Recently, a Chinese-foreign cooperative team led by the Yunnan Observatories (Master student YANG Pengcheng, Prof. ZHANG Guobao and Prof. WANG Jiancheng from Yunnan Observatories, Prof. Russell and Gelfand from New York University, Prof. Mendez from University of Groningen, Prof. LYU Ming from Xiangtan University) investigated the mechanism of ultraviolet/optical (UV/OPT) emission in black hole low-mass X-ray binary Swift J1753.5–0127 by analyzing the multi-wavelength data during the 12-yr long-term outburst. They found that the UV/OPT emission is dominated by the viscously heated accretion disk during the outburst. This work was published in Monthly Notices of the Royal Astronomical Society.

Black hole X-ray binaries (BHXBs) are binary systems consisting of a black hole and a companion star, in which black hole accretes matters from a companion star, forming radiation structures and producing broadband emission from radio and X-rays. 

Generally, the radio emission from BHXBs is believed to be produced from jets, and the X-ray emission comes from the corona. The origin of the emission in the UV/OPT band is not clearly known yet. The phenomenon of the power-law correlation between UV/OPT and X-ray emissions generally appears in the hard state in BHXBs. 

The values of the power-law index (β) indicate a different kind of emission origins. If the UV/OPT emission is due to the reprocessing of X-rays in the outer accretion disc, the β varies from ~0.3 to ~0.9; if the UV/OPT emission comes from synchrotron emission in jet, the β should be around ~0.7 over the entire UV/optical bands; if the UV/OPT emission is due to the viscously heated outer accretion disc, the expected β varies from ~0.15 to ~0.3 from optical to UV bands, and as the wavelength of the UV/OPT band decreases, the value of β increases. 

The BHXB Swift J1753.5-0127 is a typical hard state source with a 12-yr long-term outburst. The researchers studied, for the first time, the UV/OPT and X-ray flux correlations during the whole 12-yr long-term outburst with Swift data. They found that the UV/optical and X-ray emissions were strongly correlated and the power-law index increased from ~0.24 to ~0.33 as the UV/optical wavelength decreased from ~5402 ？ (V) to ~2030 ？ (UVW2), which is consistent with the theoretical expectation of the viscously heated disc model. This indicates that the UV/OPT emission is dominated by the viscously heated accretion disc. 

The researchers also found that the data deviated from the correlation were the low-intensity peaks that appeared in the X-ray band during the outburst. These low-intensity peaks were dominated by the soft X-ray component, which suggested that the deviations were driven by the increased soft X-ray emission from the inner part of the accretion disc.

This study helps us better understand the origin of UV/OPT emission in the black hole low-mass X-ray binary.

Contact:
YANG Pengcheng
Yunnan Observatories, CAS
Email: yangpengcheng@ynao.ac.cn