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 X-ray luminosity quickly decreased from LX ≈ 3 × 1037 d102 erg s−1 at peak down to LX ≈ 5 × 1033 d102 erg s−1 in quiescence (0.3−10 keV; d10 is the distance to the source in units of 10 kpc). 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 it softened considerably as the source returned to quiescence. The spectrum of the optical counterpart in quiescence showed broad emission features mainly associated with ionised 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.

Low-Mass X-ray Binaries

“…While direct observation of orbital modulation has been the prime method for identifying UCXBs [e.g., 44,45,46,37], in a significant fraction of these systems, discovery of an accreting millisecond X-ray pulsar (AMXP) has allowed estimation of orbital period through accurate timing of the X-ray pulsations [e.g., 11,47,48,49,50]. In addition to confirmed UCXBs, there are also candidate systems that are identified as such based on the X-ray evolution of their accretion outbursts [51,52], or their optical and X-ray spectral properties [53,54,55]. Furthermore, LMXBs that persistently accrete at very low rates are suspected to have small disks (since these are easier kept photo-ionized sustaining active accretion) and are hence considered good candidate UCXBs [56,57].…”

Black Hole Ultracompact X-Ray Binaries as Galactic Low-frequency Gravitational Wave Sources: The He Star Channel

“…Thus far, the number of confirmed UCXBs (with an accurately measured orbital period 80 minutes) and candidates is around 45 (Armas Padilla et al 2023). Based on the accurate detection of orbital periods, 20 sources were identified to be UCXBs in high confidence, which includes 11 persistent sources and 9 transient sources (in't Zand et al 2007;Liu et al 2007;Cartwright et al 2013;Heinke et al 2013;Pietrukowicz et al 2019;Coti Zelati et al 2021;Peng & Shen 2021;Armas Padilla et al 2023). Among the confirmed UCXBs, 19 sources were discovered to include a neutron star (NS).…”