7 research outputs found
Hairy black holes in cubic quasi-topological gravity
We construct a class of five dimensional black hole solutions to cubic
quasi-topological gravity with conformal scalar hair and study their
thermodynamics. We find these black holes provide the second example of black
hole -lines: a line of second order (continuous) phase transitions,
akin to the fluid/superfluid transition of He. Examples of isolated
critical points are found for spherical black holes, marking the first in the
literature to date. We also find various novel and interesting phase
structures, including an isolated critical point occurring in conjunction with
a double reentrant phase transition. The AdS vacua of the theory are studied,
finding ghost-free configurations where the scalar field takes on a non-zero
constant value, in notable contrast to the five dimensional Lovelock case.Comment: 29 pages, 12 figure
NuSTAR and Chandra observations of new X-ray transients in the central parsec of the Galaxy
We report NuSTAR and Chandra observations of two X-ray transients, SWIFT
J174540.7290015 (T15) and SWIFT J174540.2290037 (T37), which were
discovered by the Neil Gehrels Swift Observatory in 2016 within 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 and
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 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 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
() and large disk density ( cm).
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.Comment: 15 pages, 7 figures, accepted for publication in Ap