Here, we report the detection of three X-ray flares which are consistent with being Type I X-ray bursts. The first is detected on 2020 March 6 in concurrent NICER and NuSTAR data; the second and third, on March 9 and March 20, were covered only by NICER. All three bursts display a fast rise and exponential decay. The first two have similar lightcurves, with peaks of ~195 and ~140 cts/s above the persistent level (0.7-10 keV with NICER). The peak NuSTAR rate was ~130 cts/s/FPM (3-78 keV). The spectrum of the burst emission is usually fit well by a blackbody; a power-law provides a poor fit. Splitting the burst into peak and decay sections (see linked figure) shows cooling at a stable area. The first burst has weaker constraints and is consistent with the second. The third burst has a much brighter initial peak than the other bursts and time resolved spectroscopy shows evidence of a photospheric radius expansion. The burst reaches an unabsorbed bolometric peak flux of 7.8x10^-9 erg/s/cm^-2. Assuming that this is the Eddington luminosity of the neutron star, a distance of ~15 kpc can be inferred. During the expansion of the photosphere, the X-ray spectra show excess relative to a simple blackbody model therefore we modeled these spectra with two blackbodies after subtracting the persistent emission (similar to that by Bult et al. (2019) for SAX J1808.4-3658). These properties are all consistent with thermonuclear (Type I) X-ray bursts, which would imply that the compact object in Swift J1858.6-0814 is a neutron star.
In addition to these bursts, the NICER light curve shows occasional dips, from ~200 cts/s to around 3 cts/s. Deep, prolonged dips are found to occur with a period of roughly 78640 s. The dip phase duration is approximately 4000 s. Ingresses/egresses are similarly shaped and last ~100 s. The spectrum during ingress/egress is also consistent with absorption. These dips may be due to eclipses by the secondary star or occultation by a component of the accretion system The most recent observed ingress (time of half non-dip flux) occurred at 2020-03-16 00:42:40 UTC.
The NuSTAR observation was carried out on 2020 March 6 (MJD 58914.37). Apart from the flare, the source count rate in the NuSTAR band was relatively steady, with an average 3-78 keV count rate of ~26 cts/s/FPM. The NuSTAR FPMA+FPMB spectra are reasonably well fit by an absorbed cutoff power-law with an absorbing column density NH=4x10^(21) cm^(-2), photon index Gamma=0.96, and high energy cutoff of 5.5 keV, with a reduced chi-squared of 1.5. No hard tail above this is apparent. The source now has a 3-79 keV band flux of ~8x10^(-10) erg/cm^2/s.
Further NICER observations are planned; we encourage multi-wavelength follow-up.
We thank the NICER and NuSTAR operations teams for the rapid approval and execution of our Target of Opportunity proposals. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.
|Publication date||18 Mar 2020|
|Publication status||Published - 18 Mar 2020|
|Series||The Astronomer's telegram|
- Black hole
- Neutron star