NICER / ISS Science Nugget
for March 12, 2020

X-ray flaring in MAXI J1820+070 probes emission near black hole

A bright new X-ray source was found in March 2018 by JAXA's MAXI X-ray camera on ISS. Now known as MAXI J1820+070, it lies in the constellation of Ophiuchus, the Serpent Bearer. It was very bright for about 8 months, during which it became the second-brightest X-ray source in the sky. An optical counterpart was found and revealed a binary system consisting of a normal star in close orbit about a 7 Solar-mass black hole. Gas from the star is falling onto the black hole, releasing gravitational energy that powers emission of X-rays. Optical and radio studies indicate that it lies in the disk of our Galaxy at a distance of about 10,000 light-years.

Matter cannot fall straight onto the black hole because of its orbital angular momentum, much as the Earth doesn't fall into the Sun and the ISS doesn't fall onto the Earth. Instead, it forms an orbiting disk of gas around the black hole, with magnetic viscous forces causing it to spiral slowly inward, releasing gravitational energy into heat and thus radiation. Einstein's General Relativity dictates, however, that the orbiting disk must stop before reaching the black hole. Inside this radius, known as the Innermost Stable Circular Orbit, or ISCO, matter falls inward on short curved orbits into the black hole; this is known as the plunge region, from which negligible radiation should emerge. A rough analogy would be the top of a waterfall: if you are in a boat approaching the waterfall your speed may be steady but as you go over the edge you accelerate freely downward.

Repeated X-ray observations of MAXI J1820+070 were made with NICER and NASA's freely-flying NuSTAR telescope. Both have photon-counting detectors that are not blinded by high count rates like many other X-ray detectors in orbit, and so are ideal. The source evolved from a hard state dominated by high energy X-rays to a soft state, characterized by the distinctive thermal spectrum from the dense hot gas of the accretion disk, rising in temperature up to the ISCO. An additional, slightly hotter, component was found in all the soft state observations. A natural place to locate it is near the ISCO. Gas in the disk is so hot that it is highly ionized and is expected to be magnetized as fields are amplified in the turbulent and differentially-rotating disk flow. Magnetic fields will slow gas at the start of the plunge region allowing more gravitational energy to be released as heat and X-rays. This may be what we have seen: the last gasp of radiation as gas begins to fall freely into the black hole. This work, led by Prof. Andy Fabian of Cambridge University in the UK, has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.

X-ray spectrum of the black-hole binary MAXI J1820+070

Figure: X-ray spectrum of the black-hole binary MAXI J1820+070 in its soft state, as measured with NICER (data: black points with error bars, and model: solid black histogram and curve) and two independent NuSTAR detectors (data: red and green points with error bars, and mode: red and green histograms and curves). From top to bottom, the solid curves originating at the left edge represent thermal emission from the accretion disk, an additional high-temperature component likely arising from the final "plunge region" near the event horizon, and high-energy emission likely from hot gas in a "corona" above the disk.

Outbursts of the MAXI J1820+070 system were photographed on plates taken in 1898 and 1934. It is still active now, although at a much reduced level compared to 2018. A new bright outburst may occur in 40 years time. In the meantime, we need to rely on outbursts from other, similar objects.

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