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

Abstract

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 × 10 36 and 3 × 10 37 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 × 10 31 erg s −1 on their quiescent 2-8 keV luminosities. No pulsations, significant QPOs, 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 ∼ 10 21 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-LMXB scenario for both transients and yield the first BH spin measurements from X-ray transients in the central 100 parsec region.

The Hills Mechanism and the Galactic Center S-stars

“…The present epoch is assumed to be typical, so ∼300 massive stars form every few million years, and become black holes and neutron stars. This model approximately reproduces the observed stellar density profile in the Galactic Center, as well as the recently discovered population of black hole X-ray binaries in the central parsec (Hailey et al 2018;Mori et al 2019) via tidal capture of main-sequence stars. This model implicitly assumes the IMF in the disk is truncated near 10M , which is problematic as the hypervelocity star observed by Koposov et al (2020) is an A-type star with a few solar masses.…”

NuSTAR monitoring of MAXI J1348-630: evidence of high density disc reflection

“…However, the reflionx hd model assumes an optically thick, slab-like atmosphere of a constant density (Ross & Fabian 2005). Thus, in our implementation, the X-rays are assumed to be reflected from a single layer at a single photoionization radius (R ion , Mori et al (2019)). Therefore, following Mori et al (2019), we perform a sanity check for the self-consistency of best-fit M4 model parameters.…”

“…Thus, in our implementation, the X-rays are assumed to be reflected from a single layer at a single photoionization radius (R ion , Mori et al (2019)). Therefore, following Mori et al (2019), we perform a sanity check for the self-consistency of best-fit M4 model parameters. Assuming an isotropically emitting illuminating source (the corona), the ionisation parameter in reflionx hd model can be redefined as ξ = L nR 2 ion (where L is the illuminating luminosity, n is the hydrogen number density).…”

The high energy X-ray probe (HEX-P): Resolving the nature of Sgr A* flares, compact object binaries and diffuse X-ray emission in the Galactic center and beyond

“…3) X-ray transients: Daily Swift/XRT monitoring since 2006, combined with other frequent X-ray observations of the GC, resulted in the detection of 20 X-ray transients in the central ∼50 pc (Muno et al, 2005;Degenaar et al, 2015). Six of the GC transients were identified as NS-LMXBs through the detection of type I X-ray bursts or pulsations (Degenaar et al, 2012), while NuSTAR observations identified two recent Swift transients as outbursting BH-LMXBs (Mori et al, 2019). Otherwise, ∼10 VFXTs have been detected with peak luminosities of L X ∼ 10 34 -10 36 erg s −1 (Degenaar et al, 2015) but their identity remains elusive despite extensive investigations by Swift-XRT (Degenaar et al, 2015).…”

“…Frequent X-ray monitoring of the GC with Swift-XRT resulted in the detection of ∼20 Xray transients in the GC (Degenaar et al, 2015). Follow-up X-ray observations with Chandra, XMM-Newton and NuSTAR enabled the identification of some of those transients (Mori et al, 2013;Mori et al, 2019).…”