Characterizing the Broadband Reflection Spectrum of MAXI J1803-298 during Its 2021 Outburst with NuSTAR and NICER

Creators: Adegoke, Oluwashina K.¹; García, Javier A.^{2, 1}; Connors, Riley M. T.³; Ding, Yuanze¹; Mastroserio, Guglielmo⁴; Steiner, James F.⁵; Ingram, Adam⁶; Harrison, Fiona A.¹; Tomsick, John A.⁷; Kara, Erin⁸; Mehdipour, Missagh⁹; Fukumura, Keigo¹⁰; Stern, Daniel¹¹; Ubach, Santiago⁵; Lucchini, Matteo¹²

1. California Institute of Technology
2. Goddard Space Flight Center
3. Villanova University
4. University of Milan
5. Harvard-Smithsonian Center for Astrophysics
6. Newcastle University
7. University of California, Berkeley
8. Massachusetts Institute of Technology
9. Space Telescope Science Institute
10. James Madison University
11. Jet Propulsion Lab
12. University of Amsterdam

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Abstract

MAXI J1803-298 is a transient black hole candidate discovered in 2021 May during an outburst that lasted several months. Multiple X-ray observations reveal recurring "dipping" intervals in several of its light curves, particularly during the hard/intermediate states, with a typical recurrence period of ∼7 hr. We report analysis of four NuSTAR observations of the source, supplemented with NICER data where available, over the duration of the outburst evolution covering the hard, intermediate, and the soft states. Reflection spectroscopy reveals the black hole to be rapidly spinning (a * = 0.990 ± 0.001) with a near edge-on viewing angle (i = 70° ± 1°). Additionally, we show that the light-curve dips are caused by photoelectric absorption from a moderately ionized absorber whose origin is not fully understood, although it is likely linked to material from the companion star impacting the outer edges of the accretion disk. We further detect absorption lines in some of the spectra, potentially associated with Fe xxv and Fe xxvi, indicative of disk winds with moderate to extreme velocities. During the intermediate state and just before transitioning into the soft state, the source showed a sudden flux increase, which we found to be dominated by soft disk photons and consistent with the filling of the inner accretion disk, at the onset of state transition. In the soft state, we show that models of disk self-irradiation provide a better fit and a preferred explanation to the broadband reflection spectrum, consistent with previous studies of other accreting sources.

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Acknowledgement

The authors would like to thank the anonymous referee as well as Poshak Gandhi and Brian Grefenstette for comments that improved the clarity of the manuscript. This work was supported under NASA contract No. NNG08FD60C, and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. A.I. acknowledges support from the Royal Society. J.A.T. acknowledges partial support from the NuSTAR Guest Observer program under NASA grant 80NSSC22K0059. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Space Science Data Center (SSDC, Italy) and the California Institute of Technology (Caltech, USA).

Funding

This work was supported under NASA contract No. NNG08FD60C, and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. A.I. acknowledges support from the Royal Society. J.A.T. acknowledges partial support from the NuSTAR Guest Observer program under NASA grant 80NSSC22K0059.

Facilities

NuSTAR - The NuSTAR (Nuclear Spectroscopic Telescope Array) mission, NICER - , MAXI - JAXA Monitor of All-sky X-ray Image experiment.

Software References

XSPEC (K. A. Arnaud 1996), XSTAR (T. Kallman & M. Bautista 2001), relxill (J. García et al. 2014; T. Dauser et al. 2014).

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