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Constraining Spin and Inclination Angle of XTE J2012+381 using AstroSat and NICER

Vaibhav Sharma, Ranjeev Misra, Shivani Chaudhary, J S Yadav, Pankaj Jain

Abstract

We present a spectral analysis of a black hole X-ray binary XTE J2012+381 during its 2022 outburst, using data from NICER and AstroSat. Combining data from NICER, LAXPC20, and SXT, we extract energy spectra covering the 0.7-10.0 keV range. We model the energy spectra using a series of physical models and find that a reflection-Comptonization model provides the best fit. Given the uncertainties in the black hole mass and source distance, we investigate the stability of the inferred spectral parameters by systematically varying the black hole mass (7.26, 11, and 16.5 M$_\odot$), source distance (3.3, 5.4, and 7.5 kpc), and spectral hardening factor (1.5, 1.7, and 1.9). We find that, across most combinations of these parameters, the spin solutions consistently lie in the high-spin regime, spanning values between $\sim$0.67 and $\sim$0.998, with only a limited subset of configurations favoring lower spins. In contrast, the disk inclination angle remains well constrained over the majority of the explored parameter space, typically ranging between $\sim$50° and $\sim$65°. Only a few parameter combinations yield higher inclination values.

Constraining Spin and Inclination Angle of XTE J2012+381 using AstroSat and NICER

Abstract

We present a spectral analysis of a black hole X-ray binary XTE J2012+381 during its 2022 outburst, using data from NICER and AstroSat. Combining data from NICER, LAXPC20, and SXT, we extract energy spectra covering the 0.7-10.0 keV range. We model the energy spectra using a series of physical models and find that a reflection-Comptonization model provides the best fit. Given the uncertainties in the black hole mass and source distance, we investigate the stability of the inferred spectral parameters by systematically varying the black hole mass (7.26, 11, and 16.5 M), source distance (3.3, 5.4, and 7.5 kpc), and spectral hardening factor (1.5, 1.7, and 1.9). We find that, across most combinations of these parameters, the spin solutions consistently lie in the high-spin regime, spanning values between 0.67 and 0.998, with only a limited subset of configurations favoring lower spins. In contrast, the disk inclination angle remains well constrained over the majority of the explored parameter space, typically ranging between 50° and 65°. Only a few parameter combinations yield higher inclination values.
Paper Structure (4 sections, 2 figures, 2 tables)

This paper contains 4 sections, 2 figures, 2 tables.

Table of Contents

  1. INTRODUCTION
  2. Observations and Data reduction
  3. Spectral Analysis and Results
  4. Discussion and Conclusion

Figures (2)

  • Figure 1: Joint spectral fitting of the energy spectra from AstroSat/SXT (black), AstroSat/LAXPC (red), and NICER (green) using the final model, constant$\times$tbabs$\times$ (relconv$\times$xilconv$\times$thcomp$\times$kerrbb + thcomp$\times$kerrbb). The upper panel shows the energy spectra along with the best-fit model, while the lower panel displays the $\chi$.
  • Figure 2: Fit statistic ($\chi^2$) as a function of black hole mass, obtained using the steppar command in xspec, at three assumed source distances: 3.3 kpc, 5.4 kpc, and 7.5 kpc. In each case, the black hole mass was varied between 5 and 20 M$_\odot$. The horizontal dashed line represents the 90$\%$ confidence level.