NuSTAR shows continued X-ray activity of Swift J1858.6-0814 in an unusual spectral state
ATel #12512; J. Hare (UC Berkeley), P. Gandhi (Univ. of Southampton), J. A. Paice (Univ. of Southampton, IUCAA), J. Tomsick (UC Berkeley)
on 17 Feb 2019; 19:53 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)
Subjects: X-ray, Black Hole, Neutron Star, Transient
We report on our analysis of quicklook data (90501304001_v5) of a new 25.5 ks NuSTAR observation of the Galactic X-ray transient Swift J1858.6-0814 (ATel #12151, #12158, #12160, #12163, #12164, #12167, #12180, #12184, #12186, #12197, #12220), which has just become observable after a ~3-4 month period of being Sun constrained (ATel #12499). The observation took place starting 2019-02-14 10:19:36.000 UTC and was taken in NuSTAR's mode 06 due to the source's proximity to the Sun.
The source (hereafter, J1858) remains active with an average net observed count rate of 5.4 cts/s and 5.1 cts/s (8.1 cts/s and 8.5 cts/s after correcting for various factors e.g., vignetting, PSF) in the FPMA and FPMB detectors, respectively. The light curves show flares on ~100 s timescales varying in count rate by factors of ~5, with longer baseline variations also evident over the duration of the observation.
The source's time-averaged energy spectrum can be approximately parameterised with a strong Fe K reflection feature superposed on a power-law with a cutoff energy at ~5 keV. A Compton reflection hump also appears to be present. The observed flux is ~3.5x10^(-10) erg cm^(-2) s^(-1) in the 3.0-79.0 keV band.
Hardness-intensity analysis shows that the flux is dominated by soft photons (i.e., 3-10 keV), but the very low cutoff energy makes this a somewhat unusual spectral state.
Power spectral analyses show a narrow feature, possibly a QPO, at 0.0027 Hz (~364 s). The light curves also show visual evidence of such quasi-periodic variations. But given that the analysis is done on mode 06 quicklook data, we are further investigating its veracity. Observations by other X-ray missions are encouraged, as J1858 emerges from Sun constraint.
We thank the NuSTAR operations team for rapid approval and execution of our Target of Opportunity proposal.
