INTEGRAL/SPI observations of its 2nd brightest X-ray transient: MAXI J1820+070
Some X-ray binaries (XRBs) show transient episodes of very bright
luminosity in hard X-rays, corresponding to accretion events in the flow
surrounding the central object. These rare outburst events are the best
opportunities to understand how high-energy photons are produced in such
transient sources.
On 11 March 2018, the MAXI/GSC nova alert system detected a new X-ray
transient: MAXI J1820+070 (ATel #11399).
INTEGRAL participated to the subsequent large follow-up program and
pointed to the source from 16 to 27 March.
Further dedicated INTEGRAL observations were performed up to 10 May. The
total INTEGRAL exposure amounts to 1.4 Msec (about 16 days of exposure).
In the SPI low-energy hard X-ray band (20-50 keV), MAXI J1820+070 became
the second brightest transient XRB it had seen, with a maximum flux
level of about 4 times the Crab. The higher-energy hard X-ray (100-300
keV) flux, however, does not follow the low-energy evolution (image, top
left). As a result, the so-called hardness of the X-ray spectral
emission (i.e., the ratio of high-energy flux over low-energy flux, see
image, top right) varies dramatically during the first part of the
outburst and then slightly increases. This suggests that the hard X-ray
emission is being sustained by two separate components. This is also
illustrated in the evolution of the spectral shape (image, bottom
panel). During the rising phase of the outburst (image, bottom left
panel), the emission below 100 keV increases more rapidly than at higher
energies. In contrast, the spectral shape of this low-energy component
does not change when the luminosity subsequently decays (image, bottom
right panel). On the other hand, the higher-energy emission remains
almost unchanged, similar to what is observed in the radio (see ATel #11539).
The behaviour observed during the outburst of MAXI J1820+070 is
particularly interesting when compared to the 2015 outburst of another,
bright, transient XRB, GS2023+338 (=V404 Cygni). V404 Cygni showed huge
variability on all timescales (see, e.g., INTEGRAL
POM July 2015).
The differences in behaviour are to be understood in terms of the
configuration of the accretion flow, and so the hard X-ray domain is
essential to get information free of intrinsic absorption. Nonetheless,
both sources share common characteristics. In particular, two
independent components are required to describe the hard X-ray emission
and there are indications that, in both cases, the high-energy component
is related to the jet detected in radio.
This strengthens the new picture of XRBs emerging from the INTEGRAL
observations, that the jet contributes from radio to hard X-ray wavelengths.
"On the high-energy emissions of compact objects observed with INTEGRAL
SPI: Event selection impact on source spectra and scientific results for
the bright sources Crab Nebula, GS2023+338 and MAXI J1820+070",
J.-P. Roques & E. Jourdain,
ApJ, in press (arXiv:1811.06391)
