Nicer and AstroSat Observations of Swift J1658.2-4242 during its 2017 Outburst
Abstract
Swift J1658.2-4242 (J1658) is an X-ray transient discovered in 2017. During its only outburst seen so far, J1658 was regularly monitored with the Neutron star Interior Composition Explorer (NICER). Based on the radio/X-ray properties and an assumed distance (> 3 kpc) it was classified as a black hole candidate. We present preliminary results of the spectral and timing characteristics as measured with NICER. The spectral decomposition shows a significant thermal contribution in the form of blackbody emission, in addition to a thermal disk component, which is not expected for a black hole low-mass X-ray binary. We also present the results of a long AstroSat observation made at the beginning of the outburst. The data obtained with the Large Area X-ray Proportional Counter (LAXPC) on board AstroSat revealed the presence of a strong and sharp QPO at around 2.4 Hz. The energy-dependent time-lag spectrum shows a soft lag (soft photons trail behind hard photons) at the QPO frequency, where the fractional rms of the QPO increases with photon energy. We show that our comprehensive spectral study in the 0.4-12 keV band using all the observations made with NICER and the broadband spectra as seen by the Soft X-Ray Telescope (SXT) and LAXPC suggest that the accretor is a neutron star, rather than a black hole as suggested by previous work.
- Publication:
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AAS/High Energy Astrophysics Division
- Pub Date:
- March 2019
- Bibcode:
- 2019HEAD...1711102B