An X-ray binary consists of a dense compact object that strips material off its stellar companion, producing X-rays in the process. These binaries are surrounded by radiating accretion disks of infalling material, but they also sometimes fling matter out in powerful relativistic jets. What can their infrared emission tell us about the speed of these jets?

Looking for Lorentz Factors

Black hole X-ray binaries (BHXBs) are X-ray binaries where the accreting compact object is a black hole. The jets in BHXBs can approach the speed of light, and they can even give the false appearance of moving faster than light. This relativistic illusion is characterized by something called a Lorentz factor, Γ, which quantifies the distortions that come from moving at near light-speed.

Luckily we may have a new way of measuring these Lorentz factors: by looking at the BHXBs in infrared (IR) light. In a recent study, a group of scientists led by Payaswini Saikia (New York University Abu Dhabi, UAE) explored what a BHXB’s IR emission says about its structure and its jet’s Lorentz factor.

Eyes on IR Emissions

Unfortunately, the Lorentz factors of BHXB jets haven’t yet been well measured — which limits our understanding of how these speedy outflows may be launched, accelerated, and collimated as they are flung from the black holes.

The IR emission from a BHXB can be largely attributed to two things: the accretion disk and synchrotron emission from the jets. Saikia and collaborators explored the infrared emission from 14 BHXBs, gauging how it changed when the BHXB jets turned “on”, emitting highly energetic X-rays, and “off”, emitting less energetic X-rays. Saika and collaborators argue that when the jets were “off”, any observed IR emission could be attributed to the disk; when they were “on”, the excess IR emission was due to the jets. This framework for looking at the BHXBs allowed the authors to isolate the jet emission and characterize the Lorentz factors for some of these outflows.

Inclined to Model

To determine the Lorentz factors, Saikia and collaborators used the IR flux ratio between the states when the jets were “on” and “off”. Here disk inclination comes into play: due to a combination of disk geometry and relativistic beaming of the jet, at high and low disk inclinations, the ratio between the “on” and “off” states ought to be high. For intermediate inclinations, the ratio should be low.

Using observed ratios and disk inclinations, Saikia and collaborators were able to model and constrain the Lorentz factors for nine BHXBs for the first time, finding a range

In addition, the authors put limits on the underlying distribution of BHXB Lorentz factors and could confidently attribute the variations in excess IR emission to disk inclination and jet direction.

of Γ = 1.3–3.5 — which means the jet bulk flows are moving at 64–96% of the speed of light.

With more observations of BHXBs across the spectrum, the techniques in this work should be more widely applicable and could help us better understand these highly energetic objects.

Citation

“Lorentz Factors of Compact Jets in Black Hole X-Ray Binaries,” Payaswini Saikia et al 2019 ApJ 887 21. https://doi.org/10.3847/1538-4357/ab4a09