INTEGRAL Picture Of the Month
September 2021

(Click to download full resolution)

INTEGRAL observes a new Galactic microquasar as it wiggles its tail

In the course of the ongoing all-sky survey, the Spektr-RG (SRG) observatory keeps discovering new X-ray sources - from weak ones, that were not known simply because of their dimness, to relatively bright, that just have started shining. But, as it usually is with X-ray sources, the discovery is only the start of a long and sophisticated process of understanding what is happening in a particular source. What makes the situation much simpler is the availability of the optical counterpart of a newly discovered X-ray source.

In December 2019 the Asteroid Terrestrial-impact Last Alert System, ATLAS, survey discovered a weak optical transient, AT2019wey, near the Galactic plane. The transient did not show any interesting variability and was overlooked by the astronomical community. Four months later, the Mikhail Pavlinsky ART-XC telescope onboard SRG found a new X-ray source suspiciously close to the AT2019wey. Follow-up spectroscopic observations with several ground-based optical telescopes showed that the source is another Galactic X-ray binary system. Later on it was detected in the radio band. Eventually, sensitive interferometric observations with VLBA allowed to resolve the source as an extended one, showing that AT2019wey must be a microquasar - a special kind of X-ray binary in which part of matter from accretion disk is being ejected in two narrow opposing jets that could be seen in radio images. Extensive monitoring campaign also demonstrated another interesting trait of the AT2019wey - a weak correlation between its optical and X-ray emission. While its X-ray luminosity changed by a factor of ~20, in the course of a few months its optical brightness remained nearly the same. This is illustrated on the left part of the image above - the upper panel shows the evolution of AT2019wey's optical brightness in magnitudes, while the bottom panel shows countrates observed by different X-ray telescopes.

In the late summer of 2020 NuSTAR performed two consecutive observations of AT2019wey in ten days and found the source to be in hard-intermediate state, with both thermal and power-law components present in its energy spectrum. Interestingly, spectra were very hard, and showed some evolution, even on such short timescales. Since at this time SRG was taking a short break from an all-sky survey, we initiated a joint ART-XC/INTEGRAL/Swift observation of the AT2019wey, in order to constrain the parameters of its hard X-ray emission and trace its evolution.

Another, equally important task of the coordinated observation was to perform a cross-calibration of ART-XC against the INTEGRAL and Swift instruments which are well-established. Both of these tasks were successfully reached. It turned out that the power-law component became even softer, with even more curvature caused by reflection of energetic photons by the accretion disk itself. The observed spectra are shown in right part of the figure, with NuSTAR observations shown in gray and Swift/XRT, ART-XC and IBIS/ISGRI data plotted in blue, green and red, respectively. The overall shapes and normalizations of the spectra observed by the different telescopes also matched well, indicating that there are no major issues with the ART-XC spectral calibration.

back to the POM archive