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.
Credits:
"Peculiar X-ray transient SRGA J043520.9+552226/AT2019wey discovered with SRG/ART-XC",
I.A. Mereminskiy, A.V. Dodin, A. V., A.A. Lutovinov, A.N. Semena, V.A.
Arefiev, K.E. Atapin, A.A. Belinski, R.A. Burenin, M.V. Burlak, M.V.
Eselevich, A.A. Fedotieva, M.R. Gilfanov, N.P. Ikonnikova, R.A.
Krivonos, I.Yu. Lapshov, A.R. Lyapin, P.S. Medvedev, S.V. Molkov, K.A.
Postnov, M.S. Pshirkov, S.Yu. Sazonov, N.I. Shakura, A.E. Shtykovsky,
R.A. Sunyaev, A.M. Tatarnikov, A.Yu. Tkachenko, S.G. Zheltoukhov,
2021, Submitted to A&A for the Special Issue:
The Early Data Release of eROSITA and Mikhail Pavlinsky ART-XC on the SRG Mission https://arxiv.org/abs/2107.05588
