NICER and NuSTAR detections of Type I bursts and periodic dips in Swift J1858.6-0814

D. J. K. Buisson*, J. Hare, T. Güver, D. Altamirano, Keith C. Gendreau, Zaven Arzoumanian, P. M. Bult, Tod E. Strohmayer, N. Castro Segura, Javier A. Garcia, R.A. Remillard, J. A. Tomsick, J. Chenevez, G. K. Jaisawal, M. Ozbey Arabaci, F. Vincentelli, J. Homan, S. Guillot, M. T. Wolff, D. Chakrabarty M. Ng

*Corresponding author for this work

Research output: Other contributionNet publication - Internet publicationResearch


Swift J1858.6-0814 is a low-mass X-ray binary, whose outburst has been ongoing since October 2018 (Krimm et al. ATel #12151). Previously, the source was found to be highly variable on time scales of 10-100 s, and to have a relatively hard X-ray spectrum with strong iron features (ATel #12158, ATel #12160, ATel #12163, ATel #12167, ATel #12220; Hare et al. 2020, ApJ, 890, 57). The source has recently returned to viewable by satellite after being Sun-constrained and appears to be significantly less variable, with a much larger soft X-ray flux than previously observed (ATel #13536).
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.
Original languageEnglish
Publication date18 Mar 2020
Publication statusPublished - 18 Mar 2020
SeriesThe Astronomer's telegram
NumberATel #13563


  • X-ray
  • Binary
  • Black hole
  • Neutron star
  • Transient


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