The Accreting Black Hole Swift J1753.5-0127 from Radio to Hard X-Ray

Abstract

We report on multi-wavelength measurements of the accreting black hole Swift J1753.5–0127 in the hard state at low luminosity (L ∼ 2.7×10^(36) erg s^(−1) assuming a distance of d = 3 kpc) in 2014 April. The radio emission is optically thick synchrotron, presumably from a compact jet. We take advantage of the low extinction (E(B−V) = 0.45 from earlier work) and model the near-IR to UV emission with a multi-temperature disk model. Assuming a black hole mass of MBH = 5M⊙ and a system inclination of i = 40^◦, the fits imply an inner radius for the disk of R_(in)/R_g > 212 d3 (MBH/5M_⊙)^−1, where R_g is the gravitational radius of the black hole, and d_3 is the distance to the source in units of 3 kpc. The outer radius is R_(out)/R_g = 90,000 d_3 (MBH/5M_⊙)^(−1), which corresponds to 6.6×10^(10) d_3 cm, consistent with the expected size of the disk given previous measurements of the size of the companion’s Roche lobe. The 0.5–240 keV energy spectrum measured by Swift/XRT, Suzaku (XIS, PIN, and GSO), and NuSTAR is relatively well characterized by an absorbed power-law with a photon index of Γ = 1.722±0.003 (90% confidence error), 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 (radio to 240 keV) spectral energy distribution (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 (i.e., the jet) might dominate above 20 keV, which would constrain the break frequency to be between 2.4×10^(10) Hz and 3.6×10^(12) 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-σ level for a disk-blackbody component with a temperature of kTin = 150 +30/−20 eV and an inner radius of 5–14R_g. If this component is real, it might imply the presence of an inner optically thick accretion disk in addition to the strongly truncated (R_(in) > 212R_g) disk. We also perform X-ray timing analysis, and the power spectrum is dominated by a Lorentzian component with νmax = 0.110±0.003Hz and νmax = 0.16±0.04 Hz as measured by XIS and XRT, respectively.