The Accreting Black Hole Swift J1753.5-0127 from Radio to Hard X-Ray
(Jun 22, 2015)Abstract
(abridged) We report on multi-wavelength measurements of Swift J1753.5-0127
in the hard state at L=2.7e36 erg/s (assuming d=3 kpc) in 2014. The radio
emission is optically thick synchrotron, presumably from a compact jet. We take
advantage of the low extinction and model the near-IR to UV emission with a
multi-temperature disk model. Assuming a BH mass of M\_BH=5 Msun and a system
inclination of 40 deg, the fits imply an inner radius for the disk of
Rin/Rg>212 d\_3 (5Msun/M\_BH). The outer radius is R\_out/R\_g=90,000 d\_3
(5Msun/M\_BH), which corresponds to 6.6e10 d\_3 cm, consistent with the expected
size of the disk. The 0.5-240 keV spectrum measured by Swift/XRT, Suzaku, and
NuSTAR is relatively well characterized by a power-law with a photon index of
Gamma=1.722+/-0.003, but a significant improvement is seen when a second
continuum component is added. Reflection is a possibility, but no iron line is
detected, implying a low iron abundance. We are able to fit the entire SED with
a multi-temperature disk component, a Comptonization component, and a broken
power-law, representing the emission from the compact jet. The broken power-law
cannot significantly contribute to the soft X-ray emission, and this may be
related to why Swift J1753.5-0127 is an outlier in the radio/X-ray correlation.
The broken power-law might dominate above 20 keV, which would constrain the
break frequency to be between 2.4e10 Hz and 3.6e12 Hz. Although the fits to the
full SED do not include significant thermal emission in the X-ray band,
previous observations have consistently seen such a component, and we find that
there is evidence at the 3.1-sigma level for a disk-blackbody component with a
temperature of 150(+30)(-20) eV and an inner radius of 5-14 R\_g. If this
component is real, it might imply the presence of an inner accretion disk in
addition to the strongly truncated (R\_in>212 R\_g) disk.