NICER detection of flux increase and spectral state change in Swift 1858.6-0814

ATel #13536; Douglas J. K. Buisson, Diego Altamirano (Southampton), Ron Remillard (MIT), Zaven Arzoumanian, Keith Gendreau (NASA/GSFC), Poshak Gandhi, Federico Vincentelli (Southampton)
on 2 Mar 2020; 14:33 UT
Credential Certification: Douglas Buisson (djkb2@cam.ac.uk)

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Swift J1858.6-0814 (Krimm et al. ATel #12151) is an X-ray binary whose discovery outburst had been ongoing for over one year before entering Sun constraint for most X-ray telescopes in November 2019. The typical X-ray light curve of Swift J1858.6-0814 was described by a low-flux persistent level in addition to bright seconds-to-minutes flares where the flux changed by factors of up to ~100 (ATel #12158, #12160; Hare et al. 2020, ApJ 890, 57). Its energy spectrum was consistently hard, with a strong neutral iron K-alpha emission line and K edge (ATel #12158, #12512; Hare et al. 2020).

Since leaving Solar exclusion, NICER observed Swift J1858.6-0814 on 18th, 22nd, 26th, and 27th February. This has acquired 1.3 ks of good exposure, taken in segments of a few hundred seconds. Preliminary analysis shows that in all of the observations the source had similar flux and spectral shape. In addition, we find a significantly brighter soft X-ray flux (around 30 times the previous quiescent level), a weaker iron K-alpha line, and that the variability is much reduced. The 4-10 keV flux is 6*10^-10 erg/cm^2/s; this is consistent with the recent report of a nearby soft X-ray transient by the MAXI Team (Negoro et al., ATel #13455) and confirms that the nearby MAXI detection is Swift J1858.6-0814 rather than a new transient.

The spectrum is not trivial to identify as a canonical hard/soft state. Modelling the combined spectrum with a power-law (Gamma=1.50+-0.01, Chi^2/dof=2244/1197=1.87) shows a high-energy rollover. This may be modelled with a disc (kT=2.04+-0.02 keV, Chi^2/dof=2219/1197=1.85 with the diskbb model) but this leaves low-energy residuals. Alternatively, a cutoff power-law gives a better fit (1435/1196=1.20) but unusually hard and cold parameters for Comptonisation (Gamma=0.95+-0.04; E_Cut=6.2+-0.4 keV). Generalising the disc model to diskpbb gives p=0.625+-0.04 (kT=3.1+-0.1 keV, Chi^2/dof=1391/1196=1.16), which (being less than 0.75) may indicate a slim disc. Note that all models include Galactic absorption; values range from (1-3)*10^21 cm^-2. Before Solar exclusion, Swift J1858.6-0814 showed a strong neutral K-alpha emission line (with strength ~7e-5 photons/cm^2/s). This is not apparent in the new spectra. Fitting lines at the energies of neutral-like, XXV and XXVI Fe K-alpha gives 90% upper limits of 0.00068, 0.00036 and 0.00037 photons/cm^2/s respectively. This limit for neutral-like iron is consistent with the previous line flux but requires a reduction in the equivalent width of the line by a factor > 3.5. The variability in the 2020 observations is now modest: F_Var=4% on timescales > 0.01s and 2% on timescales > 10s. This is in contrast to variability by factors of up to several hundred in the previous state (ATel #12158, Hare et al. 2020). No QPOs are obvious in the PSD.

Further NICER observations are planned; we encourage multi-wavelength follow-up.

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.

Plot showing spectrum before and after state change