# Discovery of 146.8 s pulsations from EP J005146.9-730930, a new transient Be/X-ray binary in the SMC

F Haberl, Y Xu, C Maitra, G Vasilopoulos, W Zhang, W Yuan, C Jin, H N Yang, L Ducci, D M Kaltenbrunner, et al.

► To cite this version:

F Haberl, Y Xu, C Maitra, G Vasilopoulos, W Zhang, et al. Discovery of 146.8 s pulsations from EP J005146.9-730930, a new transient Be/X-ray binary in the SMC. Monthly Notices of the Royal Astronomical Society, 2025, 542 (2), pp.583 - 590. 10.1093/mnras/staf1224. insu-05222711

HAL Id: insu-05222711

https://insu.hal.science/insu-05222711v1

Submitted on 25 Aug 2025

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# Discovery of 146.8 s pulsations from EP J005146.9–730930, a new transient Be/X-ray binary in the SMC

F. Haberl<sup>1,★</sup> Y. Xu,<sup>2★</sup> C. Maitra<sup>1</sup> G. Vasilopoulos<sup>1,3,4</sup> W. Zhang,<sup>5</sup> W. Yuan,<sup>5,6</sup> C. Jin,<sup>5,6,7</sup> H.N. Yang<sup>1,5</sup> L. Ducci<sup>1,8</sup> D.M. Kaltenbrunner,<sup>1</sup> P. Maggi<sup>9</sup> and A. Rau<sup>1</sup>

Accepted 2025 July 22. Received 2025 July 22; in original form 2025 April 20

## ABSTRACT

Recent observations of the Small Magellanic Cloud (SMC) with *Einstein Probe (EP)* revealed a new transient X-ray source, most likely identified as Be/X-ray binary. To characterise the X-ray properties of EP J005146.9–730930 and in particular to look for pulsations in the X-ray flux, we triggered an *XMM–Newton* anticipated target of opportunity observation. To follow the flux evolution during the outburst we monitored the source for about three months with the Follow-up X-ray Telescope of *EP*. The *XMM–Newton* observation was performed on 2024 September 15 and we used the data from the European Photon Imaging Camera (EPIC) for detailed spectral and timing analyses. The EPIC X-ray spectrum is well described by an absorbed power law with photon index of $1.25 \pm 0.04$ and the timing analysis revealed pulsations with $146.79 \pm 0.03$ s. The source flux had decreased by a factor of about 10 since the observed maximum about one month before the *XMM–Newton* observation. EP J005146.9–730930 was never detected significantly during serendipitous observations before September 2024. The characteristics of the X-ray brightening suggest the source was discovered during a type II outburst reaching an X-ray peak luminosity of $\sim 2 \times 10^{37}$ erg s$^{-1}$. The trend of spectral hardening towards higher luminosities observed by *EP* suggests that the source is accreting below the critical luminosity, yielding an estimated lower limit for the pulsar magnetic field strength of $3.3 \times 10^{12}$ G. The improved X-ray position confirms the candidate Be star OGLE J005147.58–730924.7 as optical counterpart. We conclude that EP J005146.9–730930 $= SXP 146.8$ is a new Be/X-ray binary pulsar in the SMC.

**Key words:** stars: emission-line, Be–stars: neutron–pulsars: individual: EP J005146.9–730930–galaxies: individual: SMC–X-rays: binaries.

## 1 INTRODUCTION

The Small Magellanic Cloud (SMC) is well known for the large population of high-mass X-ray binaries (HMXBs) with more than 125 systems known. About half of them revealed X-ray pulsations indicating the spin period of an accretion-powered neutron star. Since the compilation of the SMC HMXB catalogue by Haberl & Sturm (2016), seven new pulse periods from known or new Be/X-ray binaries (BeXRBs, the major subclass of HMXBs) were discovered (Vasilopoulos et al. 2016, 2020; Haberl et al. 2019; Lazzarini et al. 2019; Kennea et al. 2020; Coe et al. 2023; Maitra et al. 2023), increasing the number of BeXRB pulsars to 70. The large number of HMXBs is most likely related to the recent star formation history

of the SMC which shows a maximum around 40 Myr ago. At this age the maximum occurrence of the Be phenomenon is expected (Antoniou et al. 2010).

BeXRBs are often discovered during X-ray outbursts. Periodic type-I outbursts (peak X-ray luminosities $<10^{37}$ erg s$^{-1}$) occur around the periastron passage of the neutron star and tend to last only a small fraction of the binary period. Type-II outbursts are usually stronger ($>10^{37}$ erg s$^{-1}$) and can last longer than the orbital period. For more details see the review by e.g. Reig (2011).

During recent monitoring observations of the SMC, a new transient source was discovered in data of the Follow-up X-ray Telescope (FXT) onboard the newly launched *Einstein Probe* mission (EP; Yuan et al. 2022). The source, EP J005146.9–730930, was first detected by *EP*-FXT on 2024 July 25 at an 0.5–10 keV X-ray flux of $6.1 \times 10^{-13}$ erg cm$^{-2}$ s$^{-1}$. EP J005146.9–730930 significantly brightened until August 2 ($1.1 \times 10^{-11}$ erg cm$^{-2}$ s$^{-1}$) and August

<footnote n="1"><sup>1</sup>Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, D-85748 Garching, Germany</footnote>

<footnote n="2"><sup>2</sup>Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China</footnote>

<footnote n="3"><sup>3</sup>Department of Physics, National and Kapodistrian University of Athens, University Campus Zografos, GR 15784 Athens, Greece</footnote>

<footnote n="4"><sup>4</sup>Institute of Accelerating Systems & Applications, University Campus Zografos, GR 15784 Athens, Greece</footnote>

<footnote n="5"><sup>5</sup>National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, P. R. China</footnote>

<footnote n="6"><sup>6</sup>School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, Beijing 100049, P. R. China</footnote>

<footnote n="7"><sup>7</sup>Institute for Frontier in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, P. R. China</footnote>

<footnote n="8"><sup>8</sup>Institut für Astronomie und Astrophysik, Sand 1, D-72076 Tübingen, Germany</footnote>

<footnote n="9"><sup>9</sup>Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France</footnote>

<footnote>*E-mail: fwh@mpe.mpg.de (FH), xuyj@ihep.ac.cn (YX)</footnote>
<table><caption>**Table 1.** EP-FXT monitoring observations of EP J005146.9–730930 with the best-fitting spectral parameters. Both FXTA and FXTB data were taken in the FF mode, pile-up effects were negligible at such flux levels (Zhang et al. 2025). The best-fitting spectral parameters of $N_H^{SMC}$ and $\Gamma$ are obtained from joint fits of FXTA and FXTB. The net count rates and the observed X-ray flux in the 0.2–10 keV band listed are the average values obtained by FXTA and FXTB. All uncertainties quoted are at the 90 per cent confidence level.</caption><thead><tr><th>No.</th><th>ObsID</th><th>Observation time<br>(UT)</th><th>Exposure<br>(s)</th><th>Net count rate<br>(10<sup>-2</sup> s<sup>-1</sup>)</th><th>N<sub>H</sub><sup>SMC</sup><br>(10<sup>21</sup> cm<sup>-2</sup>)</th><th>Γ</th><th>Flux<sub>0.2–10 keV</sub><br>(erg cm<sup>-2</sup> s<sup>-1</sup>)</th><th>Cstat/dof</th></tr></thead><tbody><tr><td>1</td><td>11908398338</td><td>2024-07-25 15:59–07-25 16:18</td><td>1148</td><td>3.0 ± 0.4</td><td>&lt;9.7</td><td>0.7<sup>+0.6</sup><sub>-0.3</sub></td><td>(2.3 ± 0.6)×10<sup>-12</sup></td><td>70.9/61</td></tr><tr><td>2</td><td>11908398339</td><td>2024-08-01 23:05–08-01 23:23</td><td>1075</td><td>20.6 ± 1.0</td><td>13 ± 5</td><td>1.1 ± 0.2</td><td>(1.21 ± 0.11)×10<sup>-11</sup></td><td>271.0/328</td></tr><tr><td>3</td><td>11908428547</td><td>2024-08-02 23:08–08-02 23:29</td><td>1288</td><td>23.1 ± 0.9</td><td>9.8<sup>+4.3</sup><sub>-3.8</sub></td><td>1.0 ± 0.2</td><td>(1.28 ± 0.10)×10<sup>-11</sup></td><td>312.0/412</td></tr><tr><td>4</td><td>11908398374</td><td>2024-08-14 17:24–08-14 17:56</td><td>979</td><td>55.5 ± 1.7</td><td>2.3<sup>+1.7</sup><sub>-1.5</sub></td><td>0.7 ± 0.1</td><td>(3.84 ± 0.21)×10<sup>-11</sup></td><td>486.5/595</td></tr><tr><td>5</td><td>11908428567</td><td>2024-08-15 22:56–08-15 23:19</td><td>1405</td><td>63.5 ± 1.5</td><td>3.1<sup>+1.5</sup><sub>-1.3</sub></td><td>0.8 ± 0.1</td><td>(3.66 ± 0.15)×10<sup>-11</sup></td><td>699.4/767</td></tr><tr><td>6</td><td>11908525313</td><td>2024-08-21 18:27–08-21 18:53</td><td>1545</td><td>39.9 ± 1.1</td><td>3.9<sup>+1.9</sup><sub>-1.7</sub></td><td>0.9 ± 0.1</td><td>(2.53 ± 0.13)×10<sup>-11</sup></td><td>565.2/617</td></tr><tr><td>7</td><td>11908553988</td><td>2024-08-28 16:41–08-28 16:57</td><td>969</td><td>25.0 ± 1.1</td><td>6.7<sup>+3.7</sup><sub>-3.2</sub></td><td>1.0 ± 0.2</td><td>(1.42 ± 0.12)×10<sup>-11</sup></td><td>284.5/340</td></tr><tr><td>8</td><td>11908561156</td><td>2024-08-29 18:49–08-29 19:12</td><td>1306</td><td>33.9 ± 1.1</td><td>3.5<sup>+2.5</sup><sub>-2.1</sub></td><td>0.8 ± 0.1</td><td>(1.98 ± 0.12)×10<sup>-11</sup></td><td>456.1/529</td></tr><tr><td>9</td><td>11908572422</td><td>2024-09-03 17:19–09-03 17:47</td><td>1648</td><td>21.3 ± 0.8</td><td>4.9<sup>+3.0</sup><sub>-2.7</sub></td><td>0.9 ± 0.2</td><td>(1.37 ± 0.10)×10<sup>-11</sup></td><td>355.2/451</td></tr><tr><td>10</td><td>11908609289</td><td>2024-09-12 17:23–09-12 17:44</td><td>1275</td><td>10.6 ± 0.6</td><td>15<sup>+9</sup><sub>-7</sub></td><td>1.6<sup>+0.4</sup><sub>-0.3</sub></td><td>(3.6 ± 0.5)×10<sup>-12</sup></td><td>164.3/222</td></tr><tr><td>11</td><td>11908656897</td><td>2024-09-26 19:49–09-26 20:09</td><td>1224</td><td>6.8 ± 0.5</td><td>15<sup>+10</sup><sub>-8</sub></td><td>1.6 ± 0.5</td><td>(2.3 ± 0.4)×10<sup>-12</sup></td><td>136.9/143</td></tr><tr><td>12</td><td>11908663553</td><td>2024-09-28 22:45–09-28 23:27</td><td>1571</td><td>6.3 ± 0.5</td><td>&lt;9.3</td><td>1.0<sup>+0.4</sup><sub>-0.3</sub></td><td>(3.5 ± 0.5)×10<sup>-12</sup></td><td>137.8/170</td></tr><tr><td>13</td><td>11908712722</td><td>2024-09-26 19:49–09-26 20:09</td><td>867</td><td>4.4 ± 0.5</td><td>&lt;8.5</td><td>0.7<sup>+0.6</sup><sub>-0.3</sub></td><td>(3.5 ± 0.8)×10<sup>-12</sup></td><td>57.6/68</td></tr><tr><td>14</td><td>11908731657</td><td>2024-10-16 18:05–10-16 18:30</td><td>1459</td><td>2.7 ± 0.3</td><td>&lt;15.1</td><td>1.0<sup>+0.6</sup><sub>-0.5</sub></td><td>(1.4 ± 0.3)×10<sup>-12</sup></td><td>77.9/73</td></tr><tr><td>15</td><td>11908753155</td><td>2024-10-10 23:55–10-11 00:09</td><td>1104</td><td>1.7 ± 0.3</td><td>&lt;11.5</td><td>0.6<sup>+0.7</sup><sub>-0.5</sub></td><td>(1.4 ± 0.6)×10<sup>-12</sup></td><td>40.3/32</td></tr><tr><td>16</td><td>11908754179</td><td>2024-10-22 08:23–10-22 08:39</td><td>971</td><td>2.3 ± 0.4</td><td>&lt;13.9</td><td>1.7<sup>+1.2</sup><sub>-0.5</sub></td><td>(5.7 ± 1.9)×10<sup>-13</sup></td><td>23.2/38</td></tr></tbody></table>

15 (3.5 × 10⁻¹¹ erg cm⁻² s⁻¹). It remained near this flux level until August 29 (1.8 × 10⁻¹¹ erg cm⁻² s⁻¹) as reported by Xu et al. (2024). The flux increase was also seen during monitoring observations with the Neil Gehrels Swift observatory (Swift; Gehrels et al. 2004). EP J005146.9–730930 was detected in the data of the X-ray telescope (XRT) with a maximum count rate on August 13. From the improved X-ray position the candidate Be star OGLE J005147.58–730924.7 was proposed as optical counterpart (Kennea et al. 2024).

In order to look for pulsations, we triggered one of our pre-approved anticipated target of opportunity observations with XMM–Newton. In Section 2, we describe the observations and the data analysis. The results are discussed in Section 3.

## 2 X-RAY OBSERVATIONS AND DATA ANALYSIS

### 2.1 Einstein Probe

The outburst of EP J005146.9–730930 was found by EP during a regular monitoring programme of the SMC using FXT. EP observed EP J005146.9–730930 with multiple snapshots from 2024 July 25th to October 22nd (see Table 1 for details), then further observations of the corresponding SMC regions were prohibited due to solar constraints. EP-FXT consists of two identical and co-aligned telescope units, FXTA and FXTB, both observed EP J005146.9–730930 in the full-frame (FF) mode with thin filters.

We reduced the EP-FXT data following standard procedures using the FXT Data Analysis Software (FXTDAS) v1.10 with CALDB v1.10. The source spectra were extracted using a circular region with a radius of 50 arcsec centred on the source position from FXTA and FXTB, respectively. Corresponding background spectra were selected from source-free areas with a radius of 300 arcsec within the field of view. Due to the short exposure times of the individual EP-FXT monitoring observations, no periodicity was significantly detected in any of the single-epoch observations. Therefore, we only report the energy and

flux evolution measured by EP in this paper. EP J005146.9–730930 and the nearby BeXRB SXP 172 (see below) could be resolved by FXT (on-axis half-power diameter spatial resolution of ≲30 arcsec) but not by the Wide-field X-ray Telescope onboard EP. Therefore, in this paper we only report the X-ray fluxes measured by FXT. For spectral analysis, we group the energy spectra to have a minimum of one count in each bin, and jointly fit the FXTA and FXTB spectra in XSPEC (Arnaud 1996) using C-statistics (Cash 1979) to determine the best-fitting parameters, which is preferable to χ²-statistics for low-count spectra.

#### 2.1.1 Spectral analysis

To study the spectral evolution of EP J005146.9–730930 during the outburst, we performed a spectral modelling of the multi-epoch observations taken by EP-FXT. We jointly fit the FXTA and FXTB spectra in the 0.2–10 keV band with an absorbed power-law model plus a cross-normalization constant: constant*tbabs*tbvarabs*powerlaw. The cross-normalization constant is allowed to vary freely for FXTB and is assumed to be unity for FXTA. For the absorption along the line of sight we used two components, one accounting for the foreground absorption in the Galaxy with abundances in the interstellar medium (ISM) following Wilms, Allen & McCray (2000) and atomic cross-sections from Verner et al. (1996). The Galactic column density, $N_H^{Gal}$, was taken from Dickey & Lockman (1990) and fixed in the fit at 0.56 × 10²¹ cm⁻². For the absorption local to the source and in the ISM of the SMC, we assumed elemental abundances of 1.0 for He and 0.2 solar for elements with Z > 2 (Russell & Dopita 1992) and left the column density free in the fit. The best-fitting parameters measured during the EP-FXT monitoring observations, including the intrinsic absorption column density in the SMC, $N_H^{SMC}$, the power-law photon index, $\Gamma$, the X-ray flux in the 0.2–10 keV band, and the fitting statistics are listed in Table 1. As seen from the C-statistics, the model describes the FXT energy spectra well. The flux evolution during the
<figure><img src="image_2.png" /><figcaption>**Figure 1.** X-ray flux (observed, 0.2–10.0 keV) of EP J005146.9–730930 during the EP-FXT monitoring of the outburst. The flux derived from the *XMM-Newton* (Section 2.2.1) and *Swift* (Section 2.3) observations are included.</figcaption></figure>

X-ray outburst is plotted in Fig. 1. The EP-FXT spectra together with the evolution of $N_{H}^{SMC}$ and $\Gamma$ as function of X-ray luminosity are shown in Fig. 2. The EP-FXT observation on MJD 60565.7 was conducted about three days before the *XMM-Newton* observation. The best-fitting parameters determined by EP-FXT on MJD 60565.7 could be considered as roughly consistent with the *XMM-Newton* results (see below) within errors, given that the EP-FXT parameters are less well constrained due to poorer statistics.

## 2.2 XMM-Newton

XMM-Newton carries three XRTs, two equipped with Metal Oxide Semiconductor (MOS) CCD arrays (Turner et al. 2001) and one with a pn-CCD (Strüder et al. 2001). These detectors, which comprise the European Photon Imaging Camera (EPIC), are sensitive in the 0.15–12 keV energy band. The XMM-Newton observation was performed on 2024 September 15 (for details see Table 2).

We used the XMM-Newton Science Analysis Software (SAS) 21.0¹ package to process the EPIC data. To exclude time intervals of high background during soft-proton flares, we filtered out time intervals with background rates in the 7.0–15.0 keV band above 8 and 2.5 cts ks⁻<fnref n="1" /> arcmin⁻² for EPIC-pn and EPIC-MOS, respectively. The X-ray position of EP J005146.9–730930 was determined using the standard XMM-Newton source detection task edetect_chain on the EPIC images after screening for the background flaring. The uncertainty is dominated by the remaining systematic error of 0.5 arcsec. The position is 1.1 arcsec from OGLE J005147.58–730924.7, the optical counterpart proposed by Kennea et al. (2024) and 0.63 arcsec from the Gaia DR3 position (Gaia Collaboration 2023) of the counterpart. During the XMM-Newton observation the nearby Be/XRB SXP 172 (~70 arcsec angular distance; Antoniou et al. 2009; Laycock et al. 2010) was a factor of ~2.5 brighter than EP J005146.9–730930 (see Fig. 3). Its derived X-ray position is also shifted by about the same amount and direction with respect to the Gaia DR3 position of its identified counterpart (see

Table 3). Due to the small angular distance between the two sources field distortions and rotation can be neglected. Therefore, we used SXP 172 to compute a linear astrometric correction of −0.44 arcsec and +0.52 arcsec in R.A. and Dec. for the X-ray position of EP J005146.9−730930. The resulting final X-ray coordinates for EP J005146.9−730930 are presented in Table 3, fully consistent with the Gaia position within 0.1 arcsec.

For the extraction of EPIC spectra and light curves, we defined circular regions with radii of 30 and 60 arcsec for the source and a nearby source-free background region, respectively. The EPIC spectra were grouped to a minimum of 20 counts per bin. For the timing analysis of the EPIC-pn data we used the first 11 ks of the observation, which includes some weak flares but excludes the very strong background flares during the last part of the observation. The source shows little flux variations during the observation.

### 2.2.1 EPIC spectral analysis

The X-ray spectrum of EP J005146.9–730930 during the XMM-Newton observation was analysed by simultaneously fitting the model from above to the three EPIC spectra. Again, a cross-normalization constant was included which was fixed at 1.0 for the MOS1 spectrum and allowed to vary freely for the other two spectra. This model describes the spectra well with a $\chi^2$ of 469.0 for 440 degrees of freedom. The EPIC spectra with their best-fitting model are presented in Fig. 4 and the best-fitting model parameters are summarized in Table 4. The observed flux (0.2–10.0 keV, averaged over the three instruments) was $3.43 \times 10^{-12} \text{ erg cm}^{-2} \text{ s}^{-1}$ which can be converted to an absorption-corrected luminosity of $1.85 \times 10^{36} \text{ erg s}^{-1}$, assuming a distance of 62 kpc to the SMC (Graczyk et al. 2020).

### 2.2.2 EPIC-pn timing analysis

To investigate short-term variability of EP J005146.9–730930, we created EPIC-pn light curves in the 0.2–8.0 keV energy band from the source extraction region. We selected valid pixel patterns (PATTERN 1–12) and corrected the event arrival times to the Solar system barycentre. For our search for X-ray pulsations in the light curves, we first created power spectra which revealed pulsations at a frequency of about 6.8 mHz. The power spectrum obtained from the 0.2–8.0 keV EPIC-pn light curve is presented in Fig. 5. Besides the fundamental frequency, also the first harmonic at about 13.6 mHz is visible.

Following recent work on other HMXB pulsars in the Magellanic Clouds (e.g. Vasilopoulos et al. 2017; Haberl et al. 2022), we determined the precise period and its error in a second step. For this, we used the Bayesian approach described by Gregory & Loredo (1996) in a restricted frequency range around the peak detected in the power spectrum. This results in a pulse period of $146.79 \pm 0.03$ s.

The pulse profiles in different energy bands, together with the corresponding hardness ratio (HR), are shown in Fig. 6. As already indicated by the presence of the first harmonic in the power spectrum, the pulse profile is double peaked. The earlier of the two peaks is more pronounced at energies above 2 keV, which leads to a maximum in the HR, while lower HRs are indicated during the intensity minimum. We infer a pulsed fraction of $36 \pm 8$ per cent [$0.5 \times (\text{maximum-minimum})/\text{mean}$] from the broad (0.2–8.0 keV) band. We computed the pulsed fraction as a function of energy following (Ferrigno, D’Aì & Ambrosi 2023). In particular, we used the FFT, RMS, and area methods and computed the pulsed fraction for nine energy ranges between 0.2 and 8.0 keV, with the results shown in Fig. 7. The results

<footnote n="1">¹https://www.cosmos.esa.int/web/xmm-newton/sas</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Left: EP-FXT energy spectra folded with the best-fitting models during the rising phase of the outburst (a), along with the spectral residuals (b). FXTA and FXTB spectra taken in a single epoch are plotted in the same colour, and are rebinned for visual clarity. Middle: sample EP-FXT spectra during the decaying phase of the outburst. Right: relation of the photon index (c) and the intrinsic absorption column density in SMC along the line-of-sight of EP J005146.9–730930 (f) with the absorption-corrected X-ray luminosity in 0.2–10 keV, assuming a source distance of 62 kpc. Measurements by EP, *XMM-Newton*, and Swift are plotted in black, red, and green, respectively.</figcaption></figure>

<table><caption>**Table 2.** The *XMM-Newton* observation of EP J005146.9–730930. The net exposure times used for the EPIC spectra and images after background-flare screening are listed for pn, MOS1, and MOS2, respectively.</caption><thead><tr><th>Instrument</th><th>Observation ID</th><th>Observation time (UT)</th><th>(ks)</th><th>Net exposure time<br>Fraction of total</th></tr></thead><tbody><tr><td>EPIC</td><td>0903450101</td><td>2024-09-15 16:19–22:46</td><td>9.00, 13.25, 13.22</td><td>67 per cent, 77 per cent, 76 per cent</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Figure 3.** *XMM-Newton* EPIC colour image. Red, green, and blue represent the detector background-subtracted and vignetting-corrected count rate images in the bands 0.2–1.0, 1.0–2.0, and 2.0–4.5 keV, respectively. HMXBs from Haberl & Sturm (2016) known in the field are marked with their pulse period (white), if known, or catalogue numbers otherwise. An association of the 9.1 s pulsar with source 74 still needs confirmation.</figcaption></figure>

reveal somewhat smaller pulsed fraction values below 1 keV, but no significant features are identified, in contrast to other bright outbursts in the Magellanic Clouds (Yang et al. 2025).

## 2.3 Long-term X-ray behaviour

We used the High-energy Light curve GeneraTor (HILIGT; König et al. 2022; Saxton et al. 2022)<fnref n="2" /> to investigate the long-term X-ray light curve of EP J005146.9–730930. The tool uses available catalogues to search for source detections and provides upper limits (2σ) when the source was not detected. We excluded ROSAT data because of the limited energy band and spatial resolution. We mainly obtained upper limits from archival observations performed with Swift and *XMM-Newton* before 2024 September. To determine the conversion factor for Swift-XRT count rates (0.2–12 keV) to fluxes (0.2–10 keV) we downloaded the spectrum obtained on 2024-09-05 16:12–18:08 UT (ObsID 00016793001, exposure 2.50 ks), when the flux measured by Swift was highest, from the Swift archive<fnref n="3" /> (Evans et al. 2009) and modelled it consistently to the EPIC spectra. The best-fitting model parameters are listed in Table 4. This resulted in a conversion factor of $1.02 \times 10^{-10} \text{ erg cm}^{-2} \text{ s}^{-1}/(\text{cts s}^{-1})$. For *XMM-Newton*, we used the fluxes provided by HILIGT (assuming

<footnote n="2">²http://xmmuls.esac.esa.int/upperlimitserver/</footnote>

<footnote n="3">³https://www.swift.ac.uk/user_objects/index.php</footnote>
<table><caption>**Table 3.** Astrometric alignments.</caption><thead><tr><th rowspan="2">X-ray source</th><th colspan="2">R.A. (J2000) Dec.<br>EPIC uncorrected</th><th rowspan="2">1σ Err</th><th colspan="2">R.A. (J2000) Dec.<br>Gaia DR3</th><th colspan="2">R.A. (J2000) Dec.<br>EPIC corrected</th><th rowspan="2">1σ Err</th></tr><tr><th>(h m s)</th><th>(°′′)</th><th>(h m s)</th><th>(°′′)</th><th>(h m s)</th><th>(°′′)</th></tr></thead><tbody><tr><td>EP J005146.9–730930</td><td>00 51 47.695</td><td>−73 09 25.60</td><td>0.51</td><td>00 51 47.584</td><td>−73 09 25.15</td><td>00 51 47.59</td><td>−73 09 25.1</td><td>0.1</td></tr><tr><td>SXP 172</td><td>00 51 52.126</td><td>−73 10 34.66</td><td>0.50</td><td>00 51 52.025</td><td>−73 10 34.14</td><td>00 51 52.03</td><td>−73 10 34.1</td><td>0.1</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Top: *XMM–Newton* EPIC spectra of EP J005146.9–730930 with best-fitting model as histograms (MOS1: black, MOS2: red, pn: green). Bottom: residuals in error units.</figcaption></figure>

a power law with photon index 1.5 and an absorption column density of $10^{21} \text{ cm}^{-2}$). To convert the HILIGT 0.2–12 keV fluxes to 0.2–10.0 keV band we simulated an EPIC-pn spectrum with the HILIGT model parameters and normalized it to the count rate obtained from the observed spectrum.

Because EP J005146.9–730930 was not detected by eROSITA (Predehl et al. 2021), we determined upper limits from the four eROSITA surveys, which covered the source. Due to the vicinity of SXP 172 and the size of the point spread function of ~30 arcsec (half energy width in survey mode; Merloni et al. 2024) SXP 172 contributes to the flux at the position of EP J005146.9–730930. Therefore, we extracted source and background spectra, with the background region at the same distance to SXP 172, but on the opposite side of EP J005146.9–730930 in order to take the contribution from this brighter source into account. Assuming the best-fitting spectral model from the EPIC spectra, normalized to the 2σ count rate upper limits inferred from the spectra, we computed fluxes in the 0.2–10.0 keV band. Finally, the 0.2–10.0 keV fluxes measured by EP and from the new *XMM–Newton* observation were included in the light curve, which is plotted in Fig. 8 (see also Fig. 1). It reveals the first strong X-ray outburst seen from EP J005146.9–730930 with a maximum flux a factor of at least 2340 higher than the most

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Power spectrum of EP J005146.9–730930 derived from the 0.2–8.0 keV EPIC-pn light curve with 1.2 s binning.</figcaption></figure>

stringent upper limit inferred from an *XMM–Newton* observation in 2009.

## 3 DISCUSSION

After the discovery of a new X-ray transient in the SMC by EP, we observed EP J005146.9–730930 with *XMM–Newton* during its outburst. The *XMM–Newton* observation was performed 31 d after the maximum source flux was observed by EP, when the flux had declined by a factor of ~10. The EPIC data reveal a highly significant coherent signal at a period of 146.79 ± 0.03 s, which is close to the maximum of the second peak in the pulse-period distribution of BeXRB pulsars in the SMC (Haberl & Sturm 2016). The light curve folded at the pulse period shows a double-peaked profile with pulsed fraction of 36 per cent, also typical for BeXRBs.

Based on the EP-FXT monitoring light curve (see Fig. 1), the outburst of EP J005146.9–730930 followed a fast-rise-slow-decay profile, lasting more than 90 d. From the EP-FXT spectral analysis results, the absorption-corrected X-ray flux in the 0.2–10 keV band reached at least, $L_{\text{peak}} \sim 1.8 \times 10^{37} \text{ erg s}^{-1}$, near the peak of the outburst ($L_{\text{peak}}/L_{\text{Edd}} \sim 0.1$, here the Eddington luminosity for a neutron star is taken as $1.7 \times 10^{38} \text{ erg s}^{-1}$), indicating that EP J005146.9–730930 likely went into a type-II outburst of a transient BeXRB despite the peak luminosity being at the lower end

<table><caption>**Table 4.** *XMM–Newton* and *Swift* spectral analysis results of EP J005146.9–730930. Errors on spectral fit parameters are given for 90 per cent confidence intervals.</caption><thead><tr><th>Instrument</th><th>N<sub>H</sub><sup>Gal</sup><br>(10<sup>21</sup> cm<sup>-2</sup>)</th><th>N<sub>H</sub><sup>SMC</sup><br>(10<sup>21</sup> cm<sup>-2</sup>)</th><th>Photon<br>index</th><th>F<sub>x</sub><sup>a</sup><br>(erg cm<sup>-2</sup> s<sup>-1</sup>)</th><th>L<sub>x</sub><sup>a</sup><br>(erg s<sup>-1</sup>)</th><th>χ<sub>r</sub><sup>2</sup>/dof</th></tr></thead><tbody><tr><td>XMM–Newton EPIC</td><td>0.56 fixed</td><td>6.4 ± 0.6</td><td>1.25 ± 0.04</td><td>3.43 × 10<sup>-12</sup></td><td>1.85 × 10<sup>36</sup></td><td>1.07/440</td></tr><tr><td>Swift–XRT</td><td>0.56 fixed</td><td>5.0<sup>+6.0</sup><sub>-3.5</sub></td><td>0.81 ± 0.27</td><td>1.43 × 10<sup>-11</sup></td><td>7.03 × 10<sup>36</sup></td><td>0.54/14</td></tr></tbody></table>

<footnote><sup>a</sup>Observed X-ray flux and absorption-corrected luminosity in the 0.2–10 keV band assuming a distance of 62 kpc.</footnote>
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Pulse profiles, normalized to the average count rates, obtained from folding the EPIC-pn light curves in broad, soft, and hard energy bands (top three panels). The HR (bottom) was derived as count ratio of hard to soft bands.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 7.** Pulse profile as a function of energy. The pulsed fraction is computed using three methods outlined in Ferrigno et al. (2023).</figcaption></figure>

of the distributions of typical values (Reig 2011). EP-FXT observed EP J005146.9–730930 in outburst covering the change over nearly two orders of magnitude in luminosity, enabling us to investigate the relation of its spectral properties with luminosity.

The spectral evolution of BeXRBs during type-II outbursts has been more thoroughly studied for Galactic sources (e.g. Reig 2008; Tamang et al. 2022; Thalhammer et al. 2024). The spectral behaviour of EP J005146.9–730930 is consistent with the typical cases known for Galactic sources accreting at relatively low Eddington ratios, in the sense that the energy spectra hardened when the source luminosity became higher (see Fig. 2e). Considering the most tightly constrained data points, we find that the photon index measured by EP-FXT, $\Gamma \sim 0.7–0.8$, around the peak of the outburst is significantly smaller than that obtained based on XMM-Newton, $\Gamma = 1.25 \pm 0.04$. The decrease of photon index and the increase of absorption column density could both cause spectral hardening. The hardness-intensity diagram of EP J005146.9–730930 during outburst measured by EP in Fig. 9(a) displays a similar trend as Fig. 2e, indicating that the spectral hardening behaviour towards high luminosities is model independent. Also, we plot the confidence contours in Fig. 9(b) to investigate the level of parameter degeneracy between $N_H^{SMC}$, and $\Gamma$. As an example, the best fit of the EP data on MJD 60538.0 prefers both lower $N_H^{SMC}$ and smaller $\Gamma$ when compared to that on MJD 60565.7 above $3\sigma$ confidence, with a difference of about ten times in luminosity.

The spectral evolution of EP J005146.9–730930 supports the general picture of an accreting neutron star below a certain critical luminosity that is largely determined by the pulsar magnetic field, when the deceleration of the accreting flow at the vicinity of the neutron star surface is dominated by Coulomb interactions rather than radiation pressure (Basko & Sunyaev 1976). Under such circumstances, the increase in luminosity is believed to lead to a decrease in the height of the emission zone in the accretion column above the neutron star surface, hence larger optical depth for the Comptonization process and producing harder photons (Becker et al. 2012; Reig & Nespoli 2013). For an approximate estimate, assuming the critical luminosity, $L_{crit} > L_{peak}$, and a relation of the critical luminosity with the neutron star surface magnetic field,<fnref n="4" /> we could obtain a lower limit for the magnetic field of EP J005146.9–730930 as, $B > 3.3 \times 10^{12}$ G.

There are also hints for a tentative anticorrelation between the absorption column density and the source luminosity (see Fig. 2f), which could be caused by absorption variations related to orbital geometrical effects, similar as observed during a recent giant outburst of the Galactic BeXRB EXO 2030+375 (Thalhammer et al. 2024). We note that the measurements of $N_H^{SMC}$ derived from EP J005146.9–730930 are subject to large uncertainties, as only upper limits are yielded by the last few EP observations. Based on only the data points with constrained measurements from EP, XMM-Newton, and Swift, the standard Pearson correlation coefficient between the absorption column density, $N_H^{SMC}$, and the logarithmic X-ray luminosity, $L_x$, is found to be –0.746, with a $p$-value of 0.005, indicating an anticorrelation with a confidence of ~2.79$\sigma$.

During many serendipitous X-ray observations before 2024 EP J005146.9–730930 was never significantly detected. Many short observations yielded relatively non-constraining $2\sigma$ upper limits. Formally, only one measurement was indicated as detection, but the flux is again only twice its error. Two XMM-Newton pointed observations provide stringent upper limits about three orders of magnitude below the maximum observed during outburst. Duration,

<footnote n="4">⁴The critical luminosity is derived to be $1.5 \times 10^{37} B_{12}^{16/15} \text{ erg s}^{-1}$ in Becker et al. (2012). And here, we multiply the absorption-corrected luminosity in the 0.2–10 keV band by a factor of 3 to estimate the bolometric luminosity of EP J005146.9–730930.</footnote>
<figure><img src="image_9.png" /><figcaption>**Figure 8.** Long-term X-ray flux (observed, 0.2–10.0 keV) evolution of EP J005146.9–730930. Down-arrows mark 2 σ upper limits. The evolution of the X-ray outburst with expanded time axis is shown in Fig. 1.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 9.** The hardness-intensity diagram measured by EP during the outburst of EP J005146.9–730930 (a). The HR is defined as the ratio of the count rates in the energy band of 2–10 keV and 0.2–2 keV. The quoted numbers are the average measurements by FXTA and FXTB. Confidence contours of the absorption column density, $N_H^{SMC}$, and the photon index, $\Gamma$, determined from three representative EP observations on MJD 60538.0, 60550.7, and 60565.7 (b).</figcaption></figure>

high amplitude and maximum X-ray luminosity of ~2 × 10^37 erg s⁻¹ all suggest that EP J005146.9–730930 was for the first time detected in X-rays during a type-II outburst.

## ACKNOWLEDGEMENTS

This work is based on data obtained with EP and XMM-Newton. EP is a space mission supported by the Strategic Priority Programme on Space Science of Chinese Academy of Sciences, in collaboration with ESA, MPE, and CNES (grant no. XDA15310000), the Strategic Priority Research Programme of the Chinese Academy of Sciences (grant no. XDB0550200), and the National Key R&D Programme of China (2022YFF0711500). YX acknowledges support by the Hundred Talents Programme of the Chinese Academy of Sciences. CJ acknowledges the National Natural Science Foundation of China through grant no. 12473016. LD acknowledges funding from German Research Foundation (DFG), Projektnummer 549824807. XMM-Newton is an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. The XMM-Newton project is supported by the DLR and the Max Planck Society. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.

## DATA AVAILABILITY

The data presented in the tables and figures of the paper are available upon reasonable request from the corresponding author. The data from the *XMM-Newton* observation will become available from the *XMM-Newton* Science Archive<fnref n="5" /> at the end of September 2025.

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