Abstract
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.
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.
| Original language | English |
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| Publication date | 9 Oct 2024 |
| Publication status | Published - 9 Oct 2024 |
| Series | The Astronomer's telegram |
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| Number | ATel #16859 |