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

Abstract

We report on multiwavelength measurements of the accreting black hole Swift J1753.5-0127 in the hard state at low luminosity (L ∼ 2.7 × 10³⁶ 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 multitemperature disk model. Assuming a black hole mass of M_BH = 5 M_⊙ and a system inclination of i = 40°, the fits imply an inner radius for the disk of R_in/R_g > 212d₃(M_BH/5 M_⊙)^-1, where R_g is the gravitational radius of the black hole and d₃ is the distance to the source in units of 3 kpc. The outer radius is R_out/R_g=90,000 d₃(M_BH/5 M_⊙)^-1, which corresponds to 6.6 × 10¹⁰ d₃ 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/X-ray Telescope (XRT), Suzaku (XIS, PIN, and GSO), and Nuclear Spectroscopic Telescope Array 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 multitemperature 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 × 10¹⁰ and 3.6 × 10¹² 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 3.1σ level for a disk-blackbody component with a temperature of {{kT}}_{in}={150}_-20⁺³⁰ eV and an inner radius of 5R_g-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 ν_max = 0.110 ± 0.003 Hz and ν_max = 0.16 ± 0.04 Hz as measured by XIS and XRT, respectively.