TY - JOUR

T1 - Multi-band observations of Swift J0840.7-3516

T2 - A new transient ultra-compact X-ray binary candidate

AU - Zelati, F. Coti

AU - Postigo, A. de Ugarte

AU - Russell, T. D.

AU - Borghese, A.

AU - Rea, N.

AU - Esposito, P.

AU - Israel, G. L.

AU - Campana, S.

PY - 2021/6/8

Y1 - 2021/6/8

N2 - 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 similar to 5 days, during which the X-ray luminosity quickly decreased fromL-X approximate to 3 x 10(37)d(10)(2)L Xapproximate to 3x1037d10 2erg s(-1) at peak down toL-X approximate to 5 x 10(33)d(10)(2)L Xapproximate to 5x1033d10 2erg s(-1) in quiescence (0.3-10 keV; d(10) 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.

AB - 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 similar to 5 days, during which the X-ray luminosity quickly decreased fromL-X approximate to 3 x 10(37)d(10)(2)L Xapproximate to 3x1037d10 2erg s(-1) at peak down toL-X approximate to 5 x 10(33)d(10)(2)L Xapproximate to 5x1033d10 2erg s(-1) in quiescence (0.3-10 keV; d(10) 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.

KW - methods: data analysis

KW - methods: observational

KW - techniques: spectroscopic

KW - X-rays: binaries

KW - X-rays: individuals: Swift J0840

KW - 7-3516

KW - BLACK-HOLE CANDIDATE

KW - NEUTRON-STAR

KW - ACCRETION DISKS

KW - NUSTAR OBSERVATIONS

KW - ULTRACOMPACT ORBIT

KW - RADIO

KW - PULSATIONS

KW - DISCOVERY

KW - PERIOD

KW - NICER

U2 - 10.1051/0004-6361/202140573

DO - 10.1051/0004-6361/202140573

M3 - Journal article

VL - 650

JO - Astronomy & Astrophysics

JF - Astronomy & Astrophysics

SN - 0004-6361

M1 - A69

ER -