Significant Intrinsic Absorption in Swift J1858.6-0814

ATel #12220; M. T. Reynolds, J. M. Miller, R. M. Ludlam, B. E. Tetarenko (University of Michigan)
on 16 Nov 2018; 12:20 UT
Credential Certification: Mark Reynolds (markrey@umich.edu)

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Further Swift observations of Swift J1858.6-0814 (Atel: #12151, #12158, #12163, #12167, #12180, #12184, #12186, #12197) reveal evidence for significant absorption intrinsic to the source.

In a ~1.9 ks XRT/windowed timing mode observation on 2018-11-14 (obsid: 00010955004), the source spectrum is measured to display clear signatures of obscuration, with a significant soft excess and a nominally ultra hard power-law component. Fitting the time-averaged spectrum with a model accounting for partial covering absorption, we measure
pha(apec+pcfabs(po)): N_H = 0.2E22 cm^-2; kT = 0.23+/-0.03 keV; apec_norm = (5.7+/-0.1)E-3; N_H_pcf = (1.7 +0.6 -0.5)E23 cm^-2; f_pcf = 0.88 +0.07 -0.17; gamma = 2.0+\-0.4 ; po_norm = (0.85 +2.02 -0.6)E-3; chi2/nu = 24.15/27, where the errors are at the 1 sigma level.

The observed 0.5 - 10 keV flux is measured to be abs_fx = (1.4 +0.1 -0.5)E-11 erg/s/cm^2. The unabsorbed 0.5 - 10.0 keV flux in the power-law component alone is unabs_fx_po = (4.1 +4.0 -1.7)E-11 erg/s/cm^2.

We caution that the flux uncertainties are quoted at the 90% confidence level; however, due to the current low source flux relative to the background in wt-mode, the quoted fluxes may be systematically uncertain at the 25-30% level, i.e, abs_fx ~ 1.9E-11 erg/s/cm^2; unabs_fx_po ~ 5e-11 erg/s/cm^2 if the background is ignored.

The current source properties are consistent with those observed from V404 Cyg during the 2015 outburst (e.g., Motta+ 2017, MNRAS, 468, 981) and previously from V4641 Sgr (Morningstar+ 2014, ApJ, 786, 20), and suggest that significant mass is being ejected from the inner accretion flow during the current phase of the ongoing outburst.

Further multi-wavelength observations are encouraged.