NuSTAR and Chandra observations of new X-ray transients in the central parsec of the Galaxy

Kavli Affiliate: Melania Nynka

| First 5 Authors: Kaya Mori, Charles J. Hailey, Shifra Mandel, Yve E. Schutt, Matteo Bachetti

| Summary:

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 $rsim1$ 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 $8times10^{36}$ and
$3times10^{37}$ erg/s, respectively. No X-ray outbursts have previously been
detected from the two transients and our Chandra ACIS analysis puts an upper
limit of $L_X lesssim 2 times10^{31}$ erg/s 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 $nu sim 10$ 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
($a_{*} gtrsim 0.9$) and large disk density ($n_esim10^{21}$ 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 pc region.

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