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NuSTAR and Chandra Observations of New X-Ray Transients in the Central Parsec of the Galaxy

Mori, Kaya and Hailey, Charles J. and Mandel, Shifra and Schutt, Yve E. and Bachetti, Matteo and Coerver, Anna and Baganoff, Frederick K. and Dykaar, Hannah and Grindlay, Jonathan E. and Haggard, Daryl and Heuer, Keri and Hong, Jaesub and Hord, Benjamin J. and Jin, Chichuan and Nynka, Melania and Ponti, Gabriele and Tomsick, John A. (2019) NuSTAR and Chandra Observations of New X-Ray Transients in the Central Parsec of the Galaxy. Astrophysical Journal, 885 (2). Art. No. 142. ISSN 1538-4357. doi:10.3847/1538-4357/ab4b47.

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We report NuSTAR and Chandra observations of two X-ray transients, SWIFT J174540.7−290015 (T15) and SWIFT J174540.2−290037 (T37), which were discovered by the Neil Gehrels Swift Observatory in 2016 within r ~ 1 pc of Sgr A*. NuSTAR detected bright X-ray outbursts from T15 and T37, likely in the soft and hard states, with 3–79 keV luminosities of 8 × 10³⁶ and 3 × 10³⁷ erg s⁻¹, respectively. No X-ray outbursts have previously been detected from the two transients and our Chandra ACIS analysis puts an upper limit of L_X ≾ 2 × 10³¹ erg s⁻¹ on their quiescent 2–8 keV luminosities. No pulsations, significant quasi-periodic oscillations, or type I X-ray bursts were detected in the NuSTAR data. While T15 exhibited no significant red noise, the T37 power density spectra are well characterized by three Lorentzian components. The declining variability of T37 above ν ~ 10 Hz is typical of black hole (BH) transients in the hard state. NuSTAR spectra of both transients exhibit a thermal disk blackbody, X-ray reflection with broadened Fe atomic features, and a continuum component well described by Comptonization models. Their X-ray reflection spectra are most consistent with high BH spin (a* ≳ 0.9) and large disk density (n_e ~ 10²¹ cm⁻³). Based on the best-fit ionization parameters and disk densities, we found that X-ray reflection occurred near the inner-disk radius, which was derived from the relativistic broadening and thermal disk component. These X-ray characteristics suggest the outbursting BH-low-mass X-ray binary scenario for both transients and yield the first BH spin measurements from X-ray transients in the central 100 pc region.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Mori, Kaya0000-0002-9709-5389
Bachetti, Matteo0000-0002-4576-9337
Grindlay, Jonathan E.0000-0002-1323-5314
Haggard, Daryl0000-0001-6803-2138
Jin, Chichuan0000-0002-2006-1615
Nynka, Melania0000-0002-3310-1946
Ponti, Gabriele0000-0003-0293-3608
Tomsick, John A.0000-0001-5506-9855
Additional Information:© 2019 The American Astronomical Society. Received 2019 August 14; revised 2019 October 3; accepted 2019 October 3; published 2019 November 11. This work used data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by NASA. We thank the NuSTAR Operations, Software and Calibration teams for support with the execution and analysis of these observations. We made use of the NuSTAR Data Analysis Software (NuSTAR-DAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). We acknowledge the scheduling, data processing, and archive teams from the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration (NASA) under contract NAS8-03060. G.P. acknowledges financial contribution from the agreement ASI-INAF n.2017-14-H.0. C.J. acknowledges the National Natural Science Foundation of China through grant 11873054. D.H. acknowledges support from the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant and the Canadian Institute for Advanced Research (CIFAR) Azrieli Global Scholars program. We would like to thank Daniela Huppenkothen for insightful discussions on NuSTAR timing analysis and Stingray software. We acknowledge help from Ninad Nirgudkar and Gabriel Lewis Bridges for generating the Swift/XRT light curve plots. Software: NuSTARDAS (v.1.7.1), HEAsoft (v6.21; HEASARC 2014), Stingray (Huppenkothen et al. 2019), XSPEC (v12.10.1; Arnaud 1996), SAOImage DS9 (Joye & Mandel 2003).
Group:NuSTAR, Space Radiation Laboratory
Funding AgencyGrant Number
Agenzia Spaziale Italiana (ASI)2017-14-H.0
National Natural Science Foundation of China11873054
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Canadian Institute for Advanced Research (CIFAR)UNSPECIFIED
Subject Keywords:Galactic center; X-ray transient sources; low-mass X-ray binary stars; X-ray telescopes
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Galactic center (565); X-ray transient sources (1852); low-mass X-ray binary stars (939); X-ray telescopes (1825)
Record Number:CaltechAUTHORS:20191111-074450523
Persistent URL:
Official Citation:Kaya Mori et al 2019 ApJ 885 142
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:99763
Deposited By: Tony Diaz
Deposited On:12 Nov 2019 21:51
Last Modified:16 Nov 2021 17:49

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