Probing the cyclotron resonant scattering feature in the X-ray pulsar 2S 1553-542 with NuSTAR during the 2024 outburst

ATel #16859; Manoj Mandal (MCC), Gaurava K. Jaisawal (DTU Space), Sabyasachi Pal (MCC), Rahul Sharma (RRI)
on 9 Oct 2024; 17:27 UT
Credential Certification: Manoj Mandal (manojmandal@mcconline.org.in)

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The MAXI/GSC nova-alert system detected an onset of X-ray outburst from the Be/X-ray binary pulsar 2S 1553-542 on 2024-09-06 (MJD 60559) (ATel #16835). Since then, the source has been monitored by the Fermi/GBM accreting pulsar program (ATel #16839) and NICER (ATel #16846).

We report the results from the preliminary timing and spectral analysis of the NuSTAR data 91001343002V7 of 2S 1553-542. NuSTAR observed the source on 2024-10-07 starting from 12:06:09 UT (MJD 60590.5) with a total exposure of ~33 ks, recording a net count rate of about 22 c/s in the 3−79 keV energy band. The barycentric-corrected light curve shows X-ray pulsations at a period of ~9.2850 s. The pulse profile is nearly sinusoidal and dominated by a single-peak like structure. The pulsed fraction from the profile can be estimated to be nearly 68% in the 3−79 keV energy band.

The preliminary spectral analysis indicates that the continuum spectrum can be fitted by the absorbed cutoff power law (cutoffpl) and a blackbody component. Independent of the choice of the continuum (cutoffpl or Comptt) model, the spectra show the presence of an iron line around 6.4 keV and a cyclotron absorption line feature around 27 keV. The absorption feature is modelled using a Gaussian absorption model GABS. The best-fit results provide a photon index of −0.8 ± 0.1, the blackbody temperature = 1.04 ± 0.05 keV, cutoff energy = 4.9 ± 0.2 keV, iron line energy = 6.35 ± 0.07 keV and a line width 0.34±0.06 keV, at a fixed hydrogen column density of 2.3 × 10²² cm^-2 (following Tsygankov et al. 2016, MNRAS, 457, 258). The cyclotron line energy is found to be 27.6 ± 0.6 keV of width 6.5 ± 0.7 keV. The magnetic field corresponding to the cyclotron line is estimated to be 3.1 × 10¹² G. We estimated the unabsorbed source flux to be 1 × 10^-9 ergs/cm²/s in the 3−79 keV energy band. The corresponding luminosity can be calculated to be ~4.8 × 10³⁷ erg/s assuming a source distance of 20 kpc. The observed flux is more than 1.6 times higher than the flux measured by NICER on 2024-10-03 (ATel #16846). This suggests that the pulsar is accreting around its critical luminosity level. The best-fit result provides the reduced chi-square (dof) of 1.04 (1255). All reported uncertainties have 90% confidence limits.

We thank the NuSTAR SOC team for making the ToO observation possible.