Multi-band observations of Swift J0840.7−3516: A new transient ultra-compact X-ray binary candidate
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 x display--inline-block" style="background: var(--highlight-yellow); color: inherit;">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 x display--inline-block" style="background: var(--highlight-yellow); color: inherit;">a few seconds. The spectrum of the source in the energy range 0.3-20 keV was well described by x display--inline-block" style="background: var(--highlight-yellow); color: inherit;">a thermal, blackbody-like, component plus x display--inline-block" style="background: var(--highlight-yellow); color: inherit;">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 x display--inline-block" style="background: var(--highlight-yellow); color: inherit;">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 x display--inline-block" style="background: var(--highlight-yellow); color: inherit;">a transient ultra-compact X-ray binary system. © ESO 2021. ; This research is based on observations made with the Gran Telescopio Canarias (GTC), installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias in the island of La Palma, under Director's Discretionary Time (code GTC2020-142). The data were obtained with OSIRIS, built by a Consortium led by the Instituto de Astrofísica de Canarias in collaboration with the Instituto de Astronomía of the Universidad Autónoma de México. OSIRIS was funded by GRANTECAN and the National Plan of Astronomy and Astrophysics of the Spanish Government. We are indebted to Antonio Cabrera and the GTC staff for their efforts in performing the GTC observations. The NuSTAR mission is a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory and funded by NASA. The NICER mission operates on the International Space Station and is funded by NASA. The Australia Telescope Compact Array is part of the Australia Telescope National Facility which is funded by the Australian Government for operation as a National Facility managed by CSIRO. We acknowledge the Gomeroi people as the traditional owners of the ATCA observatory site. This research made use of: the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC) and the California Institute of Technology; software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC and the High Energy Astrophysics Division of the Smithsonian Astrophysical Observatory; IRAF, distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation; APLpy (Robitaille & Bressert 2012), an open-source plotting package for Python hosted at https://aplpy.github.io/; and Astropy (Astropy Collaboration 2013, 2018), a community-developed core Python package for Astronomy. We also made use of the following software: CASA (McMullin et al. 2007), HEASOFT v. 6.28, HENDRICS v. 5.1 (Bachetti 2018), XSPEC v. 12.11.1 (Arnaud 1996). FCZ and AB are supported by Juan de la Cierva fellowships. FCZ, AB and NR are supported by the ERC Consolidator Grant 'MAGNESIA' (nr. 817661) and acknowledge funding from grants SGR2017-1383 and PGC2018-095512-BI00. AdUP acknowledges funding from a Ramón y Cajal fellowship (RyC-2012-09975). TDR acknowledges financial contribution from the agreement ASI-INAF n.2017-14-H.0. We thank support from the COST Action 'PHAROS' (CA 16124). ; With funding from the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (SEV-2017-0709). ; Peer reviewed