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NuSTAR observation of Swift J1658.2-4242

ATel #11321; Y. Xu (Caltech), J. A. Kennea, (PSU), F. A. Harrison (Caltech), K. Forster (Caltech)
on 19 Feb 2018; 17:25 UT
Credential Certification: Yanjun Xu (yanjun@caltech.edu)

Subjects: X-ray, Binary, Black Hole

Referred to by ATel #: 11322, 11336, 11342, 11358, 11375

We report on the initial analysis of a NuSTAR observation of the new X-ray transient Swift J1658.2-4242 (GCN #22416, #22417, #22419; ATEL #11306, #11307, #11310, #11311, #11318). NuSTAR observed Swift J1658.2-4242 from 2018-02-16T23:26:09 to 2018-02-17T20:51:09 with a total exposure of 33.3 ks. The time-averaged net count rates are 34.5+/-0.03 cts/s and 32.3+/-0.03 cts/s for FPMA and FPMB, respectively (~65 mCrab in the NuSTAR band). The continuum spectra measured by NuSTAR can be approximately described by an absorbed cutoff power-law model with NH = 1.3 x 10^{23} cm^{-2}, photon index of 1.3 and cut-off energy at 53 keV. A disk reflection component with a broad iron line is also required for a proper fit. Two strong dips are found in the NuSTAR light curve separated by ~30ks, indicating this source could be an eclipsing system. A peak is clearly detected in the NuSTAR power spectra, with a frequency that increases from 0.138+/-0.002 Hz to 0.166+/-0.003 Hz from the first half to the second half of the observation (excluding the dips), with FWHM of 3.5e-2+/-6e-3 Hz and 5.0e-2+/-8e-3 Hz, respectively. The features in the power spectrum are similar to QPOs detected in the hard state of accreting black holes. And there is no clear indication for pulsations in the NuSTAR data. Therefore we propose that Swift J1658.2-4242 is a black hole binary.