NICER observation of Be/X-ray binary pulsar 2S 1553-542 during its 2024 outburst

The Be/X-ray binary pulsar 2S 1553-542 has undergone several outbursts, recorded in 2008 (ATel #1373), 2015 (ATel #7018), and 2021 (ATel #14301, #14308, #14335, #14348). These observations revealed a spin period of approximately 9.28 s, along with a single peak pulse profile of a nearly sinusoidal nature (ATel #1373; Pahari & Pal 2012, MNRAS, 423, 3352). During the 2021 outburst, the pulsar was found to be accreting close to its critical luminosity level (Malacaria et al., 2022, ApJ, 927, 194). Recently, an X-ray outburst was detected from the system on 2024-09-06 (MJD 60559) by the MAXI/GSC nova-alert system (ATel #16835). The Fermi/GBM accreting pulsar program detected pulsation at 9.282 s, confirming the onset of a new outburst from 2S 1553-542 (ATel #16839).

We report the results from the preliminary timing and spectral analysis of the NICER data of the pulsar. NICER observed the source on 2024-10-03 starting from 01:30:29 UT (MJD 60586.25) with a total exposure of ~0.8 ks, recording a net count rate of about 27 c/s in the 0.5−10 keV energy band. The barycentric-corrected light curve shows X-ray pulsations at a period of 9.2823(1) s. The pulse profile consists of a single-peak feature along with an additional secondary hump. The pulsed fraction from the profile can be estimated to be nearly 60% in the 0.5−10 keV energy band.

The NICER spectrum in the 0.5−10 keV range can be modelled with an absorbed power-law model. An addition of a partial covering component further improves the fit. The best-fit results provide a photon index of 1.0 ± 0.1, a hydrogen column density of (1.2 ± 1.1) × 1022 cm-2, partial covering absorption column density of (5.5 ± 0.5) × 1022 cm-2 and its corresponding covering fraction of 0.95 ± 0.1. The NICER spectra did not show any presence of an iron line. The unabsorbed source flux is (6.0 ± 0.4) × 10-10 erg/cm2/s in the 0.5−10 keV energy band. The corresponding luminosity can be estimated to be ~2.9 × 1037 erg/s assuming a source distance of 20 kpc. This suggests that the pulsar is accreting close to its critical luminosity level. The best-fit result provides the reduced chi-square (dof) of 0.94 (400). All reported uncertainties have 90% confidence limits.

We thank the NICER team for making the ToO observation possible. NICER will keep monitoring the source in the following days.

NICER is a 0.2−12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.