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# The population of hyperluminous X-ray sources as seen by XMM-Newton

Roberta Amato, Erwan Quintin, Hugo Tranin, Andrés Gúrpide, Natalie

Webb, Olivier Godet, Gian Luca Israel, Matteo Imbrogno, Elias Kammoun,
Maitrayee Gupta

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► To cite this version:

Roberta Amato, Erwan Quintin, Hugo Tranin, Andrés Gúrpide, Natalie Webb, et al.. The population of hyperluminous X-ray sources as seen by XMM-Newton. Astronomy & Astrophysics - A&A, 2025, 701, pp.A7. 10.1051/0004-6361/202555122. hal-05193574

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https://hal.science/hal-05193574v1

Submitted on 5 Sep 2025

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# The population of hyper-luminous X-ray sources as seen by XMM-Newton

Roberta Amato<sup>1,2,*</sup>①, Erwan Quintin<sup>3,①</sup>, Hugo Tranin<sup>4,5,6①</sup>, Andrés Gúrpide<sup>7①</sup>, Natalie Webb<sup>2①</sup>, Olivier Godet<sup>2①</sup>, Gian Luca Israel<sup>1①</sup>, Matteo Imbrogno<sup>1①</sup>, Elias Kammoun<sup>8①</sup>, and Maitrayee Gupta<sup>9①</sup>

Received 11 April 2025 / Accepted 12 July 2025

## ABSTRACT

**Context.** Ultraluminous and hyper-luminous X-ray sources (ULXs and HLXs) are among the brightest astrophysical objects in the X-ray sky. While ULXs are most likely to host stellar-mass compact objects accreting at super-Eddington rates, HLXs are compelling candidates for accreting intermediate-mass black holes. However, HLXs are predominantly found in distant galaxies ($d \ge 100$ Mpc), where the chances of source confusion and misidentification with active galactic nuclei (AGNs) or other transient X-ray phenomena are high.

**Aims.** Our goal is to produce a clean sample of HLXs, by removing possible contaminants, and to characterise the spectral properties of the remaining population. This sample can then be used to determine whether different types of astrophysical objects coexist within the same class.

**Methods.** Starting with a set of 115 HLXs detected by *XMM-Newton*, we identified and removed contaminants (AGNs, X-ray diffuse emission detected as point-like, and tidal disruption event candidates) and retrieved 40 sources for which *XMM-Newton* spectra are available. We fit them with an absorbed power law model and determined their unabsorbed luminosities and hardness ratios. We constructed the hardness-luminosity diagram and compared the results with the spectral properties of the HLX prototype, ESO 243-49 HLX-1. Then we conducted a deeper analysis on a selection of promising candidates.

**Results.** The resulting HLX population spans a luminosity range from $1 \times 10^{41}$ erg s$^{-1}$ to nearly $10^{43}$ erg s$^{-1}$ and is homogeneously spread in hardness between 0.5 and 5. Half of the population display hardness ratios higher than a typical AGN and could be considered an extension of the ULX population at higher energies. We found four very soft outliers, which are characterised by steep power law spectra and no X-ray emission above 1–2 keV, similarly to ESO 243-49 HLX-1. Those with multi-epoch archival data display changes in luminosity of up to nearly two orders in magnitude.

**Conclusions.** We show that sources currently identified as HLXs can be more diverse than ULXs and disentangling among these different types of objects is not trivial with the data currently available. New observations would be beneficial to expand the current sample and uncover the true nature of many objects of this class, some of which show very similar characteristics to ESO 243-49 HLX-1.

**Key words.** catalogs – stars: black holes – X-rays: binaries

## 1. Introduction

Ultraluminous X-ray sources (ULXs) are defined as point-like, X-ray sources with luminosities exceeding $10^{39}$ erg s$^{-1}$ that are found in the off-nuclear regions of nearby galaxies (see reviews by Kaaret et al. 2017; King et al. 2023; Pinto & Walton 2023; Fabrika et al. 2021). This luminosity threshold corresponds to the Eddington limit of a $10^8$ M$_\odot$ accreting black hole (BH), under the assumption of isotropic emission. Overall, ULXs are known to host either neutron stars (NSs) or BHs, accreting matter from a companion star at super-Eddington mass transfer rates. For a few ULXs, the compact object is unequivocally a NS, as they

exhibit pulsations of the order of seconds (e.g. Bachetti et al. 2014; Israel et al. 2017a,b; Carpano et al. 2018; Fürst et al. 2016; Rodríguez Castillo et al. 2020; Sathyaprakash et al. 2019) and are commonly referred to as pulsating ULXs (PULXs). For the remaining (and the great majority of) sources, which do not show pulsations, it is not possible to determine with certainty the nature of the compact object. Nevertheless, spectral studies suggest that the hardest ULXs are more likely to host NSs, while the softest would be more consistent with hosting stellar-mass BHs (Gúrpide et al. 2021; Pintore et al. 2017; Reyero Serantes et al. 2024).

The most luminous sources, namely, those with X-ray luminosities of $\ge 10^{41}$ erg s$^{-1}$ are commonly referred to as

<footnote n="1">1 INAF – Osservatorio Astronomico di Roma, Via Frascati 33, I-00078 Monte Porzio Catone, (RM), Italy</footnote>

<footnote n="2">2 IRAP, CNRS, Université de Toulouse, CNES, 9 Avenue du Colonel Roche, 31028 Toulouse, France</footnote>

<footnote n="3">3 European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain</footnote>

<footnote n="4">4 Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB), c. Martí i Franquès, 1, 08028 Barcelona, Spain</footnote>

<footnote n="5">5 Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), c. Martí i Franquès, 1, 08028 Barcelona, Spain</footnote>

<footnote n="6">6 Institut d'Estudis Espacials de Catalunya (IEEC), c. Gran Capità, 2-4, 08034 Barcelona, Spain</footnote>

<footnote n="7">7 School of Physics & Astronomy, University of Southampton, Southampton, Southampton SO17 1BJ, UK</footnote>

<footnote n="8">8 Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Pasadena, CA 91125, USA</footnote>

<footnote n="9">9 Astronomical Institute of the Czech Academy of Sciences, Boční II 1401/1, 14100 Praha 4, Czech Republic</footnote>

<footnote>* Corresponding author: roberta.amato@inaf.it</footnote>
hyper-luminous X-ray sources (HLXs; Matsumoto et al. 2003; Gao et al. 2003). It is not clear whether they are merely an extension of the ULX class at higher luminosities, whether they represent heavier accreting objects, such as intermediate-mass BHs (IMBHs; with masses $10^2-10^5 M_\odot$; see e.g. Colbert & Mushotzky 1999), or whether they are a collection of different types of sources altogether. This doubt is strengthened by the presence of two different objects in the HLX class. The first is the PULX NGC 5907 ULX-1, which reaches a peak luminosity of $2 \times 10^{41} \text{ erg s}^{-1}$ (Israel et al. 2017a). The second is the IMBH candidate ESO 243-49 HLX-1 (Farrell et al. 2009), which reaches a peak luminosity of $2 \times 10^{42} \text{ erg s}^{-1}$ and has a mass estimate for the BH of $9 \times 10^3-9 \times 10^4 M_\odot$ (Webb et al. 2012). While NGC 5907 ULX-1 shows characteristics that are similar to other (P)ULXs (e.g Gúrpide et al. 2021), ESO 243-49 HLX-1 is more similar to BH binaries rather than ULXs (Servillat et al. 2011; Soria et al. 2021; Godet et al. 2009, 2014; Straub et al. 2014). This suggests that the HLX population must be made up of at least two different classes of objects, whose prototype could be ESO 243-49 HLX-1 and NGC 5907 ULX-1, respectively (see e.g. the recent work of MacKenzie et al. 2023).

ULXs and HLXs constitute the high-luminosity end of the X-ray luminosity function (XLF) of X-ray binaries in other galaxies, which has a power-law shape, with a break at $L_X \sim 10^{40} \text{ erg s}^{-1}$ (Tranin et al. 2024; Mineo et al. 2012). Whether this break is real, with HLXs being the tail of the ULX XLF, or whether it is due to observational biases is a matter of debate. Additionally, unlike the ULXs, HLXs seem to be equally distributed in elliptical and spiral galaxies (Tranin et al. 2024). While ULXs are typically found in galaxies within a few hundred Mpc (with many even within the first 50 Mpc; see e.g. the ULX catalogues; Kovlakas et al. 2020; Walton et al. 2022; Tranin et al. 2024), HLXs are significantly more distant, with very few sources located within 100 Mpc. Although this could be partly due to observational biases (as closer ULXs are easier to detect) and sample incompleteness, it is clear that the HLX population predominately lies beyond 100 Mpc. As a consequence, while we can obtain good-quality data for ULXs with just a few tens of ks of observations with current X-ray facilities, much longer exposures are needed for HLXs (or for ULXs at very high distances, $d > 100 \text{ Mpc}$), in spite of their higher intrinsic luminosities. Hence, HLXs remain overall less well studied than ULXs, with the exception of the aforementioned ESO 243-49 HLX-1, which has been repeatedly observed by XMM-Newton and Chandra and still is extensively monitored by Swift (e.g. Webb et al. 2023; Lin et al. 2020).

The aim of this paper is to take a deeper look at the HLX population to investigate their nature, on the basis of their luminosity and spectral hardness, to see if different populations can be identified. We collected a sample of 115 HLXs and HLX candidates observed by XMM-Newton from the most recent catalogue of ULXs and HLXs of Tranin et al. (2024) (Sect. 2). We then identified and excluded several contaminants and fitted the spectra of the remaining sources to build the hardness-luminosity diagram (HLD, Sect. 3). We carried out a more detailed analysis on ESO 243-49 HLX-1 (Sect. 3) and on a few selected promising sources (Sect. 5). Our conclusions are presented in Sect. 7.

## 2. Sample selection

We considered the HLX sample presented in the catalogue of Tranin et al. (2024), which collects ULXs and HLXs from the three main X-ray catalogues, namely Chandra CSC2 (Evans et al. 2010, 2019), Swift/XRT 2SXPS (Evans et al. 2020),

and XMM-Newton 4XMM-DR12 (Webb et al. 2020). In the catalogue, the association of the sources with their host galaxies was made through cross-correlation with the compilation of galaxy catalogues GLADE (Dálya et al. 2018). According to the significance of their association, the authors divided the sources into three categories: robust, weak, and galaxy pair candidates. Robust sources have no detected counterpart in other wavelengths, reducing their chances of being background active galactic nuclei (AGNs) or dwarf satellite galaxies, while both weak and galaxy pairs candidates have association with a detected optical counterparts (for more details, see Tranin et al. 2024).

The automated probabilistic method developed by Tranin et al. (2022) guarantees a contamination rate $\le 2\%$ of the ULX sample alone. However, for HLXs, this method can produce significant biases, with sources misclassified as stars or AGNs when associated with strong optical counterparts. Hence, those authors applied the following further selection criteria for a more secure HLX identification: (1) a mean observed X-ray luminosity of $\ge 10^{41} \text{ erg s}^{-1}$ and a signal-to-noise ratio of $S/N \ge 3$; (2) no evident contaminant from visual inspections; (3) and when an optical counterpart is present (weak sources and galaxy pairs), the redshifts between the host galaxy and the counterpart must be consistent (at $1\sigma$). The catalogue does not include information on the peak luminosity of each source (intra-observational or long-term), nor upper limits on missed detections. Our sample is therefore made up of sources with average HLX luminosities determined over the course of a single observation. Less luminous X-ray sources undergoing sporadic hyper-luminous outbursts might be excluded.

The collection consists of 191 sources, 11 of which are observed in all three X-ray catalogues. In this sample, the contamination rate among robust and weak HLX candidates is expected to be of $\sim 30\%$ (Tranin et al. 2024). With the aim of carrying out a systematic spectral analysis, we decided to use only data from the 4XMM-DR12 catalogue<fnref n="1" />, which contributes for the majority of the sources in the HLX sample (115 vs 75 for CSC2 and 13 for 2SXPS).

## 3. Spectral analysis

The spectral properties of the HLXs and HLX candidates reported in the X-ray catalogues were obtained from spectral fits with simple models (power laws or black bodies, not suited to model ULX-like spectra) and not corrected for the Galactic and local absorptions. This is the case of the 4XMM catalogue, which estimates fluxes in different energy bands using an absorbed power law with fixed values of the equivalent hydrogen column, $N_H = 3 \times 10^{20} \text{ cm}^{-2}$, and photon index of $\Gamma = 1.7$ for all sources (Webb et al. 2020). Therefore, we decided to carry out a more refined spectral analysis of the data to obtain more accurate flux measurements and hardness ratios (HR), as this approach has proven fruitful to categorise and investigate the nature of ULXs (e.g. Gúrpide et al. 2021).

First, we retrieved all available source spectra for all three EPIC cameras on board XMM-Newton (pn, MOS1 and MOS2; Strüder et al. 2001; Turner et al. 2001) from the 4XMM-DR12 archive. The XMM-Newton Pipeline Processing System (PPS)<fnref n="2" /> only generates spectra for sources with $\ge 100$ counts, which are then rebinned to a minimum of five bins ($\ge 20$ counts per bin). Out of the 115 XMM-Newton HLX candidates, we retrieved

<footnote n="1">¹ http://xmm-catalog.irap.omp.eu/</footnote>

<footnote n="2">² https://www.cosmos.esa.int/web/xmm-newton/pipeline</footnote>
the spectra of 64 sources, together with their corresponding background spectra and ancillary files (arfs), while the canned response matrices files (rmfs, update 2021-12-09) were downloaded from the *XMM-Newton* online repository<fnref n="3" />.

Before performing the spectral fits, we inspected the candidates labelled as galaxy pairs to verify their classification. We found that 13 of them are actually confirmed or candidate AGNs, positionally coincident with a galaxy, often in pairs or clusters of galaxies. The remaining three sources were very faint and their spectra were dominated by the background, so we decided to exclude them from the current sample. We were left with 48 sources.

We simultaneously fit all three EPIC instrument spectra within the python interface of Xspec (Arnaud 1996), PyXspec<fnref n="4" /> (version 1.2.2, with Xspec version 12.10.1), with abundances from Wilms et al. (2000) and cross-sections from Verner et al. (1996). All the spectra were fitted in the 0.3–10 keV energy range and the goodness of the fits was evaluated with a chi-squared statistics. We used the same absorbed power law model (tbabs×pow) as in the 4XMM catalogue, but leaving $N_H$, $\Gamma$, and the power law normalisation free to vary. When the best-fit value of the absorption column was below the Galactic hydrogen column density along the line of sight (HI4PI Collaboration 2016), as computed through the python package gdpyc (Ruiz 2018), we fixed it to the average HI4PI value. In total, 32 spectra had a free $N_H$ and 16 fixed. We rejected all the fits with $\chi^2_{\text{red}} > 2$ and/or null hypothesis probability (n.h.p.) <0.01 (cf. Appendix A) and the background-dominated ones (ObsIDs. 0691590101a and 0502690601). Among the rejected sources, one is ESO 243-49 HLX-1 (cf. Sect. 4), and another is a tidal disruption event (TDE) candidate (4XMM J215022.4-055109; Lin et al. 2018). We refined two further fits with bad statistics either by freezing the $N_H$ to the HI4PI value (ObsIDs. 0201901801 and 0110870101) or adding a cross-calibration constant between the pn and MOS data (ObsID.0110870101 as well). Finally, we obtained 40 statistically acceptable spectral fits, one for each source.

We computed the unabsorbed fluxes in the 0.3–1.5 keV (soft) and 1.5–10 keV (hard) energy ranges together with the corresponding hard-to-soft ratio, as typically done for ULXs (see e.g. Gúrpide et al. 2021). From the broadband unabsorbed flux, computed in the energy range 0.3–10 keV, we derived the unabsorbed luminosities, using the distances of the host galaxies of Tranin et al. (2024), derived from GLADE. The resulting HLD is shown in Fig. 1, while Fig. 2 displays the distribution of the absorption columns and photon indices. The median $N_H$ is $1.1 \times 10^{21} \text{ cm}^{-2}$, while the median photon index is 1.85. As a result of fitting with free parameters, the broadband fluxes for two sources turned out to be lower than those reported in the 4XMM catalogue and below the HLX threshold of $10^{41} \text{ erg s}^{-1}$, leading to their reclassification as ULXs. Those sources can be considered extreme ULXs (eULXs; Gladstone 2013). We retained them as objects of interest because they are typically hard and are considered good candidates for IMBHs.

## 4. ESO 243-49 HLX-1

Compared to other HLXs, ESO 243-49 HLX-1 has been well studied and observed in recent decades, with different missions,

making it possible to now study the evolution of this source on yearly timescales (e.g. Webb et al. 2023; Lin et al. 2020). In particular, a number of authors demonstrated that the spectral evolution of ESO 243-49 HLX-1 is similar to that of an X-ray BH binary, with transitions from high-soft to low-hard states (Godet et al. 2009; Servillat et al. 2011; Godet et al. 2012; Webb et al. 2012; Straub et al. 2014; Soria et al. 2021). Hence, to achieve a more complete understanding of the behaviour of this source and looking for similarities within our sample of sources, we considered all archival *XMM-Newton* observations of ESO 243-49 HLX-1 (see Table 1; noting that only ObsID.0801310101 is included in our sample, as it is the only observation present in the catalogue of Tranin et al. 2024). Following the previous analysis (cf. Sect. 3), we retrieved all EPIC spectra of ESO 243-49 HLX-1 from the 4XMM-DR12 and fit the spectra of each observation individually. ObsID.0204540201 has no pn spectrum because the source falls on a CCD gap. In ObsID.0844030101, the source was detected only in the EPIC-pn, most likely because of its faintness and a short exposure. In this observation, the spectrum has $\le 100$ counts in the energy range 0.3–10 keV and the background dominates above 1 keV; therefore, we decided to exclude it from the analysis. ObsID.089180201 had no spectrum available, implying that although the source was detected the spectrum had less than 100 counts. To our knowledge, this is the first time that the results of the fit of the 2017 dataset are presented in a paper.

The simple power-law fit already showed the spectrum to be more complex, as the fit was statistically rejected ($\chi^2 > 2$). This is due to the presence of a soft excess at ~0.7 keV, typically modelled with an additional thermal component (Servillat et al. 2011; Godet et al. 2009). We tried the fit with a simple black body and with a multi-temperature disc black body (diskbb). The question of whether these are the best models and the resulting implications are beyond the scope of this paper. The objective is to have a good enough description of all ESO 243-49 HLX-1 spectral states to allow for a comparison with all the HLX candidates in the HLD. Between the two models, the one including the black body returned statistically good fits for all the spectra. The best-fit results are reported in Table 2. We also computed the HR as in Sect. 3 and the unabsorbed X-ray luminosities, assuming a distance of the host galaxy of 90.94 Mpc, as in Tranin et al. (2024). We plot the spectral evolution of ESO 243-49 HLX-1 on the HLD in Fig. 1 (red squares and arrows). We also note that the spectrum of ESO 243-49 HLX-1 is significantly dominated by the background at energies $\ge 2$ keV ($\ge 5$ keV for ObsIDs. 0560180901 and 0693060401). However, we decided to perform the fit in the usual energy band 0.3–10 keV to try to constrain the photon index of the power law and be consistent with the analysis of the sample (Sect. 3).

## 5. Analysis of selected candidates

The majority of our sample has HRs in the range 0.5–5, with a few outliers. Two sources stand out for their high HR values ($\ge 10$): 4XMM J050252.9-755108 (ObsID.0762800101) and 4XMM J050117.1-242808 (ObsID.0110870101). Both source spectra have few spectral bins (four and five, respectively) in the energy range 0.3–10 keV, so we caution the reader about the reliability of these data point. The *XMM-Newton* position of the first one matches those of three Gaia counterparts, one is suggested to be a galaxy (with a probability of 1.0) and possibly even the host galaxy (distance of 367 Mpc), while the other two stars (probabilities $\ge 0.99$) of magnitude ~20 (Gaia Collaboration 2020).

<footnote n="3">³ See https://www.cosmos.esa.int/web/xmm-newton/epic-response-files</footnote>

<footnote n="4">⁴ https://heasarc.gsfc.nasa.gov/xanadu/xspec/python/html/index.html</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Hardness-luminosity diagram (HLD) of the sample of HLX candidates considered in this work. Blue filled dots are for sources with a robust association to the host galaxy and empty orange dots for weak associations. The red vertical area indicates the position on the HLD of a typical AGN, with a power law index between 1.8 and 2 (fluxes computed with WebPIMMS; see https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3pimms/w3pimms.pl). The grey dashed area corresponds to the parameter space covered in Gúrpide et al. (2021). The dotted line indicates the conventional HLX luminosity threshold of $1 \times 10^{41} \text{ erg s}^{-1}$. The long-term spectral evolution of ESO 243-49 HLX-1 is shown in red squares and arrows. The four most interesting candidates analysed in this work (cf. Sect. 5) are marked with red circles and numbered. For those with available data, the long-term evolution or the HR range is reported with grey data points and arrows.</figcaption></figure>

<table><caption>**Table 1.** Log of archival XMM-Newton observations of ESO 243-49 HLX-1.</caption><thead><tr><th>ObsID.</th><th>Date</th><th>EPIC instr.</th><th>Exposure times (ks)</th><th>EPIC counts</th></tr></thead><tbody><tr><td>0204540201</td><td>2004-11-23</td><td>MOS1/MOS2</td><td>21.6/21.6</td><td>2640 ± 66</td></tr><tr><td>0560180901</td><td>2008-11-28</td><td>pn/MOS1/MOS2</td><td>35.3/49.9/49.9</td><td>14698 ± 130</td></tr><tr><td>0655510201</td><td>2010-05-14</td><td>pn/MOS1/MOS2</td><td>74.1/104.4/104.5</td><td>1185 ± 48</td></tr><tr><td>0693060401</td><td>2012-11-23</td><td>pn/MOS1/MOS2</td><td>43.5/50.7/50.8</td><td>3385 ± 66</td></tr><tr><td>0693060301</td><td>2013-07-04</td><td>pn/MOS1/MOS2</td><td>108.4/120.2/120.3</td><td>2469 ± 63</td></tr><tr><td>0801310101</td><td>2017-05-17</td><td>pn/MOS1/MOS2</td><td>110.8/124.5/127.5</td><td>42910 ± 221</td></tr><tr><td>0844030101</td><td>2021-01-01</td><td>pn</td><td>17.7</td><td>121 ± 16</td></tr><tr><td>0891802101</td><td>2021-11-16</td><td>-</td><td>14.8</td><td>58 ± 12</td></tr></tbody></table>

**Notes.** The reported EPIC instruments are those where the source was detected and a spectrum was extracted. The exposure times correspond to the duration of the observation after the subtraction of the flaring background, except for ObsID.0891802101, where no spectrum was available. The EPIC counts refer to the estimated source counts in all EPIC detectors in the 0.2–12 keV band.

The second source has a robust association with the galaxy Gaia DR3 2960239864411377536 (Gaia Collaboration 2020), at a distance of 975 Mpc. The lack of any additional X-ray or multiwavelength data prevented us from deducing more details of the nature of these two sources.

The softest sources in our sample are particularly interesting candidates. For each of them, we conducted a deeper analysis, presented in the following subsections, incorporating X-ray and multiwavelength data from other missions (when available).

Information about the host galaxies was obtained from the SIM-BAD Astronomical Database<fnref n="5" />.

### 5.1. 4XMM J231818.7-422237

This source is associated with the galaxy 2MASX J23181913-4222355 (at 260 Mpc) and lies at 5 kpc (~5") from the galaxy

<footnote n="5">⁵ https://simbad.cds.unistra.fr/simbad/</footnote>
<table><caption>**Table 2.** Best-fit values of *XMM-Newton* spectra of ESO 243-49 HLX-1 used in this work.</caption><thead><tr><th>ObsID.</th><th>N<sub>H</sub><br>(10<sup>20</sup> cm<sup>-2</sup>)</th><th>T<sub>disc</sub><br>(keV)</th><th>Γ</th><th>HR</th><th>Unabs. L<sub>X</sub><br>(10<sup>41</sup> erg s<sup>-1</sup>)</th><th>χ<sup>2</sup>/d.o.f.<sup>b</sup></th><th>n.h.p.</th><th>State</th></tr></thead><tbody><tr><td>0204540201</td><td>15<sup>+14</sup><sub>-10</sub></td><td>0.09<sup>+0.12</sup><sub>-0.02</sub></td><td>3.4 ± 0.6</td><td>0.080 ± 0.006</td><td>7.3 ± 0.4</td><td>45.81/41</td><td>0.279</td><td>High/soft</td></tr><tr><td>0560180901</td><td>1.52<sup>a</sup></td><td>0.134 ± 0.003</td><td>2.53<sup>+0.18</sup><sub>-0.19</sub></td><td>0.160 ± 0.004</td><td>3.82 ± 0.06</td><td>144.62/150</td><td>0.609</td><td>High/soft</td></tr><tr><td>0655510201</td><td>1.52<sup>a</sup></td><td>0.20 ± 0.02</td><td>0.9<sup>+0.7</sup><sub>-0.8</sub></td><td>1.48 ± 0.18</td><td>0.225 ± 0.019</td><td>79.74/60</td><td>0.045</td><td>Low/hard</td></tr><tr><td>0693060401</td><td>1.52<sup>a</sup></td><td>0.127<sup>+0.013</sup><sub>-0.014</sub></td><td>2.08 ± 0.19</td><td>0.66 ± 0.03</td><td>0.91 ± 0.03</td><td>86.61/82</td><td>0.343</td><td>Interm.</td></tr><tr><td>0693060301</td><td>1.52<sup>a</sup></td><td>0.16<sup>+0.04</sup><sub>-0.03</sub></td><td>2.0<sup>+0.2</sup><sub>-0.3</sub></td><td>0.83 ± 0.06</td><td>0.284 ± 0.015</td><td>110.39/109</td><td>0.445</td><td>Low/hard</td></tr><tr><td>0801310101</td><td>3.1 ± 1.3</td><td>0.142 ± 0.004</td><td>2.9 ± 0.16</td><td>0.1393 ± 0.0017</td><td>5.36 ± 0.05</td><td>334.74/281</td><td>0.015</td><td>High/soft</td></tr></tbody></table>

**Notes.** The HR is the ratio of the unabsorbed fluxes in the energy bands 1.5–10 keV and 0.3–1.5 keV. Unabsorbed luminosities are computed in the 0.3–10 keV band. (a) Parameter frozen to the Galactic absorption (HI4PI Collaboration 2016). (b) d.o.f. = degrees of freedom.

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Histograms of the distributions of the best-fit values of the absorption column densities (top) and photon indices (bottom) from the spectral fits of all the source on the HLD in Fig. 1.</figcaption></figure>

centre. The host has been spectrally identified as a passive galaxy by the 6dF DR3 survey (Jones et al. 2009), excluding the possibility of a low-luminosity AGN. The estimated unabsorbed flux of this source in the 0.3–10 keV band is $(1.6^{+0.7}_{-0.5}) \times 10^{-13}$ erg cm$^{-2}$ s$^{-1}$, which results in an unabsorbed luminosity of $L_X = (1.3^{+0.6}_{-0.4}) \times 10^{42}$ erg s$^{-1}$ if it is indeed associated with this galaxy. Hence, this candidate would classify as a bona fide HLX.

However, a visual inspection of the field of view of the source revealed that it also lays in the halo of a closer galaxy, NGC 7582, which is a barred spiral galaxy with a Seyfert 2 AGN at ~17.6 Mpc (Liu & Bregman 2005), 5.1 kpc away from the centre. If it were indeed hosted in NGC 7582, the estimated unabsorbed X-ray luminosity of the source would be of $6 \times 10^{39}$ erg s$^{-1}$, in the ULX regime.

There are a total of five *XMM-Newton* archival observation of the field of view of 4XMM J231818.7-422237, taken in the years 2001, 2005, 2007, 2016, and 2018, but the source is detected only in the 2005 observation (ObsID.0204610101). The spectral fit of this source (Fig. 3) resulted in $N_H = (1.3 \pm 0.4) \times 10^{21}$ cm$^{-2}$ and $\Gamma = 5.0 \pm 0.4$ ($\chi^2$/d.o.f.=157.1/122). With an HR of $(7.5 \pm 0.4) \times 10^{-3}$, it is the softest source of the sample. The addition of a thermal component (a black body or a multi-temperature disc black body) to the power law model did not improve the fit. An absorbed single-thermal component model returned an unacceptable fit (n.h.p.<0.01).

The upper limits from the other observations, computed using the FLIX sensitivity estimator (Webb et al. 2023), lead to a long-term variability that is greater by more than a factor of 10. The source is also variable on short timescales in its only detection, showing oscillations with an amplitude of a factor of ~5 on a typical timescale of ~20 ks. For this reason, the source was noted to be of particular interest in Lin et al. (2013). The 0.2–1.0 keV EPIC pn light curve, obtained after a standard filtering similar to that presented in Sect. 4, is shown in Fig. 4. The corresponding normalised excess variance (computed as described in Vaughan et al. 2003) is $0.11 \pm 0.03$. There is no significant spectral change linked to this variability. A search for pulsations from this source has already been performed in the framework of the EXTraS project (De Luca et al. 2021) and no signal has been found. A more sophisticated search, accounting for instance for orbital or first period derivative corrections, is beyond the aims of this paper. Here, we computed the upper limit on the pulsed fraction (PF), defined as the modulation amplitude divided by the average of that modulation, for the EPIC-pn time series (noting that the MOS cameras have lower temporal resolution), with the software package *Stingray* (Bachetti et al. 2022; Huppenkothen et al. 2019). Assuming a sinusoidal pulsed signal (typical for ULXs, e.g. Bachetti et al. 2014), for a power of 40 and 1 harmonic, we obtained a PF $\le 40\%$, at a confidence level of 95%.

## 5.2. 4XMM J161534.3+192707

This source is associated with the radio galaxy NGC 6099, at 133 Mpc (the source lies 17.54" from the galaxy centre) in the galaxy cluster 1RXS J161531.4+192754. It is also present in the CSC2 catalogue, thanks to a *Chandra* observation taken in 2009 and centred on NGC 6099. It was already classified as an extreme luminous X-ray source candidate in the
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Fit of the EPIC spectrum of 4XMM J231818.7-422237. Top panel: pn (blue), MOS1 (orange), and MOS2 (green) data points and best-fit absorbed power law model (black line). Bottom panel: Residuals of the fit. The spectrum has been rebinned for a better visualisation. The background becomes dominant above $\ge$1 keV.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Short-term background subtracted EPIC pn 0.2–1 keV lightcurve of 4XMM J231818.7-422237 (ObsID.020461010), binned at 2 ks.</figcaption></figure>

catalogue of Gong et al. (2016), with an absorbed luminosity of ~3 × 10⁴⁰ erg s⁻¹, in the energy range 0.3–8 keV. Recently, it was reported as an IMBH candidate by Chang et al. (2025). The field of view of 4XMM J161534.3+192707 was observed twice with XMM-Newton in 2012 (ObsID.0670350301) and 2023 (ObsID.0923030101). The source was detected in the 2012 observation and its spectrum is present in our sample. The spectral fit results in an unabsorbed X-ray luminosity $L_X = (6.7_{-0.7}^{+0.9}) \times 10^{42}$ erg s⁻¹ (0.3–10 keV range). The best-fit parameters are $N_H = (1.86_{-0.17}^{+0.18}) \times 10^{21}$ cm⁻² and $\Gamma = 4.0_{-0.11}^{+0.12}$, with an HR of $(4.11 \pm 0.11) \times 10^{-2}$ (χ²/d.o.f. = 177.6/141, n.h.p. = 0.02). The best fit is shown in Fig. 5. The use of a thermal component instead of or in addition to the power law did not significantly improve the fit statistics. The resulting unabsorbed luminosity is two to three times higher than that estimated by Chang et al. (2025), who used different fitting models. As for the previous

candidate, the EPIC-pn observation did not show any pulsations and we estimated an upper limit on the PF of ~25%.

In the 2023 observation, the source is not detected by the XMM-Newton source detection pipeline, likely because of a crowded field within diffuse emission, but it is clearly visible by eye in the EPIC images. Hence, we manually extracted the source spectrum. First, we reprocessed the non-calibrated data with the Science Analysis System (SAS), version 21.0.0, and current calibration files (CCF) released in November 2023. For the EPIC-pn and MOS1 images, we extracted the source spectrum from a circular region centred on the source coordinates, with a radius of 8'' to prevent contamination from the diffuse emission of the host galaxy. The background spectrum was extracted from a circular region of ~20'' enclosing the X-ray nebular region surrounding the source, with the subtraction of two circular regions for the HLX candidate and the host galaxy. The source is not visible in the MOS2 data, where the diffuse emission dominates. We rebinned the pn and MOS1 spectra with a minimum of 20 counts per bin and fitted them together with the usual absorbed power law model, in the energy range 0.3–2 keV (the X-ray background dominates at energies $\ge$2 keV). The lower bound of the absorption was unconstrained and we fixed $N_H$ to the best-fit value of the 2012 XMM-Newton observation. We obtained a photon index of $\Gamma = 5.5_{-0.7}^{+0.9}$, with a $\chi^2$/d.o.f. = 24.26/23. The unabsorbed flux is $F_X = (6.0 \pm 1.0) \times 10^{-14}$ erg cm⁻² s⁻¹, corresponding to $L_X = (1.3 \pm 0.2) \times 10^{41}$ erg s⁻¹, in the 0.3–10 keV range. The corresponding HR is $(3.4 \pm 0.8) \times 10^{-3}$. The 2023 detection is not in the EXTraS archive. A preliminary inspection of the power density spectrum revealed no significant peaks, which is expected given the source's low count statistics (~500 counts). The upper limit on the PF was ~70%.

For the Chandra observation of 2009, we first reprocessed the data with the chandra_repro script of the Chandra data analysis software CIAO, v. 4.15.1, with CALDB v. 4.10.4 (Fruscione et al. 2006). We extracted the source and background spectra with the CIAO script specextract, selecting a circular region of radius 0.8'' centred on the source coordinates for the source spectrum and a circular region of radius 10'' on the same CCD in an area free of other X-ray sources for the background spectrum. We fitted the background subtracted spectrum with the same absorbed power law model, using Cash statistics (Cash 1979), since the source has 56 net counts in the 0.5–8 keV range. The $N_H$ was unconstrained and we fixed it to the best-fit value of the 2012 XMM-Newton observation. We obtained a photon index of $\Gamma = 4.5 \pm 0.8$ and an unabsorbed flux of $F_X = (7.8 \pm 1.9) \times 10^{-14}$ erg cm⁻² s⁻¹ (C-stat/d.o.f. = 31.51/44), corresponding to $L_X = (1.7 \pm 0.4) \times 10^{41}$ erg s⁻¹, in the 0.3–10 keV range, and consistent with that of the 2023 XMM-Newton observation. The corresponding HR is $(1.8 \pm 0.6) \times 10^{-2}$. The best-fit values of the 2009 and 2023 spectra, where the source was caught in a dimmer state (see Fig. 5), are consistent or marginally consistent with the results of Chang et al. (2025). 4XMM J161534.3+192707 displays a clear long-term variability (see Fig. 1) and it is notably soft, suggesting similarities with ESO 243-49 HLX-1.

In further support of this similarity, we identified an optical counterpart in an archival Hubble Space Telescope (HST) exposure from September 2023 (Proposal 17177, PI: I. Chilingarian). Figure 6 presents a colour composite of HST observations, revealing a point-like source located precisely at the intersection of the XMM-Newton and Chandra error circles. The source is also tentatively visible in Legacy Survey DR9 images, with consistent magnitudes in both datasets: $m_{F300X} = 26.1 \pm 0.5$,
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Best fit of the *XMM-Newton* (2012 in blue and 2023 in green) and *Chandra* (2009 in orange) spectra of 4XMM J161534.3+192707 (top panel) and residuals of the fits (bottom panel). The 2023 *XMM-Newton* and *Chandra* spectra were rebinned for a better visualisation.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** HST cutout centred on the position of 4XMM J161534.3+192707. The colour composite is based on HST observations in the F300X, F475W, and F814W filters taken in September 2023. The error circles represent the position and uncertainty reported in the *Chandra* (cyan) and *XMM-Newton* (red) catalogues.</figcaption></figure>

$m_{F475W} = 24.8 \pm 0.4$, and $m_{F814W} = 24.3 \pm 0.4$. Assuming the source is an unresolved star cluster associated with NGC 6099 (its high optical luminosity prevents it from being a single star), with a mass-to-light ratio of ~2 $M_{\odot}/L_{\odot}$ and a Galactic extinction of $A_V = 0.13$, its estimated mass would be ~$6 \times 10^6 M_{\odot}$. Alternatively, the optical emission can originate from an irradiated accretion disc (see Chang et al. 2025, for a detailed discussion on both possibilities).

### 5.3. 4XMM J160553.3+324734

This source is associated with the galaxy Gaia DR3 1323128966899690752, ~570 Mpc from Earth. There is no counterpart in any other wavelength. It has been detected with *XMM-Newton* six times, from 2003 to 2016, but a spectrum is only available for one observation, taken in 2015 (ObsID.0764460201). The fit returned the best values of $\Gamma = 3.5 \pm 0.4$, with a fixed $N_H = 2.5 \times 10^{20} \text{ cm}^{-2}$

($\chi^2$/d.o.f. = 19.24/12, n.h.p.=0.08), an unabsorbed flux of ($9.2 \pm 1.2) \times 10^{-15} \text{ erg cm}^{-2} \text{ s}^{-1}$, corresponding to a luminosity $L_X = (3.6 \pm 0.5) \times 10^{41} \text{ erg s}^{-1}$. The corresponding HR was ($8.8 \pm 1.6) \times 10^{-2}$ and it falls among the softest sources, very close on the HLD to the position of ESO 243-49 HLX-1 when in the high-soft state. No pulsation was detected for this source and the PF upper limit is ~75%. The six *XMM-Newton* detections report an absorbed flux as computed in the 4XMM catalogue between $3.3 \times 10^{-15} \text{ erg cm}^{-2} \text{ s}^{-1}$ and $1.3 \times 10^{-14} \text{ erg cm}^{-2} \text{ s}^{-1}$. With no other information on its spectral variability, it is hard to classify the source, but its luminosity and hardness would suggest a nature similar to that of ESO 243-49 HLX-1.

### 5.4. 4XMM J161604.0-223726

4XMM J161604.0-223726 is a relatively hard source located in the outskirts of the galaxy ESO 516-9, at a distance of ~110 Mpc. It has already been identified as a ULX/HLX candidate thanks to several catalogue studies (Earnshaw et al. 2019; Bernadich et al. 2022; Tranin et al. 2024). There are two *XMM-Newton* observations of this object, two days apart in 2008, which do not show any significant variability; eROSITA also provides a more recent unconstraining upper limit of $1.61 \times 10^{-13} \text{ erg cm}^{-2} \text{ s}^{-1}$ in the 0.2–5 keV band. The *XMM-Newton* spectrum is consistent with a power law with $\Gamma = 2.3_{-0.6}^{+0.8}$ ($N_H = (1.9_{-1.4}^{+1.8}) \times 10^{21} \text{ cm}^{-2}$, $\chi^2$/d.o.f. = 32.36/21, n.h.p.=0.05), with a resulting unabsorbed flux of $(2.3_{-0.5}^{+1.7}) \times 10^{-14} \text{ erg cm}^{-2} \text{ s}^{-1}$ and a corresponding luminosity of $L_X = (3.4_{-0.7}^{+2.5}) \times 10^{40} \text{ erg s}^{-1}$ in the 0.3–10 keV range, if it is indeed associated with ESO 516-9. In spite of being below the conventional luminosity threshold for HLXs, this source occupies the same HLD space as ESO 243-49 HLX-1 when in the low-hard state, suggesting yet another source of the same type. Also this source does not display any pulsating signal, with a PF upper limit of ~80% (the source has less than 250 counts).

There is no clear multi-wavelength counterpart to the X-ray source within the extent of ESO 516-9. The only apparent possible counterpart would be the infrared source WISE J161603.96-223726, which was detected as separate from the galaxy’s extended emission by the automatic pipeline (notably through its significantly softer emission, with a brighter emission in bands 3 and 4 of WISE) – although its separation from the galaxy’s emission is not visually clear. This object could possibly be a background AGN behind the foreground galaxy. Spatially resolved optical spectra (e.g. through an integral field unit) are needed to assess for instance the presence of emission at a higher redshift around the location of the X-ray source (which would indicate it is a background AGN).

## 6. Discussion

We investigated the population of HLXs observed by *XMM-Newton* and collected in Tranin et al. (2024) to determine whether HLXs are simply ‘more extreme’ ULXs and, as such, whether they share the same properties of ULXs in terms of spectral shape and evolution; alternatively, we considered whether they might make up a different category of astrophysical sources with different properties. After retrieving the spectra of individual sources and removing those securely identified as contaminants (mostly galaxy pairs), we fit the remaining ones with an absorbed power law to build the HLD. The final sample consists of 40 individual sources. As expected, the fluxes and HRs we obtained are slightly different from those in the 4XMM-DR12 catalogue, since we left the fit parameters free
to vary (cf. Sect. 3). This also impacts the luminosities of the selected sources, which are slightly different than those reported in Tranin et al. (2024).

The space on the HLD occupied by the HLX sample overlaps with the space where a typical AGN with $\Gamma = 2$ would fall (vertical red area in Fig. 1, cf. Sect. 3), highlighting the intrinsic difficulties in disentangling the two classes of objects. As a result, some AGNs can contaminate ULX and HLX catalogues (Tranin et al. 2024; Walton et al. 2022; Kovlakas et al. 2020). This issue becomes more problematic at greater distances, due to increasing difficulties in resolving objects close to each other. Typically, HLXs are detected at greater distances than ULXs, increasing the chances of ambiguous associations. From our study, we can confirm that 10 out of 64 initial spectra belong to AGNs in galaxy pairs wrongly associated with the companion galaxy, unresolved groups of galaxies, or intra-galactic diffused emission. In total, 25% of the retrieved spectra were of sources labelled as galaxy pairs in the catalogue of Tranin et al. (2024) and were excluded from the analysis presented here. These spectra significantly differ from the spectra of the final sample in shape and number of counts. In fact, AGNs typically reach luminosities higher than $10^{41}$ erg s$^{-1}$ and are the targets of long observations, so that their spectra have higher statistics, with a spectral shape that significantly deviates from a simple power law. It cannot be excluded that other AGNs remain in the final sample, especially if their spectra have low statistics. Nonetheless, the sample presented here can be considered one of cleanest produced to date.

The bulk of the sample consists of sources with HRs in the range 0.5–5 and luminosities between ~$10^{41}$ erg s$^{-1}$ and $10^{43}$ erg s$^{-1}$, with no particular distribution between sources with robust or weak associations to their host galaxies as in the original catalogue. Approximately 50% of sources are harder than typical AGNs, hinting at the possibility that half the sample might actually be an extension of the ULX population towards higher luminosity. Indeed, when they are fitted with the same model, ULX spectra are typically harder than the HLX sample analysed in this work, with $\Gamma < 2.3$ (Gladstone et al. 2009). On the other hand, roughly 40% of the HLX candidates have spectra with $HR \le 1$, indicative of a significant soft part of the population. These candidates could represent a distinct class of objects, rather than typical ULXs, that encompasses astrophysical phenomena reaching hyper-luminous levels, such as TDEs.

A prime example would be ESO 243-49 HLX-1, present in the catalogue, interpreted by many authors as an IMBH, with mass estimations in the range $10^4-10^5$ M$_{\odot}$ (e.g. Webb et al. 2012), undergoing a repetitive partial TDE (Godet et al. 2014; Webb et al. 2014). There is also one other source in the sample, 4XMM J215022.4-055109, interpreted as a TDE around an IMBH in an off-centre star cluster (Lin et al. 2018). This suggests that other TDE candidates can hide among the softest HLXs. Indeed, we found four more candidates showing properties similar to ESO 243-49 HLX-1. The most striking is 4XMM J161534.3+192707 (cf. Sect. 5.2), which stands out as the most luminous and the second softest source of the sample. It occupies an area on the HLD close to ESO 243-49 HLX-1 in the high-soft state and displays a clear long-term variability (see Fig. 1), with a difference in luminosity of almost two orders of magnitudes. It is associated to an optical counterpart, which can be interpreted as a star cluster. Therefore, the interpretation of 4XMM J161534.3+192707 as a partial TDE, similar to ESO 243-49 HLX-1, is plausible (as also concluded in the recent work of Chang et al. 2025). However, the evolution of the HR

of 4XMM J161534.3+192707 differs from that of ESO 243-49 HLX-1 and is more akin to the behaviour of hard ULXs, which transition from a hard, high-luminosity state to a softer, dimmer state (see examples in Gúrpide et al. 2021).

Two other soft candidates, 4XMM J160553.3+324734 (Sect. 5.3) and 4XMM J161604.0-223726 (Sect. 5.4), occupy the same position in the HLD of ESO 243-49 HLX-1 in the high-soft and low-hard states, respectively. The available flux estimates of 4XMM J160553.3+324734 do not suggest changes in luminosities by any substantial orders of magnitude, but this might be due to the limited coverage of the source. For both sources, the lack of more high-quality data prevents a more secure classification. Lastly, the softest source of the sample, 4XMM J231818.7-422237 is an ambiguous case because it could comprise yet another soft HLX or SSUL source, based on its possible association with its host galaxy (cf. Sect. 5.1). The pronounced variability in the source light curve is reminiscent of the temporal variability of other TDEs. Unfortunately, the lack of a long-term coverage of the source prevents us to advance more secure claims, as done for example for the TDE Swift J1644+57, which alternates between normal and dipping states, suggestive of a jet precession scenario (Jin 2021).

None of the selected candidates show pulsations, which should not be surprising, considering that known PULXs have on average harder spectra (see e.g. the HLD in Gúrpide et al. 2021), compared to the candidates analysed here. Furthermore, the limited source statistics and the transient nature of the pulsed signals hinder the detection of pulsations. Notably, the prototype of this class, ESO 243-49 HLX-1, does not exhibit pulsations despite having higher count statistics. Taken together, these considerations suggest that very soft HLXs may constitute a distinct class of objects, differing from ‘standard’ ULXs and possibly representing IMBH and/or TDEs.

The work presented in this paper is subject to a few biases and limitations. First, the sample is limited to those sources that have average HLX luminosity levels during an observation (see Sect. 2). Hence, sources that occasionally surpass the $10^{41}$ erg s$^{-1}$ luminosity threshold for a shorter time segment than the duration of the observation are automatically excluded. Second, the sample is comprehensive of XMM-Newton data alone. However, considering the lower-quality of the spectra collected by the other two X-ray missions, Swift/XRT and Chandra, and considering the lower number of their HLX candidates (88 vs. 115 of XMM-Newton), the inclusion of their spectra in the present analysis probably would not have lead to substantial changes in the final results.

## 7. Conclusions

This work investigates the spectral properties of a clean sample of HLXs as observed with XMM-Newton and aims at discerning the presence of different type of sources within the same class. By performing spectral fits and correcting for the absorption along the line of sight, this is the first time that the intrinsic (i.e. unabsorbed) fluxes and luminosities of these objects have been studied. The final sample is also the cleanest sample of XMM-Newton HLXs produced to date, since the AGN contaminants have been identified and removed, not only on the basis of their association with the host galaxies, but also on their spectral properties. The resulting HLX population has luminosities in the range $10^{41}-10^{43}$ erg s$^{-1}$ and HR between 0.5 and 5, and it is overall softer than the population of ULXs, with no striking evidence for sub-populations. The only exceptions are a few very soft sources, which have similar spectral and temporal proper-
ties to ESO 243-49 HLX-1. This nature suggests that they differ from ULXs and might instead resemble other types of sources, such as partial TDEs.

**Acknowledgements.** This research has made use of data obtained from the *Chandra* Data Archive and the *Chandra* Source Catalogue, and software provided by the *Chandra* X-ray Center (CXC) in the application package CIAO; of observations obtained with *XMM-Newton*, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA; of PyXspec, a Python interface to the XSPEC spectral-fitting program. The authors thank M. Coriat for his assistance in providing the data. RA and GLI acknowledge financial support from INAF through grant “INAF-Astronomy Fellowships in Italy 2022 – (GOG)”.

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**Appendix A: Excluded sources**

We collect here a few notes on all the spectra that were excluded from the final dataset because of a bad fit, determined by either a $\chi^2_{red} > 2$, or n.h.p. < 0.01, or both. In a few cases, more complex models than a power law are needed to correctly fit the data, as for the high-count spectra of a few misclassified AGN present in the catalogue. In other cases, even with a low number of counts, the spectral shape significantly differ from a power law. Those spectra typically belong to spurious detections (e.g. intra-galaxy X-ray emission, detected as point-like sources) and are hence excluded from the sample. We recall that all spectra have at least 100 counts and are binned to have at least five bins of 20 counts each, suited to be fit with Gaussian statistics.

4XMM J114630.0+202728 (ObsID.0301900601) The automated fit resulted in a $\chi^2_{red} > 2$. We then manually fit the data and obtained an acceptable $\chi^2_{red}$, but with an extremely high and unconstrained photon index. We try also to fix the $N_H$ value to the Galactic one, but obtained again a $\chi^2_{red} > 2$. This source is hence excluded from our sample. The source position matches that of the HII galaxy NAME Z 127-051 N, so the X-ray emission is probably due to the galaxy itself.

4XMM J163818.7-642201 and 4XMM J062648.7-543208 (ObsIDs.0093620101 and 0400010301) These sources lie within significant X-ray diffuse emission, resulting in an intrinsic difficulties in separating the contribution of the diffuse emission to the source spectrum. 4XMM J163818.7-642201 belongs to a group of galaxies (ref. galaxy ESO 101-4). 4XMM J062648.7-543208 is detected both in the 4XMM-DR12 and the CSC2 catalogues and it is associated with the galaxy LEDA 19057, though at radial distance of ~28 arcsec. Both sources are excluded from the sample.

4XMM J215022.4-055109 (ObsID.0823360101) The fit resulted in a n.h.p. of only 0.01 and the source was thus excluded from our sample. Moreover, the source is already known in the literature as a TDE candidate (Lin et al. 2018), hinting to the chance that other TDE candidates might hide among HLXs.

4XMM J221755.3-354629 (ObsIDs.0201902901) The automated fit resulted in a high reduced chi-squared and even fixing the $N_H$ to the HI4PI map value did not improve it. Hence, we removed this source from the dataset. Overall the source is very soft and it lies in the outskirts of the galaxy LEDA 644746.

4XMM J011028.2-460422 (ObsID.0801310101) This is ESO 243-49 HLX-1 (Farrell et al. 2009). The automated fit resulted in $\chi^2_{red}$(d.o.f.)= 1.959(132) and a n.h.p. of the order of $10^{-10}$ (see also Table 1). In this observation the source is in soft state, in the HLX regime. More details about this source are given in Sect.4.
## Appendix B: Dataset

We report in Table B.1 a subset of all the data entries used in this work, with the X-ray luminosities and HRs obtained from the fit. The complete sample is available upon request.

<table><caption>**Table B.1.** HLX candidates analysed in this work.</caption><thead><tr><th></th><th>ObsID</th><th>IAUNAME</th><th>Exposure Times (ks)</th><th>HR</th><th>X-ray Luminosity (10<sup>41</sup> erg s<sup>-1</sup>)</th><th>Distance (Mpc)</th><th>Subsample</th></tr></thead><tbody><tr><td>1</td><td>0744410101</td><td>4XMM J192133.3+441322</td><td>26.7/34.3/34.3</td><td>4.3 ± 0.5</td><td>30<sup>+4</sup><sub>-3</sub></td><td>357</td><td>robust</td></tr><tr><td>2</td><td>0044350101</td><td>4XMM J003416.6-212430</td><td>18.0/-/-</td><td>4.54 ± 1.5</td><td>11<sup>+7</sup><sub>-5</sub></td><td>269</td><td>weak</td></tr><tr><td>3</td><td>0651460101</td><td>4XMM J210536.0-522300</td><td>37.3/-/-</td><td>1.38 ± 0.3</td><td>0.6 ± 0.2</td><td>154</td><td>robust</td></tr><tr><td>4</td><td>0555970401</td><td>4XMM J145753.6-113959</td><td>63.0/-73.6</td><td>0.8 ± 0.2</td><td>4.6<sup>+1.8</sup><sub>-1.4</sub></td><td>486</td><td>weak</td></tr><tr><td>5</td><td>0306630201</td><td>4XMM J123115.8+105320</td><td>84.0/-/98.1</td><td>1. ± 0.2</td><td>7.8<sup>+1.4</sup><sub>-1.3</sub></td><td>488</td><td>weak</td></tr><tr><td>6</td><td>0204610101</td><td>4XMM J231818.7-422237</td><td>80./87.9/88.0</td><td>0.0075 ± 0.0004</td><td>13<sup>+6</sup><sub>-4</sub></td><td>260</td><td>robust</td></tr><tr><td>7</td><td>0093550401</td><td>4XMM J233342.0-151523</td><td>17.8/21.3/22.2</td><td>1.08 ± 0.11</td><td>8.1<sup>+0.9</sup><sub>-0.8</sub></td><td>289</td><td>weak</td></tr><tr><td>8</td><td>0827010901</td><td>4XMM J225931.7-344113</td><td>32.2/43.7/42.2</td><td>0.8 ± 0.09</td><td>9.4<sup>+2.2</sup><sub>-1.2</sub></td><td>263</td><td>robust</td></tr><tr><td>9</td><td>0724690401</td><td>4XMM J015728.3-614120</td><td>10.7/-/-</td><td>2.6 ± 0.7</td><td>1.0<sup>+0.4</sup><sub>-0.3</sub></td><td>88</td><td>robust</td></tr><tr><td>10</td><td>0743650801</td><td>4XMM J025645.2+413508</td><td>23.2/-/-</td><td>2.2 ± 0.5</td><td>6.3<sup>+3.2</sup><sub>-1.3</sub></td><td>281</td><td>robust</td></tr><tr><td>11</td><td>0670350301</td><td>4XMM J161534.3+192707</td><td>13.0/16.1/16.1</td><td>0.041 ± 0.001</td><td>67<sup>+9</sup><sub>-7</sub></td><td>133</td><td>robust</td></tr><tr><td>12</td><td>0744390301</td><td>4XMM J215057.1-073535</td><td>19.6/-/-</td><td>2.3 ± 0.8</td><td>13.1<sup>+1.3</sup><sub>-5.7</sub></td><td>617</td><td>weak</td></tr><tr><td>13</td><td>0109910101</td><td>4XMM J140106.8-111128</td><td>39.2/-/-</td><td>1.5 ± 0.5</td><td>3.14<sup>+1.87</sup><sub>-1.08</sub></td><td>379</td><td>robust</td></tr><tr><td>14</td><td>0675470501</td><td>4XMM J221014.5-121119</td><td>13.2/-/-</td><td>2.85 ± 1.02</td><td>12<sup>+5</sup><sub>-4</sub></td><td>403</td><td>weak</td></tr><tr><td>15</td><td>0502630201</td><td>4XMM J085403.2+200746</td><td>49.6/54.7/54.8</td><td>1.12 ± 0.15</td><td>1.7<sup>+0.4</sup><sub>-0.3</sub></td><td>132</td><td>weak</td></tr><tr><td>16</td><td>0505210401</td><td>4XMM J062536.4-535105</td><td>19.4/-/-</td><td>2.1 ± 0.8</td><td>3.9<sup>+2.6</sup><sub>-1.6</sub></td><td>247</td><td>robust</td></tr><tr><td>17</td><td>0762800101</td><td>4XMM J050252.9-755108</td><td>11.7/-/-</td><td>21 ± 7</td><td>49<sup>+38</sup><sub>-23</sub></td><td>367</td><td>weak</td></tr><tr><td>18</td><td>0800761301</td><td>4XMM J062447.9-372122</td><td>17.2/-/21.2</td><td>1.5 ± 0.2</td><td>1.8 ± 0.3</td><td>139</td><td>robust</td></tr><tr><td>19</td><td>0404050101</td><td>4XMM J074735.9+554115</td><td>96.3/-/-</td><td>2.3 ± 0.5</td><td>7.2<sup>+2.1</sup><sub>-1.5</sub></td><td>152</td><td>weak</td></tr><tr><td>20</td><td>0203391001</td><td>4XMM J130307.6-154225</td><td>13.9/16.8/16.8</td><td>2.0 ± 0.4</td><td>27<sup>+6</sup><sub>-5</sub></td><td>324</td><td>robust</td></tr><tr><td>21</td><td>0803910101a</td><td>4XMM J161342.6+470314</td><td>79.6/-/-</td><td>2.9 ± 1.2</td><td>3.2<sup>+3.6</sup><sub>-1.3</sub></td><td>400</td><td>robust</td></tr><tr><td>22</td><td>0604310401</td><td>4XMM J120335.1-040648</td><td>25.1/-/30.3</td><td>2.8 ± 0.2</td><td>27 ± 2</td><td>287</td><td>weak</td></tr><tr><td>23</td><td>0723100401</td><td>4XMM J125256.2-132756</td><td>60.5/-/-</td><td>3.8 ± 1.3</td><td>33<sup>+20</sup><sub>-14</sub></td><td>595</td><td>robust</td></tr><tr><td>24</td><td>0552670301</td><td>4XMM J055239.0-363936</td><td>83.9/-/-</td><td>0.46 ± 0.16</td><td>3.4<sup>+2.5</sup><sub>-1.4</sub></td><td>200</td><td>robust</td></tr><tr><td>25</td><td>0300530101</td><td>4XMM J125917.9+281107</td><td>-/-/25.0</td><td>1.22 ± 1.09</td><td>5<sup>+8</sup><sub>-4</sub></td><td>405</td><td>weak</td></tr><tr><td>26</td><td>0764460201</td><td>4XMM J160553.3+324734</td><td>105.3/-/-</td><td>0.088 ± 0.016</td><td>3.6 ± 0.5</td><td>573</td><td>robust</td></tr><tr><td>27</td><td>0203170301</td><td>4XMM J123307.1+000758</td><td>77.9/102.9/103.2</td><td>0.48 ± 0.08</td><td>17<sup>+1</sup><sub>-4</sub></td><td>617</td><td>robust</td></tr><tr><td>28</td><td>0782330301</td><td>4XMM J110106.5+102943</td><td>92.3/104.4/105.4</td><td>2.06 ± 0.12</td><td>3.19 ± 0.17</td><td>171</td><td>weak</td></tr><tr><td>29</td><td>0305980301</td><td>4XMM J042058.6-484008</td><td>10.9/15.5/14.8</td><td>0.76 ± 0.08</td><td>58<sup>+10</sup><sub>-7</sub></td><td>546</td><td>weak</td></tr><tr><td>30</td><td>0555650301</td><td>4XMM J161604.0-223726</td><td>89.1/98.7/103.6</td><td>0.65 ± 0.11</td><td>0.34<sup>+0.25</sup><sub>-0.07</sub></td><td>111</td><td>robust</td></tr><tr><td>31</td><td>0691590101b</td><td>4XMM J222324.0-290444</td><td>54.7/-/-</td><td>1.4 ± 0.6</td><td>1.0<sup>+0.8</sup><sub>-0.5</sub></td><td>293</td><td>robust</td></tr><tr><td>32</td><td>0107260101</td><td>4XMM J132811.0-313503</td><td>35.2/-/-</td><td>2.24 ± 1.12</td><td>1.5<sup>+0.9</sup><sub>-0.7</sub></td><td>184</td><td>weak</td></tr><tr><td>33</td><td>0404410101</td><td>4XMM J011459.1+002519</td><td>37.5/-/49.8</td><td>1.1 ± 0.2</td><td>1.3<sup>+0.5</sup><sub>-0.3</sub></td><td>198</td><td>robust</td></tr><tr><td>34</td><td>0083151201</td><td>4XMM J050107.9-384029</td><td>86.6/-/-</td><td>1.9 ± 0.6</td><td>5.8<sup>+2.5</sup><sub>-1.9</sub></td><td>288</td><td>weak</td></tr><tr><td>35</td><td>0500940201</td><td>4XMM J101040.4+534719</td><td>23.6/-/-</td><td>0.7 ± 0.3</td><td>20<sup>+12</sup><sub>-8</sub></td><td>643</td><td>robust</td></tr><tr><td>36</td><td>0149160201</td><td>4XMM J083235.2-225804</td><td>25.1/27.9/29.3</td><td>0.7 ± 0.1</td><td>1.7<sup>+0.7</sup><sub>-0.3</sub></td><td>118</td><td>weak</td></tr><tr><td>37</td><td>0784610101</td><td>4XMM J194744.7+704645</td><td>21.8/-/-</td><td>0.8 ± 0.3</td><td>24-9<sup>+14</sup></td><td>878</td><td>robust</td></tr><tr><td>38</td><td>0803910101b</td><td>4XMM J161347.9+470809</td><td>78.9/-/-</td><td>3.37 ± 1.15</td><td>1.4<sup>+0.6</sup><sub>-0.5</sub></td><td>322</td><td>robust</td></tr><tr><td>39</td><td>0201901801</td><td>4XMM J130256.1-022930</td><td>17.7/-/-</td><td>1.6<sup>+0.3</sup><sub>-0.5</sub></td><td>7.3<sup>+1.5</sup><sub>-1.4</sub></td><td>378</td><td>weak</td></tr><tr><td>40</td><td>0110870101</td><td>4XMM J050117.1-242808</td><td>20.3/-/24.7</td><td>7<sup>+30</sup><sub>-4</sub></td><td>87<sup>+232</sup><sub>-35</sub></td><td>1040</td><td>robust</td></tr></tbody></table>

**Notes.** For each candidate we indicate the XMM-Newton observation ID, IAU designation, exposure times for the EPIC-pn/MOS1/MOS2, hardness ratio, X-ray luminosity, host galaxy distance, and 'robust' or 'weak' classification label.