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arXiv · 2102.07319

The Peculiar X-ray Transient Swift J0840.7-3516: an Unusual Low Mass X-ray Binary or a Tidal Disruption Event?

Megumi Shidatsu·Wataru Iwakiri·Hitoshi Negoro·Tatehiro Mihara·Yoshihiro Ueda·Nobuyuki Kawai·Satoshi Nakahira·Jamie A. Kennea·Phil A. Evans·Keith C. Gendreau·Teruaki Enoto·Francesco Tombesi
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Abstract

We report on the X-ray properties of the new transient Swift J0840.7$-$3516, discovered with Swift/BAT in 2020 February, using extensive data of Swift, MAXI, NICER, and NuSTAR. The source flux increased for $\sim 10^3$ s after the discovery, decayed rapidly over $\sim$ 5 orders of magnitude in 5 days, and then remained almost constant over 9 months. Large-amplitude short-term variations on time scales of 1--$10^4$ s were observed throughout the decay. In the initial flux rise, the source showed a hard power-law shaped spectrum with a photon index of $\sim 1.0$ extending up to $\sim 30$ keV, above which an exponential cutoff was present. The photon index increased in the following rapid decay and became $\sim 2$ at the end of the decay. A spectral absorption feature at 3--4 keV was detected in the decay. It is not straightforward to explain all the observed properties by any known class of X-ray sources. We discuss the possible nature of the source, including a Galactic low mass X-ray binary with multiple extreme properties and a tidal disruption event by a supermassive black hole or a Galactic neutron star.

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Megumi Shidatsu, Wataru Iwakiri, Hitoshi Negoro, Tatehiro Mihara, Yoshihiro Ueda, Nobuyuki Kawai, Satoshi Nakahira, Jamie A. Kennea, Phil A. Evans, Keith C. Gendreau, Teruaki Enoto, Francesco Tombesi. 2021-02-15. The Peculiar X-ray Transient Swift J0840.7-3516: an Unusual Low Mass X-ray Binary or a Tidal Disruption Event?. https://doi.org/10.3847/1538-4357%2Fabe6a1

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