The X-ray view of black-hole candidate Swift J1842.5-1124 during its 2008 outburst

¹ Department of Physics and Institute of Theoretical Physics, Nanjing Normal University, 210023 Nanjing, PR China
e-mail: wengss@njnu.edu.cn
² Laboratory for Particle Astrophysics, Institute of High Energy Physics, 100049 Beijing, PR China

Received: 6 April 2016
Accepted: 20 June 2016

Abstract

Context. The spectral and temporal evolution during X-ray outbursts give important clues on the accretion process and radiation mechanism in black-hole X-ray binaries (BH XRBs).

Aims. A set of Swift and RXTE observations were executed to monitor the 2008 outburst of the black-hole candidate Swift J1842.5-1124. We investigate these data to explore the accretion physics in BH XRBs.

Methods. We carry out a comprehensive spectral and timing analysis on all the available pointing observations, including fitting both X-ray spectra and power density spectra, measuring the optical and near-ultraviolet flux density. We also search for correlations among the spectral and timing parameters.

Results. The observed properties of Swift J1842.5-1124 are similar to other BH XRBs in many respects, for example the hardness-intensity diagram and hardness-rms diagram. The type-C quasi-periodic oscillations (QPOs) were observed as the source started to transit from the low-hard state to the high-soft state. The frequency of QPOs correlate with intensity and the hard component index, and anti-correlate with the hardness and the total fractional rms. These relations are consistent with the Lense-Thirring precession model. The estimated U-band flux changed with the X-ray flux, while the flux density at the V-band remained 0.26 mJy. These results imply that the X-ray reprocessing or the tail of thermal emission from the outer disk contributes a significant fraction of the U-band radiation; alternatively, the companion star or the jet dominates the flux at longer wavelengths.