Payaswini Saikia
Compact, continuously launched jets in black hole X-ray binaries (BHXBs) produce radio to optical–infrared (OIR) synchrotron emission. These jets are launched in the hard X-ray state and are quenched in the soft state. The compact jets are spatially resolved in a few cases using VLBI radio observations. One of the basic properties of these jets is the bulk Lorentz factor, which is notoriously difficult to measure, with to date only weak constraints for a few BHXBs.
In this poster, we present simple models to constrain the Lorentz factor of the compact jets in several BHXBs using the amplitude of the jet fade and recovery at OIR wavelengths over state transitions. The accretion disc tends to dominate the OIR emission throughout BHXB outbursts, but in some sources such as GX 339–4, there is an infrared (IR) excess above the disk component due to synchrotron emission. This IR excess is observed in many BHXBs when the jet is present in the hard spectral state. Here, we use simple models to explain why some BHXBs have prominent IR excesses and some do not. We show that the amplitude of the IR excess (quantified by the amplitude of the IR quenching or recovery over the transition from/to the hard state) can be explained by the inclination-dependent beaming of the jet synchrotron emission and the projected area of the accretion disk. Furthermore, using the amplitude of the jet fade and recovery over state transitions and the known orbital parameters, we constrain for the first time the bulk Lorentz factor range of compact jets in several BHXBs, and show that all the well-constrained Lorentz factors lie in the range of 1.3–3.5.
Motivation : Compact, continuously launched jets in stellar-mass black holes or black hole X-ray binaries (BHXBs) produce radio to optical--infrared (OIR) synchrotron emission. They are not spatially resolved except in a few cases using Very Large Baseline interferometry radio observations. One of the basic properties of these jets is the bulk Lorentz factor, which is notoriously difficult to measure, with to date only weak constraints for a few BHXBs. In this project, we adopt simple models to constrain the Lorentz factor of the compact jets in several BHXBs using infrared observations.
Method : The black hole accretion disc tends to dominate the OIR emission throughout the life-cycle of a BHXB. But in some sources such as GX 339-4, there is an infrared excess due to synchrotron emission produced in the jets. In other BHXBs this IR excess is very faint or absent. Here, we investigate why some BHXBs have prominent IR excesses and some do not.
Model and data : Theoretically, the jet luminosity should correlate with the inclination angle if the emission is subjected to relativistic beaming. Indeed, we find that some of the sources with the brightest IR excesses compared to their discs are systems with a low inclination angle (e.g. 4U 1543-47 and MAXI J1836-194). We find that the amplitude of the IR excess is expected to be the highest for very low, or very high, inclination BHXBs. For high inclination systems, the disc is almost edge-on, which reduces the disc emission but not the jet emission if the jet is not highly beamed, which can lead to relatively bright IR excesses. For intermediate inclination angles (~30 - ~60 deg), no bright IR excess is expected for any Lorentz factor, and indeed all of the BHXBs with inclination angles within this range do not have prominent IR excesses.
Results : Using the amplitude of the IR excess and the known orbital parameters, we constrain for the first time the Lorentz factor of several BHXBs, and show that all the well-constrained Lorentz factors lie in the range of 1.3–3.5. These results are strongly supportive of the IR excess being produced by synchrotron emission in a relativistic outflow and demonstrate how useful OIR monitoring of BHXB is for studying jet properties.