 January to August. The vertical dashed lines indicate the dates of the Fermi-GBM (MJD 59672; 2022 April 3) and Swift-BAT short bursts (MJD 59676; 2022 April 7), respectively, while the blue shaded region corresponds to where pulsed radio emission was undetected. The gray horizontal shading corresponds to the standard deviation about the baseline X-ray flux of $1.7 \times 10^{-11} \text{ erg s}^{-1} \text{ cm}^{-2}$.</figcaption></figure>

Melatos et al. (2020),<sup>23</sup> which tracks the spin frequency ($\nu$) and spin-down rate ($\dot{\nu}$) of the magnetar over time. This enabled robust integer pulse numbering to be applied to each ToA and provided estimates of any changes in $\nu$ and $\dot{\nu}$ at the time of the outburst.

Once the phase-connected solution was obtained, we then used the glitch plugin to tempo2 (Edwards et al. 2006; Hobbs et al. 2006) to perform independent local fits to $\nu$ and $\dot{\nu}$. The results of our local fits are shown alongside the HMM tracking in Figure 3. Both sets of spin frequency/spin-down-rate measurements generally agree with one another and display approximately the same secular evolution. A timing anomaly with clear jumps in both $\nu$ and $\dot{\nu}$ occurred in coincidence with the high-energy outburst, with the HMM recovering step changes of $\Delta\nu = (9 \pm 4) \times 10^{-7} \text{ Hz}$ and $\Delta\dot{\nu} = (-1.6 \pm 0.5) \times 10^{-12} \text{ s}^{-2}$. There is also a clear, secular increase in the spin-down rate of the magnetar ($\dot{\nu}$ is becoming more negative) over time, where $\dot{\nu}$ appears to follow an approximately linear trend over the first 120 days post-outburst. This is also reflected by the downward curve in the evolution of $\nu$ over time. Increasing spin-down rates are also commonly observed in the postoutburst behavior of other magnetars, including recent outbursts in the radio-loud magnetars PSR J1622−4950 (Camilo et al. 2018) and XTE J1810−197 (Caleb et al. 2022). A linear fit to the spin-down evolution gives an apparent $\dot{\nu} = -1.95(5) \times 10^{-19} \text{ s}^{-3}$. Similar behavior was previously observed in 1E 1547.0–5408 following the 2008 outburst (Israel et al. 2010).

In order to constrain both the longer-term rotational properties of 1E 1547.0–5408 and the outburst-induced changes in its timing better, we modeled the anomaly as a pulsar glitch where

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Top panel shows the prefit Parkes/NICER timing residuals of 1E 1547.0–5408 from 2021 June to 2022 August. Changes in the magnetar spin frequency and spin-down rate with respect to their nominal values (0.478836 Hz and $-3.63 \times 10^{-12} \text{ s}^{-2}$, respectively, on MJD 58350) are shown in the middle and bottom panels respectively, where the points correspond to local fits to the ToAs. Purple shading is the $\nu$ and $\dot{\nu}$ posterior distributions that were recovered by the HMM glitch detector at each epoch. Outliers in the local fits to $\dot{\nu}$ arise from the limited number of ToAs. The vertical shaded region and dashed lines are the same as in Figure 2.</figcaption></figure>

the corresponding change in rotation phase is given by

$$ \phi_g(t) = \Delta\phi_g + \Delta\nu_p(t - t_g) + \frac{1}{2}\Delta\dot{\nu}_p(t - t_g)^2 \\ + \frac{1}{6}\Delta\dot{\nu}_p(t - t_g)^3 - \Delta\nu_d\tau_d e^{-(t-t_g)/\tau_d}. \quad (3) $$

Here, $t_g$ is the glitch epoch, $\Delta\phi_g$ is a phase jump to account for residual ambiguities in the rotation phase of the magnetar after the event, $\Delta\nu_p$ is the permanent step change in the spin frequency, $\Delta\dot{\nu}_p$ the step change in the spin-down rate, and $\Delta\nu_d$ is the exponentially recovered change in the spin frequency that occurred over a characteristic timescale, $\tau_d$. We also included a glitch-induced second spin-frequency derivative ($\Delta\dot{\nu}_p$) term to account for the postoutburst linear increase in $\dot{\nu}$ that we discussed above.

The gap in our timing between the last Parkes detection on 2022 March 16 and the first NICER observation on April 9 makes it difficult to determine the precise glitch epoch. Since magnetar glitches are often associated with radiative events (Dib & Kaspi 2014), we use the date of the short burst detected by Fermi-GBM from the direction of 1E 1547.0–5408 as the nominal glitch epoch (i.e., $t_g = \text{MJD 59672}$; 2022 April 3; see Section 3.3.3). Note, the exact choice of glitch epoch between April 1 and April 7 has a strong impact on the recovered $\Delta\nu_p$, which we discuss further below. Using tempo2 and the FITWAVES plugin, we fit the glitch parameters alongside a set of 10 harmonically related sine waves to account for strong timing noise in the residuals. This returned values of $\Delta\nu_p = 2.0(1) \times 10^{-6} \text{ Hz}$, $\Delta\dot{\nu}_p = -1.23(2) \times 10^{-14} \text{ s}^{-2}$, and $\Delta\dot{\nu}_p = -1.86(3) \times 10^{-19} \text{ s}^{-3}$. A significant $\Delta\nu_d$ of $-2.1(1) \times 10^{-6} \text{ Hz}$ was recovered. The negative value of

<footnote n="23"><sup>23</sup> https://github.com/ldunn/glitch_hmm</footnote>
<table><caption>**Table 2**
Timing and Pulsar Parameters for 1E 1547.0–5408 from Our Joint Parkes/ NICER Timing</caption><thead><tr><th></th><th></th></tr></thead><tbody><tr><td colspan="2"><strong>Pulsar parameters</strong></td></tr><tr><td>R. A. (J2000)<sup>a</sup></td><td>15<sup>b</sup>50<sup>m</sup>54<sup>s</sup> 12386</td></tr><tr><td>Decl. (J2000)<sup>a</sup></td><td>-54°18′24″ 1141</td></tr><tr><td>ν (Hz)</td><td>0.478836(3)</td></tr><tr><td>ν̇ (s<sup>-2</sup>)</td><td>-3.63(3) × 10<sup>-12</sup></td></tr><tr><td>μ<sub>α</sub> cos δ (mas)<sup>a</sup></td><td>4.8</td></tr><tr><td>μ<sub>δ</sub> (mas)<sup>a</sup></td><td>-7.9</td></tr><tr><td>Timing epoch (MJD TDB)</td><td>58350</td></tr><tr><td>Position epoch (MJD TDB)</td><td>54795</td></tr><tr><td>DM (pc cm<sup>-3</sup>)</td><td>697(1)</td></tr><tr><td>RM (rad m<sup>-2</sup>)</td><td>-1847.6(5)</td></tr><tr><td>Time span (MJD)</td><td>59368–59808</td></tr><tr><td>Number of ToAs</td><td>202</td></tr><tr><td>Solar system ephemeris</td><td>DE436</td></tr><tr><td>log<sub>10</sub>(A<sub>r</sub>) (yr<sup>3/2</sup>)</td><td>-5.1(2)</td></tr><tr><td>β<sub>r</sub></td><td>5.7(8)</td></tr><tr><td colspan="2"><strong>Glitch parameters</strong></td></tr><tr><td>t<sub>g</sub> (MJD)</td><td>59672</td></tr><tr><td>Δν<sub>p</sub> (Hz)</td><td>2(1) × 10<sup>-7</sup></td></tr><tr><td>Δν̇<sub>p</sub> (s<sup>-2</sup>)</td><td>-2.4(1) × 10<sup>-12</sup></td></tr><tr><td>Δν̇<sub>p</sub> (s<sup>-3</sup>)</td><td>-2.0(1) × 10<sup>-19</sup></td></tr></tbody></table>

**Note.** Values in parentheses are the 1σ uncertainties on the last digit.

<sup>a</sup> Fixed to values from Deller et al. (2012).

Δν<sub>d</sub> signifies a resolved rise in the spin frequency over a period of τ<sub>d</sub> = 13(1) days as opposed to a postglitch decay.

To verify the existence of this apparent delayed rise in ν, we sampled the posterior distributions for each parameter using the TempoNest Bayesian inference plugin to tempo2 (Lentati et al. 2014). This also allowed us to model the timing noise of the magnetar simultaneously as a power-law (PL) red-noise process. The power spectral density of the red-noise model is given by

$$P(f) = \frac{A_r^2}{12\pi^2} \left( \frac{f}{1 \text{ yr}} \right)^{-\beta_r} \text{yr}^{-3}, \quad (4)$$

where A<sub>r</sub> is the red-noise amplitude (in units of yr<sup>3/2</sup>) and β<sub>r</sub> the spectral index. Two separate fits were performed: one that included a resolved rise in the spin frequency, and another where Δν<sub>d</sub> is fixed to zero for an instantaneous (unresolved) glitch. Comparing the recovered log-evidence from the resulting fits, we obtain a Bayes' factor of ln B = -2.8 in favor of a resolved rise. The negative value indicates there is no significant evidence for the presence of a resolved versus an instantaneous change in the spin frequency. Our results for the unresolved glitch model are summarized in Table 2.

We present the one- and two-dimensional posterior distributions for the glitch and timing noise parameters in Figure 4. There is a strong correlation between Δν̇<sub>p</sub> and Δν̇<sub>p</sub>, indicating these two parameters induce a similar signal across the relatively short postglitch timing of 1E 1547.0–5408. While Δν̇<sub>p</sub> agrees with the value recovered by the HMM at the 68% confidence interval, the TempoNest derived value of Δν<sub>p</sub> is smaller by a factor of ~4.5. As for the postoutburst rotational

<figure><img src="image_7.png" /><figcaption>**Figure 4.** One- and two-dimensional posterior distributions for the glitch and red noise in 1E 1547.0–5408. The darker and lighter shaded regions represent the 68% and 95% confidence intervals, respectively.</figcaption></figure>

evolution, our recovered value of Δν̇<sub>p</sub> is consistent with that inferred from a simple linear fit to ν̇ over time.

The difference in the Δν<sub>p</sub> values is a direct result of our choice of the glitch epoch and the large increase in the spin-down rate. This is apparent in the TempoNest runs where we assumed earlier and later values of t<sub>g</sub>, where earlier glitch epochs result in a larger recovered value of Δν<sub>p</sub> to match the postoutburst timing. Going the other way, setting the glitch epoch to the date of the Swift-BAT short burst (MJD 59676; 2022 April 7) results in a value of Δν<sub>p</sub> that is consistent with zero at the 68% confidence interval, i.e a pure spin-down glitch. While several glitches in other magnetars have been associated with minimal instantaneous changes in ν (e.g., the 2012 outburst of 1E 1048.01–5937; Archibald et al. 2015), the lack of ToAs between the radio shutdown of 1E 1547.0–5408 and beginning of our NICER monitoring prevents us from ruling out a nonzero Δν<sub>p</sub>.

### 3.3. High-energy Outburst

#### 3.3.1. Spectral Analysis

We employed Xspec version 12.12.1 for our X-ray spectral analysis. We used the tbabs model to account for the line-of-sight absorption toward the source, along with the Wilms et al. (2000) solar abundances. We binned all spectra to have 5 counts per bin and used the W-statistic (C-stat in Xspec) to estimate the best-fit model parameters and their associated uncertainties. For the NICER spectra, however, we used the pgstat statistic, which is a more appropriate choice given the modeled nature of the background (Remillard et al. 2022). Finally, we added a numerical factor to account for any cross-calibration uncertainties when fitting spectra from different instruments simultaneously. We find a ≲2% and ≲10% deviation between the two NuSTAR spectra and the NICER/ Swift-XRT and NuSTAR spectra, respectively, commensurate with the expected values (Madsen et al. 2015).
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Upper panel: broadband $\nu F_\nu$ spectra of 1E 1547.0–5408 derived during the 2022 April outburst (NICER+NuSTAR, red), 2019 February (XRT+NuSTAR, black), and 2016 April (XRT+NuSTAR, green). The solid lines represent the best-fit model consisting of two BB and a PL. A flux enhancement is clearly present at soft X-rays coincident with the most recent outburst, in contrast to the stable nonthermal hard X-ray tail. Lower panel: deviation of the data from the best-fit model in units of $\sigma$.</figcaption></figure>

We fit the NuSTAR and NICER observations taken on 2022 April 21 simultaneously, which provides the highest S/N broadband spectral shape of 1E 1547.0–5408 during this most recent outburst. We also fit the preoutburst 2016 and 2019 simultaneous Swift-XRT and NuSTAR observations (Coti Zelati et al. 2020) to quantify the source spectral variability better. We modeled all spectra in the 1–70 keV range using a three-component construction: two absorbed blackbodies (BBs; $BB_{\text{warm}}$ and $BB_{\text{hot}}$) plus a PL, suitably representative of the magnetar broadband X-ray spectral shape (Götz et al. 2006; Kuiper et al. 2006; den Hartog et al. 2008a, 2008b; Enoto et al. 2017). We linked the absorption hydrogen column density $N_H$ among all the spectra and left the rest of the model parameters free to vary. We establish the goodness of fit by running the Xspec goodness command to simulate 1000 spectra drawn from the best-fit model and compare their fit statistic to the observed data. We find that 16% of the spectra have a fit statistic smaller than the actual value, implying that the model adequately explains the data. We measure a best-fit $N_H = (3.67 \pm 0.02) \times 10^{22} \text{ cm}^{-2}$, a factor of 1.75 times higher than the value predicted by the $N_H$–DM relation of He et al. (2013). We display the source spectrum and best-fit model in Figure 5, and present the best-fit parameters and their $1\sigma$ uncertainties in Table 3.

We find that the 2022 April 21 observation, 2.5 weeks after the GBM short burst detection (2 weeks after the confirmed BAT detection), exhibits a flux that is about 50% larger than the preoutburst level. This increase is solely due to an increase in the soft 0.5–10 keV X-ray flux (Figure 5), and particularly seen in the $BB_{\text{hot}}$ contribution. The flux of this component has increased by a factor 3 and 5 since 2016 and 2019, respectively. On the other hand, the increase in the $BB_{\text{warm}}$ component is a factor 1.2 and 1.3 larger compared to 2016 and 2019, respectively. Finally, the hard X-ray PL flux is consistent between all three observations at the $1\sigma$ level. This implies that either the hard X-ray flux was not impacted by this most recent outburst, or it followed a steeper temporal decay trend compared to the soft X-ray flux.

While the source exhibited a change in the overall flux, the broadband spectral curvature in 2022 remained relatively stable

compared to the previous epochs. We find no significant variability in the temperature of either of the two BBs, nor in the hardness of the PL component. This points to an increase in the overall area of the hot thermally emitting region as the origin of the soft X-ray flux increase (Table 3).

To study the long-term high-energy evolution of the 1E 1547.0–5408 outburst, we modeled the NICER and Swift-XRT spectra in the 1–6 keV range (beyond 6 keV the data for both instruments were mainly background dominated). We used a simple absorbed BB model and fixed the hydrogen column density $N_H$ to the best-fit value as derived above. This model sufficed to describe all the spectra except for the NICER observation on 2022 April 9 (ID 5020300101; Table 1) which required a second BB component to result in a statistically acceptable fit. The resulting 0.5–10 keV absorption-corrected flux evolution is shown in the upper panel of Figure 2. At the start of the NICER monitoring on 2022 April 9, we find a source flux that is a factor 3.5 larger than the preoutburst value, as established with Swift-XRT on 2022 March 5 (green circles in Figure 2, upper panel; see also Coti Zelati et al. 2020). This confirmed that 1E 1547.0–5408 was experiencing yet another high-energy outburst, its first since its major 2009 outburst. The source flux decayed back to the same preoutburst persistent flux level on 2022 April 26, i.e., 17 days later, and remained there until the end of our monitoring in 2022 August. We find that the bulk of the flux increase throughout the outburst occurred at energies >2 keV, consistent with the result of the NICER+NuSTAR broadband modeling.

### 3.3.2. X-Ray Timing Analysis

Given the complexity of the glitch timing model (Section 3.2), we fold the NICER barycenter-corrected events with a (strictly) postglitch variant of the combined radio/X-ray timing solution in Table 2. To account for red noise in the timing residuals, we fit for spin-frequency derivatives up to $d^5 \nu/d\tau^5$, at which order the residuals are dominated by white noise. Due to the limited exposure of most individual NICER observations, we merged several consecutive ones until we accumulated about 6000 counts in the 1.5–6 keV interval. This ensured a proper sampling of the pulse shape in two energy bands, namely 1.5–3 keV and 3–6 keV. During the outburst decay phase (2022 April 9–26), the stretch of each interval that fulfilled our criterion was on average 2 days, and increased to 10 days after the source returned to its preoutburst flux level. We present these pulse profiles in Figure 6. Each row displays the pulsations per time interval starting from the date of the initial NICER observation on 2022 April 9 (day 0), while the left and right panels show the 1.5–3 keV and 3–6 keV pulse profiles, respectively; note that the pulse in red is from the NICER observation that is simultaneous with the NuSTAR one for 2022 April 21.

The soft 1.5–3 keV pulsations are quasi-sinusoidal throughout our monitoring campaign. On the other hand, the 3–6 keV profile exhibits pronounced variability. For instance, during the outburst decay phase, there is an evident shoulder leading to the pulse peak. Moreover, the peak is slightly broader early in the outburst and displays complex shape, likely formed of two or more peaks. This complexity is confirmed with NuSTAR as we show below. The 3–6 keV pulse shape turns quasi-sinusoidal starting on day 17 from the first NICER observation, coincident with the return of the flux to the preoutburst level (Figure 2). We derive the rms pulsed fraction (rms PF) for each of the light
<table><caption>**Table 3**
Broadband X-Ray Spectral Results</caption><thead><tr><th>Date</th><th>kT<sub>warm</sub><br>(keV)</th><th>R<sup>2</sup><sub>warm</sub><br>(km<sup>2</sup>)</th><th>kT<sub>hot</sub><br>(keV)</th><th>R<sup>2</sup><sub>hot</sub><br>(km<sup>2</sup>)</th><th>Γ</th><th>F<sub>warm</sub>/F<sub>hot</sub>/F<sub>PL</sub><br>(10<sup>-12</sup> erg s<sup>-1</sup> cm<sup>-2</sup>)</th><th>F<sub>S</sub>/F<sub>H</sub><br>(10<sup>-12</sup> erg s<sup>-1</sup> cm<sup>-2</sup>)</th><th>L<sub>X</sub><br>(10<sup>34</sup> erg s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>2016-03-23</td><td>0.59 ± 0.03</td><td>2.3<sup>+0.7</sup><sub>-0.5</sub></td><td>1.16<sup>+0.1</sup><sub>-0.08</sub></td><td>0.03<sup>+0.02</sup><sub>-0.01</sub></td><td>0.2 ± 0.1</td><td>14.1<sup>+0.9</sup><sub>-0.8</sub> / 2.9<sup>+0.7</sup><sub>-0.6</sub> / 8.9<sup>+0.5</sup><sub>-0.5</sub></td><td>17.2<sup>+0.9</sup><sub>-0.8</sub> / 8.7<sup>+0.4</sup><sub>-0.4</sub></td><td>6.3<sup>+0.2</sup><sub>-0.2</sub></td></tr><tr><td>2019-02-15</td><td>0.67 ± 0.02</td><td>1.3 ± 0.2</td><td>1.4 ± 0.2</td><td>0.008 ± 0.004</td><td>0.0 ± 0.1</td><td>13.7<sup>+0.5</sup><sub>-0.5</sub> / 1.7<sup>+0.5</sup><sub>-0.3</sub> / 8.7<sup>+0.7</sup><sub>-0.6</sub></td><td>15.5<sup>+0.5</sup><sub>-0.5</sub> / 8.7<sup>+0.5</sup><sub>-0.5</sub></td><td>5.9<sup>+0.2</sup><sub>-0.1</sub></td></tr><tr><td>2022-03-21</td><td>0.64<sup>+0.04</sup><sub>-0.05</sub></td><td>2.0<sup>+0.5</sup><sub>-0.3</sub></td><td>1.0 ± 0.1</td><td>0.09<sup>+0.14</sup><sub>-0.06</sub></td><td>0.4 ± 0.2</td><td>17<sup>+2</sup><sub>-3</sub> / 6<sup>+3</sup><sub>-2</sub> / 9.0<sup>+1</sup><sub>-1</sub></td><td>23<sup>+1</sup><sub>-1</sub> / 8.7<sup>+0.8</sup><sub>-0.8</sub></td><td>7.7<sup>+0.3</sup><sub>-0.3</sub></td></tr></tbody></table>

Note. The BB emitting areas and luminosity are derived assuming a distance of 4.5 kpc (Tiendo et al. 2010). F<sub>S</sub> and F<sub>H</sub> are the 0.5–10 keV and 10–70 keV absorption-corrected fluxes, respectively.

<figure><img src="image_9.png" /></figure>

**Figure 6.** Time-resolved NICER pulse profiles of 1E 1547.0–5408 in days from 2022 April 9. We show the pulsations in the energy range 1.5–3 keV (left column) and 3–6 keV (right column). The red profile corresponds to the NICER observation simultaneous with NuSTAR taken on 2022 April 21. All profiles are displayed using 15 phase bins per rotational cycle, with the exception of the red profile, which exhibits 30 phase bins per pulse period (see text for details). Two cycles are shown for clarity.

curves in Figure 6 (see, e.g., Woods et al. 2004 for details). For this purpose, we fit each background-subtracted pulse profile to a Fourier series including three harmonics, which was sufficient to provide an acceptable fit to the data. We find PFs ranging

<figure><img src="image_10.png" /></figure>

**Figure 7.** The 1E 1547.0–5408 pulse profiles as derived with NuSTAR at different energies (top to bottom). See the text for more details.

from 19% to 25% and 29% to 34% in the 1.5–3 keV and 3–6 keV energy bands, respectively, with a typical $1\sigma$ uncertainty of about 3%. This implies a lack of significant variability in the PF at soft X-rays concurrent with the outburst flux enhancement.

The NuSTAR observation of 1E 1547.0–5408 on 2022 April 21 caught the tail end of the outburst. We display the pulse profiles during this observation in three separate energy bands, namely, 3–6 keV, 6–10 keV, and 10–15 keV (Figure 7). We confirm the NICER results which show, in the energy range 3–6 keV, a nonsymmetric profile with a shoulder leading to a multipeaked pulse maximum; this fine structure in the peak is reminiscent of that seen during the 2020 outburst of SGR 1830-0645 (Younes et al. 2022b). Similar pulse profile morphology is apparent in the 6–10 keV range: the pulse exhibits considerable asymmetry with a shoulder leading to the pulse peak. All these features are clearly absent from the 2016 and 2019 NuSTAR monitoring; the pulse profile in the same energy range shows a purely sinusoidal form (Coti Zelati et al. 2020; Figure 5). On the other hand, the pulse profile at high energies between 10 and 25 keV exhibits a quasi-sinusoidal shape, consistent with the result of the earlier NuSTAR observations. Note, however, the lower S/N in this band
compared with that at energies below 10 keV. We do not detect significant pulsations at energies >25 keV, despite the detection of the source at the ≳10σ confidence level.

Finally, we measure the NuSTAR energy-resolved rms PF following the same recipe we utilized for NICER. We find an increasing trend at soft energies, from about 30% and up to 45% at 10 keV. The PF drops to 30% in the energy range 10–25 keV. Moreover, the latter profile appears slightly offset from the soft-energy ones (Figure 7), yet a formal measurement is complicated by the difference in shape between the profiles. Lastly, we derive a 3σ upper-limit of 25% on the PF at energies >25 keV. These elements are broadly consistent with the 2016 and 2019 NuSTAR findings of Coti Zelati et al. (2020).

### 3.3.3. Short Burst Searches

The 22 day discrepancy between the time of the first radio nondetection, which occurred on 2022 March 16, and the BAT trigger on 2022 April 7 prompts the question on whether the radio disappearance genuinely preceded the trigger of the high-energy outburst. With the absence of any sensitive X-ray observations of 1E 1547.0–5408 during this gap, we relied on burst detection to gauge the source activity qualitatively. Typically, the rate of hard X-ray bursts from an active magnetar is maximal at the start of an outburst, declining thereafter. This behavior is observed during burst storms, e.g., 1E 2259+596 (Gavriil et al. 2004) and SGR 1935+2154 (Younes et al. 2020), as well as over the course of weeks following the magnetar outburst onset, e.g., SGR 1830–0645 (Younes et al. 2022a) and Swift J1818.0–1607 (Hu et al. 2020). Hence, if the outburst indeed began at the time of the radio nondetection, more bursts should be detected from the direction of 1E 1547.0–5408 by Fermi-GBM during the 20 day interval.

We searched the CTTE data for untriggered bursts in the energy range 8–100 keV following the procedure detailed in Gavriil & Kaspi (2004) and Younes et al. (2020). In summary, we binned the light curve of each GBM detector separately, at a temporal resolution $t_r$. We search each light curve in consecutive intervals of 100 s, overlapping by 10 s. We note the deviation of the number of counts $x$ in each bin from the long-term average rate in the light curve, assuming a Poisson probability density function $P(x)$. A potential burst is flagged if $P(x) < 0.01/N$, where $N$ is the number of bins per light curve (typically 100/$t_r$). The flagged bins are removed and the procedure is repeated until no more bins are found. We chose $t_r$ to be 8, 32, and 128 ms to sample the full range of magnetar burst durations. We confirm our GBM search results with a Bayesian blocks search technique as detailed in Lin et al. (2020). We only inspect potential bursts that were found in two or more detectors; single detector enhancements are typically due to particle background rather than an astrophysical event. Moreover, we rejected all bursts when 1E 1547.0–5408 was behind the earth for Fermi, and those that were registered only in NaI detectors with an angle >60° from the source since their effective area drops significantly at such large observing angles. The source-identification process, however, is complicated by another magnetar, SGR J1555–5402, which is 40′ from 1E 1547.0–5408 and was also active at this time. This separation is not distinguishable by GBM, but is easily resolved in BAT images.

We find six candidate bursts that may originate from 1E 1547.0–5408 between 2022 March 16 and April 7, including one that corresponds to a GBM trigger on 2022

April 3 (trigger 220403928). BAT detected a count rate increase in two cases, in addition to the short burst detected on 2022 April 7. One of these was identified as the aforementioned SGR J1555–5402, and the other coincidence has a negative value (consistent with zero) at the 1E 1547.0–5408 location and excludes it as the source of the rate increase. Hence, the apparent lack of any bursting activity associated with 1E 1547.0–5408 over the course of three weeks following the last radio detection suggests that the high-energy outburst genuinely commenced later. However, we cannot establish the precise date of the outburst, and tentatively place it sometime in early April. Future continuous monitoring of X-ray bright radio-loud magnetars is crucial for revealing the dynamic between the radio and X-ray emission at the start of an outburst.

## 4. Discussion

Prior to the 2009 outburst, 1E 1547.0–5408 was considered to be a so-called transient magnetar, similar to XTE J1810−197 (Ibrahim et al. 2004), with a historical quiescent (absorbed, 0.1–10 keV) flux of (0.3–2.1)×10⁻¹² erg s⁻¹ cm⁻² (Gelfand & Gaensler 2007). However, the 2009 outburst was associated with a transition of 1E 1547.0–5408 into a persistent magnetar state where it exhibits a significantly higher steady-state X-ray flux than the pre-2006 quiescent state (Coti Zelati et al. 2020). On account of this increased persistent emission, the 2022 outburst, appearing to be both less intense and shorter lived than the 2008 outburst, is likely an artifact of the much higher baseline X-ray flux. Indeed the maximum 0.5–10 keV flux of (6.0 ± 0.4) × 10⁻¹¹ erg s⁻¹ cm⁻² is comparable to the observed peak flux of ~6 × 10⁻¹¹ erg s⁻¹ cm⁻² during the 2008 outburst (Israel et al. 2010). Had this previous outburst occurred more recently, the X-ray flux of 1E 1547.0–5408 would have fallen below the current baseline of ~1.7 × 10⁻¹¹ erg s⁻¹ cm⁻² after a similarly short 10–20 day timescale. Were it not for our triggered NICER observations, the enhanced X-ray emission from this outburst may have been missed entirely. A smaller outburst may not have even produced a detectable X-ray enhancement.

Magnetar X-ray outbursts are generally attributed to crustal shifts due to stresses on the surface from internal B-field decay. These shifts could also either modify or enhance toroidal twists in the morphology of external magnetic field lines (see Turolla et al. 2015 and references therein). The soft X-ray thermal emission arises due to either internal energy deposition in the outer crust (Beloborodov & Li 2016; Akgün et al. 2018; Lander & Gourgouliatos 2019), bombardment of the surface by accelerated particles in the twisted magnetospheric field loops (Beloborodov 2009), mechanical stresses slowly building in the crust (see Lander et al. 2015; Kojima 2022 and references therein) or a combination of all three. In this model, a more complex pulse shape is expected during the outburst due to the emergence of new hot spots on the surface (e.g., Archibald et al. 2017b; Younes et al. 2022b), commensurate with the 3–6 keV pulse profile morphology displayed in Figure 7. As the outburst abates, the source surface heat map resorts back to its long-term, quasi-sinusoidal configuration. If surface bombardment is active, the evolving surface temperature profile likely is accompanied by a migration of field-line footprints associated with the twisting and untwisting of the magnetospheric field.
The hard X-ray tail above 10 keV is believed to arise from resonant inverse Compton scattering (RICS) of the soft surface thermal photons by relativistic electrons accelerated in twisted loops (Baring & Harding 2007; Nobili et al. 2008; Wadiasingh et al. 2018). Figures 5 and 7 clearly display a lack of any variability in the hard X-ray emission concurrent with the elevated soft thermal emission during this outburst of 1E 1547.0–5408. This disconnect between soft and hard X-ray bands is to be expected. Detailed models of RICS emission generally favor the production of a signal at quasi-equatorial locales of modest altitudes, where scattering kinematics between the electrons and the soft X-ray photons from the surface readily access the cyclotron resonance in the cross section (Wadiasingh et al. 2018). Thus changes in the field-line geometry near their footprints (e.g., due to a localized crustal stress) and moreover near the polar regions likely have only a small impact on the RICS emission, as do alterations of the surface hotspot size and temperature. Evidence for some separation of magnetic colatitudes for hard X-ray signals and the surface soft X-ray ones may be suggested by the modest 0.1–0.3 phase lag of the peak between the 3 and 6 keV bands and the 10–25 keV one in Figure 7. Thus changes in pulsed radio emission emanating from in or near the open field region might naturally not be accompanied by modifications to the hard X-ray signal.

Generally, pulsed radio emission from magnetars is closely linked to the outburst mechanism: it switches on following the onset of an outburst before gradually fading into radio silence as the neutron star settles into X-ray quiescence (e.g., Camilo et al. 2016; Scholz et al. 2017; but see Zhu et al. 2020; Younes et al. 2023). Unlike other radio-loud magnetars, persistent radio pulses have been detected from 1E 1547.0–5408 since the quasi-stabilization of its X-ray flux to the current high state in ~2013 (Coti Zelati et al. 2020; F. Camilo et al. 2023, in preparation). As shown in Figure 2, radio pulses were detected up until sometime between 2022 March 16 and 26, and were not seen again until April 19. An initial disappearance and reactivation of the radio emission of 1E 1547.0–5408 was also reported following the 2009 outburst (Burgay et al. 2009; Camilo et al. 2009).

The 2016 magnetar-like outburst of the high magnetic field strength pulsar PSR J1119–6127 was also associated with a shutdown in the persistent radio emission, which was redetected around two weeks after the initial X-ray outburst (Dai et al. 2018). Moreover, simultaneous X-ray and radio observations of PSR J1119–6127 revealed that magnetar-like short bursts were associated with a sudden disappearance of pulsed radio emission (Archibald et al. 2017a). This correlation was interpreted as a burst-induced quenching of the radio emission arising from electron–positron pairs leaking from the fireball trapped above the surface into the particle-acceleration gaps, thereby increasing the plasma density within these regions and suppressing the acceleration of charges that precipitate radio emission. A similar phenomenon would provide a natural explanation for the disappearance of radio pulses from 1E 1547.0–5408 if it occurred after the onset of the high-energy outburst. Yet, the dearth of Fermi-GBM and Swift-BAT burst candidates up to early 2022 April implies a delayed onset of the high-energy outburst with respect to the disappearance of the radio pulses. An earlier onset necessitates an unlikely scenario in which all short bursts emitted over an extended period of time were missed by all large FoV hard

X-ray and gamma-ray monitors. Alternatively, a slow undetected rise in high-energy activity could explain the observed delay between the radio disappearance and delayed onset of short burst detections. Such behavior was previously seen in the 2002 outburst of 1E 1048.1–5937 (Archibald et al. 2020).

Small-scale twisting of the magnetosphere from crustal fracturing and motion of the magnetic footprints can lead to a decrease or complete failure of the charged particle currents responsible for producing the pulsed radio emission in addition to variations in the optical depth of the magnetosphere (Beloborodov 2009; Timokhin 2010; Hu et al. 2022). Regardless of the exact underlying physics, any alteration of the magnetosphere must have been short lived, as we did not detect any substantial change in the average radio pulse profile shape or polarization properties of 1E 1547.0–5408 after reactivation. This is markedly different to the behavior of other radio-loud magnetars, which typically display a broad range of profile variations and a marked increase in radio flux in the immediate aftermath of an outburst (e.g., Scholz et al. 2017; Dai et al. 2018, 2019). Future observations of similar high-energy outbursts both in 1E 1547.0–5408 and other highly magnetized pulsars will shed further light on the radio-quenching mechanism.

Our joint radio and X-ray observations revealed that the 2022 outburst resulted in a significant spin-up event in the timing of 1E 1547.0–5408, which bears a strong resemblance to a pulsar glitch. The gap in our timing coverage due to the disappearance of pulsed radio emission, low observing cadence of Swift, and beginning of our triggered NICER observations means the exact epoch of the event is unknown, but is constrained to between MJD 59661 and 59672 (2022 March 23–April 3). Glitch-like timing anomalies and subsequent rapid changes in $\dot{v}$ are common feature of the postoutburst timing of magnetars (e.g., Dib & Kaspi 2014; Archibald et al. 2020), the latter of which are often ascribed to torque variations imparted by twists in the magnetosphere (Thompson et al. 2002). Unlike rotation-powered pulsars, the majority of glitches in magnetars are associated with radiative outbursts that are believed to be caused by either starquakes (Thompson & Duncan 1995) or magnetic reconnection events (Lyutikov 2002). In the star-quake model, the gradual untwisting of the internal magnetic field induces an enormous strain on the rigid outer crust of the neutron star. Eventually the tensile strength of the crust is overcome, leading to a sudden plastic deformation or even fracturing of a small region of the crust. This triggers an outward exchange of angular momentum from the superfluid core to the crust via unpinning of vortices at the crust–core boundary (Ruderman et al. 1998). Modeling the timing anomaly in 1E 1547.0–5408 as a glitch, we obtained permanent step changes in $v$ and $\dot{v}$ of 0.2(1) µHz and $-2.4(1) \times 10^{-12} \text{ s}^{-2}$, respectively. The permanent change in the spin frequency is small when compared to most glitches in magnetars (see Figure 2 of Fuentes et al. 2017). On the other hand, the increase in the spin-down rate is among the largest recorded in any pulsar (Lower et al. 2021a; Basu et al. 2022).

Following the glitch, we also detected a secular increase in the spin-down rate that continued up until at least our last NICER observation. We inferred a $\dot{v}$ of $-1.95(5) \times 10^{-19} \text{ s}^{-3}$ from a simple linear fit to the $\dot{v}$ time series and $-2.0(1) \times 10^{-19} \text{ s}^{-3}$ from our glitch fit in Section 3.2. This corresponds to a net increase in $\dot{v}$ of $-2.54 \times 10^{-12} \text{ s}^{-2}$ over the 147 days of postoutburst timing. A similar linear trend was
observed following the 2008 outburst (Israel et al. 2010), where the resulting $\dot{v} = -4.8(9) \times 10^{-18} s^{-3}$ was an order of magnitude larger than we observed after the 2022 outburst. Secular increases in the spin-down rate are a common feature of outbursts in magnetars and high B-field pulsars and are often attributed to the magnetosphere becoming increasingly twisted up to a maximum value after a few 10s to 100s of days (Thompson et al. 2002; Beloborodov 2009). After reaching the maximum twist, the field bundle then begins to unwind, causing a turnover in the spin-down evolution as the magnetar relaxes back toward the preoutburst state. A global alteration of the twist can be ruled out in this 2022 event given the lack of hard X-ray evolution, indicating that either a small-scale local twist in bundles of magnetic field lines near the active pole or transient particle winds may be responsible for the secular increase in $\dot{v}$ (Harding et al. 1999).

Spin-down rates peak anywhere between $-10^{-13}$ and $-10^{-11} s^{-2}$ depending on the strength of the outburst. Given the lack of a turnover in the postoutburst $\dot{v}$ evolution of 1E 1547.0–5408, it is likely the maximum twist imparted by the 2022 outburst or peak in the outflowing particle flux has yet to be reached. The recovery back to the initial spin-down state varies between magnetars and from one outburst to the next. Occasionally, the initial turnover is followed by an extended period of “noisy” spin-down evolution, including quasi-periodic variations such as those seen in 1E 1048.0–5937 (Archibald et al. 2015) and Swift J1818.0–1607 (Rajwade et al. 2022). These variations are reminiscent of the spin-down state-switching process seen among rotation-powered pulsars (Lyne et al. 2010), and often not associated with any changes in high-energy emission. Such behavior remains to be seen in the current postoutburst timing of 1E 1547.0–5408.

## 5. Conclusions

We have presented the detection of a new outburst of 1E 1547.0–5408 that began in 2022 April. The outburst was marked by an increased soft X-ray emission and the disappearance of pulsed radio emission, both of which recovered back to their preoutburst properties ~3 weeks later. The disappearance of the radio pulses prior to the outburst onset is surprising, and could point to an undetected slow rise in high-energy activity. Such behavior could arise from small-scale crustal fracturing, where variations in the magnetospheric currents and optical depth resulted in a quenching of the radio emission mechanism. Interestingly, we find no change to the hard nonthermal tail concurrent with the soft X-ray and radio variability. Our joint Parkes/NICER timing measurements revealed the presence of a glitch-like timing event in the rotation of the magnetar. A significant instantaneous increase in the spin-down rate was also measured, which continued to increase at a rate of $-2.75 \times 10^{-19} s^{-3}$ over the first 147 days after the outburst. Small-scale shifts in magnetospheric geometry in the lead up to the main quake may have been responsible for the quenching of pulsed radio emission. These results highlight the importance of conducting high-cadence monitoring of radio-loud magnetars such as 1E 1547.0–5408 at both radio and X-ray frequencies. Future outbursts with greater coverage around the initial trigger time may be able to resolve the radio quenching, delayed rise in spin frequency, and initial increase in X-ray flux. Such observations would provide an even clearer picture of the magnetar outburst mechanism. Given some similarities between this latest event and the 2008

outburst, it is possible that 1E 1547.0–5408 has entered a new state of increased activity. In this case, the 2022 outburst may therefore be a precursor to additional outbursts over the coming years.

We thank George Hobbs and Rahul Sengar for generously giving us some of their Parkes observing time, and a special thanks to James Green and Phil Edwards for granting Parkes director discretionary time. We also thank the anonymous referee for their helpful comments. The Parkes radio telescope (Murriyang) is part of the Australia Telescope National Facility (https://ror.org/05qajvd42) which is funded by the Australian Government for operation as a National Facility managed by CSIRO. We acknowledge the Wiradjuri people as the traditional owners of the Observatory site. Radio data reduction and analysis were performed on the OzSTAR national HPC facility, which is funded by Swinburne University of Technology and the National Collaborative Research Infrastructure Strategy (NCRIS). A portion of this work was supported by NASA through the NICER mission and the Astrophysics Explorers Program. G.Y.’s research is supported by an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Oak Ridge Associated Universities under contract with NASA. L.D. is supported by the Australian Research Council Centre of Excellence CE170100004 (OzGrav). M.G.B. acknowledges the generous support of the National Science Foundation through grant AST-1813649. Z.W. acknowledges support by NASA under award number 80GSFC21M0002. S.G. acknowledges the support of the National Centre for Space Studies (CNES).

Software: dspsr (van Straten & Bailes 2011), HEAsoft (v6.30.1; HEASARC 2014), psrarchive (Hotan et al. 2004; van Straten et al. 2012), NuSTARDAS (v2.1.2), DM_phase (Seymour et al. 2019), PRESTO (Ransom 2011), RMNest (Lower et al. 2022), tempo2 (Edwards et al. 2006; Hobbs et al. 2006), TempoNest (Lentati et al. 2014), XSPEC (v12.12.1; Arnaud 1996).

## ORCID iDs

Marcus E. Lower https://orcid.org/0000-0001-9208-0009

George Younes https://orcid.org/0000-0002-7991-028X

Paul Scholz https://orcid.org/0000-0002-7374-7119

Fernando Camilo https://orcid.org/0000-0002-1873-3718

Liam Dunn https://orcid.org/0000-0002-1769-6097

Simon Johnston https://orcid.org/0000-0002-7122-4963

Teruaki Enoto https://orcid.org/0000-0003-1244-3100

David M. Palmer https://orcid.org/0000-0001-7128-0802

Matthew G. Baring https://orcid.org/0000-0003-4433-1365

Keith Gendreau https://orcid.org/0000-0001-7115-2819

Ersin Göğüş https://orcid.org/0000-0002-5274-6790

Sebastien Guillot https://orcid.org/0000-0002-6449-106X

Alexander J. van der Horst https://orcid.org/0000-0001-9149-6707

Chin-Ping Hu https://orcid.org/0000-0001-8551-2002

Chryssa Kouveliotou https://orcid.org/0000-0003-1443-593X

Lin Lin https://orcid.org/0000-0002-0633-5325

Christian Malacaria https://orcid.org/0000-0002-0380-0041

Rachael Stewart https://orcid.org/0000-0002-0254-5915

Zorawar Wadiasingh https://orcid.org/0000-0002-9249-0515
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