EDP Sciences logo
Subscriber Authentication Point
Search
Menu
Home All issues Volume 693 (January 2025) A&A, 693 (2025) A158 Full HTML
Open Access
Issue
A&A
Volume 693, January 2025
Article Number A158
Number of page(s) 6
Section Stellar structure and evolution
DOI https://doi.org/10.1051/0004-6361/202452333
Published online 14 January 2025

© The Authors 2025

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication.

1. Introduction

Currently, 146 millisecond pulsars (MSPs) have been detected by the Fermi Gamma-ray Space Telescope since its launch in 2008 (Ray 2024). A considerable number of the MSPs are so-called spider pulsars, which are divided into the ‘redback’ (RB) and ‘black widow’ (BW) subclasses (Roberts 2013; Manchester 2017). These are close binary systems with orbital periods of ≲1 d in which the face side of a tidally locked low-mass companion star is heated and ablated by the high-energy pulsar wind. The BWs have degenerate companions with masses of ≲0.05 M⊙, while companions of RBs are non-degenerate and heavier (0.1–1 M⊙). In such systems, masses of neutron stars (NSs) can exceed 2 M⊙, which is considerably higher than the canonical NS mass of 1.4 M⊙ (e.g. Nieder et al. 2020; Romani et al. 2022; Thongmeearkom et al. 2024). This makes studies of spiders particularly interesting for constraining the still poorly known properties of the superdense matter inside NSs.

At the present time, about 20 RBs and 40 BWs have been discovered in the Galactic disc (Strader et al. 2019; Swihart et al. 2022), and similar numbers have been found in globular clusters (Freire 2024). Unfortunately, the material evaporated from a companion often obscures the pulsar radio emission for part of the system’s orbit, which makes it difficult to detect pulsations and therefore to find new spiders in radio surveys. Nevertheless, identification is still possible through multi-wavelength investigations of likely counterparts to the unassociated Fermi sources, especially in the optical and X-rays (e.g. Salvetti et al. 2017). Notably, such searches have resulted in the discovery of several promising spider pulsar candidates (e.g. Li et al. 2021; Swihart et al. 2021, 2022; Halpern 2022; Karpova et al. 2023; Zyuzin et al. 2024).

The unassociated Fermi source 4FGL J1544.2−2554 (hereafter J1544) has recently been proposed as an MSP candidate based on its γ-ray spectral and timing properties, with a rather high probability of 0.75 (Mayer & Becker 2024). Using the data obtained by the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) aboard the Spektrum-Roentgen-Gamma (SRG) space observatory, the authors found a likely X-ray counterpart to J1544: 4eRASS J154415.4−255531. The counterpart spatially coincides with a star (Gaia catalogue ID 6235002859670996352) that could be a binary companion of the proposed MSP candidate (see figure 7 in Mayer & Becker 2024). According to the Gaia Data Release 3 (DR3) catalogue, its magnitude is G = 20.6, and the coordinates are RA(2016) = 15h44m15 . s $ \overset{\text{ s}}{.} $455 and Dec(2016) = −25°55′32 . ″ $ \overset{\prime \prime }{.} $688 (Gaia Collaboration 2016, 2023).

We also found this companion candidate in the Panoramic Survey Telescope and Rapid Response System survey (Pan-STARRS; Flewelling et al. 2020), Zwicky Transient Facility (ZTF; Masci et al. 2019), and Legacy Surveys DR 10 (Dey et al. 2019) catalogues. In these surveys, the candidate shows a strong variability at a level of ≳1 mag. Combined with the detected X-ray and γ-ray emission, such a strong optical variability implies that J1544 is a spider pulsar. In this case, its optical brightness is expected to be modulated with the orbital period of the binary system. However, the available optical data are too sparse and noisy to be used for a periodicity search.

To clarify the nature of this source, we have thus carried out dedicated time series optical photometric observations. In this work, we report the period of the system and estimate its parameters based on light curve analysis. We conclude that J1544 is indeed a likely member of the spider class. The paper is organised as follows: Observations and data reduction are described in Sect. 2, data analysis is presented in Sect. 3, a discussion is provided in Sect. 4, and the conclusions are in Sect. 5.

2. Observations and data reduction

We performed time series photometric observations of the J1544 field in the Johnson-Cousins B, V, and Rc bands with the ‘Rueda Italiana’ instrument attached to the 2.1-m telescope at the Observatorio Astronómico Nacional San Pedro Mártir (OAN-SPM) in Mexico on June 8–13, 2024. The weather conditions were either photometric or clear with an effective seeing between 1 . ″ $ \overset{\prime \prime }{.} $5 and 2 . ″ $ \overset{\prime \prime }{.} $1 (derived as the full width at the half maximum of a point source spatial profile in an image). All images were taken with exposures of 400 s. The log of observations is given in Table 1. Standard data reduction of the raw frames was carried out using the IRAF package. To calculate the astrometric solution, we used a set of reference stars from the Gaia DR3 catalogue. The formal rms uncertainties of the resulting astrometric fit were ΔRA ≲ 0 . ″ $ \overset{\prime \prime }{.} $023 and ΔDec ≲ 0 . ″ $ \overset{\prime \prime }{.} $027. The Landolt standards PG 1323 and PG 1657 (Landolt 1992) were observed for the photometric calibration. We employed the aperture photometry to extract the target light curves, and a differential technique was applied to eliminate the variations due to the changing weather conditions using three non-variable bright field stars as references.

Table 1.

Log of the J1544 observations.

3. Data analysis and results

Examples of the images of the J1544 field obtained in the Rc band are presented in Fig. 1. They demonstrate that the likely optical counterpart to the γ-ray source is strongly variable.

thumbnail Fig. 1.

Individual 1 . ′ $ \overset{\prime }{.} $5 × 1 . ′ $ \overset{\prime }{.} $5 Rc-band images of the J1544 field obtained with the OAN-SPM 2.1-m telescope near the maximum (left) and minimum (right) brightness phases. The optical counterpart candidate is shown with an arrow. The circle with a radius of 3″indicates the 1σ position uncertainty of 4eRASS J154415.4−255531 proposed as the X-ray counterpart (Mayer & Becker 2024).

To search for its periodicities, we performed a Lomb–Scargle periodogram analysis (Lomb 1976; Scargle 1982) of the barycentre-corrected data in the Rc band where the most numerous set of data points was obtained after combining the data from six nights. The resulting power spectrum is presented in Fig. 2. The highest peak corresponds to the frequency 8.81 d−1, that is, the period1Pph = 2.724(13) h. The second harmonic of the periodicity can also be clearly seen in the power spectrum at the frequency 17.62 d−1.

thumbnail Fig. 2.

Lomb-Scargle periodogram. The highest peak corresponding to the best period is enlarged in the inset.

The B, V, and Rc-band light curves of J1544 folded with the period are shown in Fig. 3. They have a single broad peak per period with a total brightness variation of ≳2.5 mag. Near the minimum brightness phase the source drops below the detection limits of about 23 mag. The shapes of the light curves appear to be symmetric, and the amplitudes of their brightness modulation are similar to those observed in other spider MSPs.

thumbnail Fig. 3.

Light curves of the J1544 counterpart candidate obtained with the OAN-SPM 2.1-m telescope and result of their modelling. Top: Light curves folded with a period of 2.724 h. The best-fitting model is shown with solid lines. Bottom: Fit residuals derived as the difference between the observed (O) and the calculated (C) magnitudes for each data point in terms of the magnitude error σ.

We fitted the folded light curves with the symmetric direct heating model to derive the parameters of the presumed spider system. In this model, the primary is an NS that irradiates and heats a low-mass secondary. Each surface element of the secondary has a black-body spectrum with an effective temperature varying from element to element. (For details, see Zharikov et al. (2013, 2019), Kirichenko et al. (2024).)

The fitted parameters were the distance to the system D, the pulsar mass Mp, the mass ratio of the binary components q, the system orbit inclination i, the effective irradiation factor Kirr defining the companion heating, the companion Roche lobe filling factor f, and the ‘night-side’ temperature Tn of the companion in respect to the pulsar. The orbital period was fixed at the measured value Pph. With b = 22 . ° 586 $ b=22{{\overset{\circ}{.}}}586 $, J1544 is a high Galactic latitude object. According to the 3D dust map by Green et al. (2019), the interstellar reddening E(B − V) in this direction quickly increases with the distance and reaches its maximum value of ≈0.23 mag at 0.2 kpc. The colour and magnitude values of our source suggest that it is likely a more distant system. We thus fixed E(B − V) at this maximum value. The gradient descent method was utilised to find the minimum of the χ2 function. The results of the fit with 1σ uncertainties of the parameters are presented in Table 2, and the best-fitting model is shown with the solid lines in Fig. 3. As can be seen from the figure, the multi-colour light curves are well described by the model. The inferred uncertainties are statistical and should be considered preliminary due to the lack of data points. For instance, in the absence of data points near the minimum brightness phase, the ‘night-side’ temperature value is a prediction based on extrapolation of the best-fit light curves at phases around the brightness peak. This also affects the resulting inclination angle. In addition, there is covariance between the distance and the Roche lobe filling factor. An independent distance constraint would be valuable, as it can be used as a prior to obtain a convincing filling factor.

Table 2.

The light curve fitting results for J1544.

4. Discussion

We performed the first multi-band time series photometry of the likely optical counterpart to the γ-ray source J1544. We found that its brightness is highly modulated with the period 2.724 h and an amplitude of ≳2.5 mag. Its light curves demonstrate a single broad peak per period. Our findings are consistent with what is observed for spider systems where the heating of the companion face side by the energetic pulsar wind dominates over the tidal distortion of the companion (e.g. Draghis et al. 2019; Kandel et al. 2020; Mata Sánchez et al. 2023). We thus propose that J1544 is a very promising candidate of being a spider system.

The high 2–4 mag amplitude of the J1544 variability is more typical for BWs than for RBs, whose typical modulation is ≲1 mag2. The presumed orbital period is also more appropriate for BWs, as about two dozen of the confirmed members of this family have orbital periods of less than 3 h, while only one RB – PSR J1748−2446A3 – with such a short period is known (Strader et al. 2019; Swihart et al. 2022; Freire 2024). The estimated ‘night-side’ temperature of the companion, ∼3000 K, is also typical for BWs, which usually have Tn of 1000–3000 K, while RBs are hotter, with Tn of 4000–6000 K (Turchetta et al. 2023). The difference between the ‘night-side’ and the maximum ‘day-side’ temperatures is significant, ΔT ∼ 4000 K, which is again an attribute of BW systems (e.g. Mata Sánchez et al. 2023). Redbacks typically have ΔT of several hundred Kelvins (Koljonen & Linares 2023). The Roche lobe filling factor of the J1544 companion, f = 0.65, is rather low for BWs and RBs, which usually have f ≳ 0.8 (see e.g. Strader et al. 2019; Mata Sánchez et al. 2023); however, it is not unique. There are systems with similar values, such as the RB J1431−4715, with f = 0.73(4) (de Martino et al. 2024), and the BWs J0023+0923, with f = 0.5(1), and J2256−1024, with f = 0.4(2) (Mata Sánchez et al. 2023). The obtained irradiation factor is similar to the values derived for other spider systems (e.g. Zharikov et al. 2019; Kirichenko et al. 2024; Zyuzin et al. 2024).

Spider pulsars show a bimodal distribution of the companion masses with a gap in the range 0.07–0.1 M⊙ between the BW and RB systems (Swihart et al. 2022). From the light curve modelling, the J1544 companion has M c = 0 . 102 − 0.033 + 0.053 $ M_{\mathrm{c}}=0.102^{+0.053}_{-0.033} $ M⊙, which is consistent with both subclasses. The derived mass of the putative pulsar is very uncertain.

According to the Fermi Large Area Telescope 14-Year Point Source Catalog (4FGL-DR4; Ballet et al. 2023), the J1544 flux in the 0.1–100 GeV range is Fγ = 8.4(7)×10−12 erg s−1 cm−2. The eROSITA count rate of the J1544 counterpart candidate is 33.08 cts ks−1 in the 0.2–2.3 keV range (Mayer & Becker 2024). Using WebPIMMS4, we estimated the unabsorbed flux in the 0.5–10 keV band, FX ≈ 6.2 × 10−14 erg s−1 cm−2 or ≈1.5 × 10−13 erg s−1 cm−2. Here, we assumed the absorbed power-law spectral model with the photon indices Γ = 2.5 or 1.4, which are the average values for the BWs and RBs, respectively (Swihart et al. 2022). The absorbing column density NH = 2 × 1021 cm−2 was derived from the reddening value using the empirical relation of Foight et al. (2016). For the distance of 2.13 kpc obtained from the light curve fitting, the luminosities are LX ≈ 3.4 × 1031 erg s−1 (for Γ = 2.5) or 8 × 1031 erg s−1 (for Γ = 1.4) and Lγ ≈ 4.5 × 1033 erg s−1. The X-ray to γ-ray flux ratio is ∼0.01. All of these values are typical for spider pulsars (Strader et al. 2019; Swihart et al. 2022).

5. Conclusions

Based on our optical data and the available γ-ray and X-ray data, we propose that J1544 is a very promising candidate of being a spider pulsar at a distance of ≈2.1 kpc. The uncertainties of the derived mass of the putative MSP companion do not allow us to determine whether it belongs to the RB or BW spider subclasses. However, if it is a member of the RB family, it could have the shortest orbital period among the RBs known in the Galactic field. The model used for the light curve fitting predicts brightness variations with the orbital phase at the levels ΔB ≈ 6 mag, ΔV ≈ 4.8 mag, and ΔRc ≈ 4 mag. Confirmation of this prediction through deeper optical observations near the minimum brightness phase would make J1544 the second RB with such a strong brightness variability, along with PSR J2339−0533 mentioned above. In the BW case, J1544 could be among the BWs with the most massive companions. On the other hand, it might also represent a bridge between the two spider subclasses.

Targeted searches for periodic millisecond pulsations related to the pulsar spin in the radio and γ-ray domains are necessary to confirm the spider nature of J1544. Detection of the binary radio pulsar J1544−2555 using MeerKAT was reported by the TRansients And PUlsars with MeerKAT (TRAPUM) collaboration on their website5. It has a spin period of 2.39 ms and a dispersion measure of 25.8 pc cm−3. No other information, such as coordinate uncertainties or orbital period, is presented there; however, it is most likely the same source as the one considered in this work. Measurement of the binary period in the radio would confidently prove this. Optical spectroscopy and multi-band photometry by a large-aperture telescope would be useful for measuring the radial velocity curve of the presumed spider companion, for detection of the source and obtaining its colours near the minimum brightness phase and thus for constraining the parameters of the binary system and the ‘night-side’ temperature of the companion with higher precision. Deeper studies in X-rays are also encouraged.

Data availability

Photometric measurements in the B, V, and Rc bands are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/693/A158

1

The period uncertainty was derived as the half width at half maximum of the highest peak in the periodogram. We also used archival ZTF and Pan-STARRS data to better constrain the period. (See Appendix A for details.).

2

This does not include RB PSR J2339−0533, which has the modulation amplitude Δg ≈ 6 mag (Kandel et al. 2020).

3

PSR J1748−2446A has Pb = 1.8 h (Lyne et al. 1990). However, it is located in a globular cluster and thus has a formation history different to spider pulsars in the Galactic field.

Acknowledgments

We thank the anonymous referee for useful comments which helped us to improve the paper. Based upon observations carried out at the Observatorio Astronómico Nacional San Pedro Mártir (OAN-SPM), Baja California, México. We thank the daytime and night support staff at the OAN-SPM for facilitating and helping obtain our observations. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST–1238877, the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation. Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grants No. AST-1440341 and AST-2034437 and a collaboration including current partners Caltech, IPAC, the Oskar Klein Center at Stockholm University, the University of Maryland, University of California, Berkeley, the University of Wisconsin at Milwaukee, University of Warwick, Ruhr University, Cornell University, Northwestern University and Drexel University. Operations are conducted by COO, IPAC, and UW. The work of AVK, DAZ and YAS (data reduction, periodicity search) was supported by the baseline project FFUG-2024-0002 of the Ioffe Institute. DAZ thanks Pirinem School of Theoretical Physics for hospitality. SVZ acknowledges DGAPA-PAPIIT grant IN119323. AYK acknowledges the DGAPA-PAPIIT grant IA105024.

References

  1. Ballet, J., Bruel, P., Burnett, T. H., Lott, B., & The Fermi-LAT collaboration 2023, arXiv e-prints [arXiv:2307.12546] [Google Scholar]
  2. de Martino, D., Phosrisom, A., Dhillon, V. S., et al. 2024, A&A, 691, A36 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  3. Dey, A., Schlegel, D. J., Lang, D., et al. 2019, AJ, 157, 168 [Google Scholar]
  4. Draghis, P., Romani, R. W., Filippenko, A. V., et al. 2019, ApJ, 883, 108 [NASA ADS] [CrossRef] [Google Scholar]
  5. Flewelling, H. A., Magnier, E. A., Chambers, K. C., et al. 2020, ApJS, 251, 7 [NASA ADS] [CrossRef] [Google Scholar]
  6. Foight, D. R., Güver, T., Özel, F., & Slane, P. O. 2016, ApJ, 826, 66 [Google Scholar]
  7. Freire, P. 2024, Pulsars in globular clusters, https://www3.mpifr-bonn.mpg.de/staff/pfreire/GCpsr.html, (accessed June 27, 2024) [Google Scholar]
  8. Gaia Collaboration (Prusti, T., et al.) 2016, A&A, 595, A1 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  9. Gaia Collaboration (Vallenari, A., et al.) 2023, A&A, 674, A1 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  10. Green, G. M., Schlafly, E., Zucker, C., Speagle, J. S., & Finkbeiner, D. 2019, ApJ, 887, 93 [NASA ADS] [CrossRef] [Google Scholar]
  11. Halpern, J. P. 2022, ApJ, 932, L8 [Google Scholar]
  12. Kandel, D., Romani, R. W., Filippenko, A. V., Brink, T. G., & Zheng, W. 2020, ApJ, 903, 39 [NASA ADS] [CrossRef] [Google Scholar]
  13. Karpova, A. V., Zyuzin, D. A., Shibanov, Y. A., & Gilfanov, M. R. 2023, MNRAS, 524, 3020 [NASA ADS] [CrossRef] [Google Scholar]
  14. Kirichenko, A. Y., Zharikov, S. V., Karpova, A. V., et al. 2024, MNRAS, 527, 4563 [Google Scholar]
  15. Koljonen, K. I. I., & Linares, M. 2023, MNRAS, 525, 3963 [NASA ADS] [CrossRef] [Google Scholar]
  16. Landolt, A. U. 1992, AJ, 104, 340 [Google Scholar]
  17. Li, K.-L., Jane Yap, Y. X., Hui, C. Y., & Kong, A. K. H. 2021, ApJ, 911, 92 [NASA ADS] [CrossRef] [Google Scholar]
  18. Lomb, N. R. 1976, Ap&SS, 39, 447 [Google Scholar]
  19. Lyne, A. G., Manchester, R. N., D’Amico, N., et al. 1990, Nature, 347, 650 [Google Scholar]
  20. Manchester, R. N. 2017, JApA, 38, 42 [NASA ADS] [Google Scholar]
  21. Masci, F. J., Laher, R. R., Rusholme, B., et al. 2019, PASP, 131, 018003 [Google Scholar]
  22. Mata Sánchez, D., Kennedy, M. R., Clark, C. J., et al. 2023, MNRAS, 520, 2217 [CrossRef] [Google Scholar]
  23. Mayer, M. G. F., & Becker, W. 2024, A&A, 684, A208 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  24. Nieder, L., Clark, C. J., Kandel, D., et al. 2020, ApJ, 902, L46 [NASA ADS] [CrossRef] [Google Scholar]
  25. Ray, P. 2024, Public List of LAT-Detected Gamma-Ray Pulsars, https://confluence.slac.stanford.edu/display/GLAMCOG/Public+List+of+LAT-Detected+Gamma-Ray+Pulsars, (accessed June 26, 2024) [Google Scholar]
  26. Roberts, M. S. E. 2013, IAU Symp., 291, 127 [NASA ADS] [Google Scholar]
  27. Romani, R. W., Kandel, D., Filippenko, A. V., Brink, T. G., & Zheng, W. 2022, ApJ, 934, L18 [NASA ADS] [CrossRef] [Google Scholar]
  28. Salvetti, D., Mignani, R. P., De Luca, A., et al. 2017, MNRAS, 470, 466 [NASA ADS] [CrossRef] [Google Scholar]
  29. Scargle, J. D. 1982, ApJ, 263, 835 [Google Scholar]
  30. Strader, J., Swihart, S., Chomiuk, L., et al. 2019, ApJ, 872, 42 [Google Scholar]
  31. Swihart, S. J., Strader, J., Aydi, E., et al. 2021, ApJ, 909, 185 [NASA ADS] [CrossRef] [Google Scholar]
  32. Swihart, S. J., Strader, J., Chomiuk, L., et al. 2022, ApJ, 941, 199 [NASA ADS] [CrossRef] [Google Scholar]
  33. Thongmeearkom, T., Clark, C. J., Breton, R. P., et al. 2024, MNRAS, 530, 4676 [CrossRef] [Google Scholar]
  34. Tonry, J. L., Stubbs, C. W., Lykke, K. R., et al. 2012, ApJ, 750, 99 [Google Scholar]
  35. Turchetta, M., Linares, M., Koljonen, K., & Sen, B. 2023, MNRAS, 525, 2565 [CrossRef] [Google Scholar]
  36. Zharikov, S., Tovmassian, G., Aviles, A., et al. 2013, A&A, 549, A77 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  37. Zharikov, S., Kirichenko, A., Zyuzin, D., Shibanov, Y., & Deneva, J. S. 2019, MNRAS, 489, 5547 [CrossRef] [Google Scholar]
  38. Zyuzin, D. A., Kirichenko, A. Y., Karpova, A. V., et al. 2024, MNRAS, 527, 6712 [Google Scholar]

Appendix A: Periodicity search adding archival data

We performed periodicity search adding available r-band data from the Pan-STARRS DR 2 (6 data points) and ZTF DR 22 (16 data points) catalogues to the OAN-SPM Rc-band data. The Legacy Surveys DR 10 does not have measurements in this band. The archival data are sparse, but they allowed us to cover a time range of about 11 yr. We transformed the archival magnitudes into the Rc-band magnitudes using equations from Tonry et al. (2012). Searching for periodicity using the combined data set resulted in the largest peak in the power spectrum near the same frequency as without the archival data. To obtain the best period value and its uncertainty, we simulated 2000 light curves with magnitudes scattered around the central values of the measured points, assuming a normal distribution with the standard deviation determined by the magnitude uncertainty. Times of measurements were uniformly distributed within the exposure times of individual frames. To each simulated light curve we applied the Lomb-Scargle periodogram method and obtained the period distribution. The mean value and standard deviation of the latter were taken as the best period and its uncertainty, Pph = 2.723884(1) h. This Pph is consistent with that derived in Sec. 3 using only the OAN-SPM data, while its uncertainty is much smaller due to the much longer time interval covered by the archival data. The light curves folded with this period are shown in Fig. A.1. One can see that the archival data are in agreement with the OAN-SPM data although the Pan-STARRS and ZTF surveys were able to detect J1544 only near its maximum brightness phase and with significantly higher magnitude uncertainties.

thumbnail Fig. A.1.

Light curves of the J1544 counterpart candidate folded with a period of 2.723884 h. The data from different instruments and filters are marked by various colours as indicated in the legend (PS ≡ Pan-STARRS).

All Tables

Table 1.

Log of the J1544 observations.

In the text
Table 2.

The light curve fitting results for J1544.

In the text

All Figures

thumbnail Fig. 1.

Individual 1 . ′ $ \overset{\prime }{.} $5 × 1 . ′ $ \overset{\prime }{.} $5 Rc-band images of the J1544 field obtained with the OAN-SPM 2.1-m telescope near the maximum (left) and minimum (right) brightness phases. The optical counterpart candidate is shown with an arrow. The circle with a radius of 3″indicates the 1σ position uncertainty of 4eRASS J154415.4−255531 proposed as the X-ray counterpart (Mayer & Becker 2024).
In the text
thumbnail Fig. 2.

Lomb-Scargle periodogram. The highest peak corresponding to the best period is enlarged in the inset.
In the text
thumbnail Fig. 3.

Light curves of the J1544 counterpart candidate obtained with the OAN-SPM 2.1-m telescope and result of their modelling. Top: Light curves folded with a period of 2.724 h. The best-fitting model is shown with solid lines. Bottom: Fit residuals derived as the difference between the observed (O) and the calculated (C) magnitudes for each data point in terms of the magnitude error σ.
In the text
thumbnail Fig. A.1.

Light curves of the J1544 counterpart candidate folded with a period of 2.723884 h. The data from different instruments and filters are marked by various colours as indicated in the legend (PS ≡ Pan-STARRS).
In the text

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.