Multi-band observations of Swift J0840.7-3516: a new transient ultra-compact X-ray binary candidate

Abstract

We report on multi-band observations of the transient source Swift J0840.7−3516, which was detected in outburst in 2020 February by the Neil Gehrels Swift Observatory. The outburst episode lasted just ∼5 days, during which the observed X-ray flux quickly decreased from ∼ 1.8 × 10 −9 erg cm −2 s −1 at peak down to ∼ 1.3 × 10 −13 erg cm −2 s −1 in quiescence (0.3-10 keV). Such a marked and rapid decrease in the flux was also registered at UV and optical wavelengths. In outburst, the source showed considerable aperiodic variability in the X-rays on timescales as short as a few seconds. The spectrum of the source in the energy range 0.3-20 keV was well described by a thermal (blackbody-like) component plus a non-thermal (power law-like) component, and softened considerably as the source returned to quiescence. The spectrum of the optical counterpart in quiescence showed broad emission features associated mainly with ionized carbon and oxygen, superposed on a blue continuum. No evidence for bright continuum radio emission was found in quiescence. We discuss possible scenarios for the nature of this source, and show that the observed phenomenology points to a transient ultra-compact X-ray binary system.

Ultra-compact X-ray binaries with He star companions

“…It has been suggested that all confirmed UCXBs contain NS accretors, but the nature of accretors has not been identified exclusively in the observations (e.g. Sazonov et al 2020;Coti Zelati et al 2021). Note that UCXBs with BH accretors have not been confirmed, but there is a known moderately strong BH UCXB candidate 47 Tuc X9 in our Galaxy (see Bahramian et al 2017).…”

“…So far, there are about 40 known UCXBs or candidates, in which 15 of them have been identified with high confidence on the basis of the observed orbital periods (including 10 persistent sources and 5 transient sources; e.g. in 't Zand, Jonker & Markwardt 2007;Liu, van Paradijs & van den Heuvel 2007;Heinke et al 2013;Cartwright et al 2013;Coti Zelati et al 2021). It has been suggested that all confirmed UCXBs contain NS accretors, but the nature of accretors has not been identified exclusively in the observations (e.g.…”