The Hard State of the Highly Absorbed High Inclination Black Hole Binary Candidate Swift J1658.2-4242 Observed by NuSTAR and Swift

TL;DR

This paper analyzes the spectral and timing properties of the black hole binary candidate Swift J1658.2-4242, revealing a high inclination, rapid spin, and evidence of outflows, using NuSTAR and Swift observations.

Contribution

It provides the first detailed spectral and timing analysis of Swift J1658.2-4242, including spin measurement, inclination angle, and detection of outflow signatures in the low/hard state.

Findings

Black hole is rapidly spinning with a*>0.96

Inner disk viewed at high inclination of ~64°

Detection of an ionized Fe K absorption line indicating outflows

Abstract

We present a spectral and timing analysis of the newly reported Galactic X-ray transient Swift J1658.2-4242 observed by NuSTAR and Swift. The broad-band X-ray continuum is typical of a black hole binary in the bright hard state, with a photon index of $\Gamma=1.63\pm0.02$ and a low coronal temperature of $kT_{\rm e}=22\pm1$ keV, corresponding to a low spectral cutoff well constrained by NuSTAR. Spectral modeling of the relativistic disk reflection features, consisting of a broad Fe K$\alpha$ line and the Compton reflection hump, reveals that the black hole is rapidly spinning with the spin parameter of $a^{*}>0.96$, and the inner accretion disk is viewed at a high inclination angle of $i=64^{+2}_{-3}{^\circ}$ (statistical errors, 90% confidence). The high inclination is independently confirmed by dips in the light curves, which can be explained by absorbing material located near the disk plane temporarily obscuring the central region. In addition, we detect an absorption line in the NuSTAR spectra centered at $7.03^{+0.04}_{-0.03}$ keV. If associated with ionized Fe K absorption lines, this provides evidence for the presence of outflowing material in the low/hard state of a black hole binary candidate. A timing analysis shows the presence of a type-C QPO in the power spectrum, with the frequency increasing from $\sim0.14$ Hz to $\sim0.21$ Hz during the single NuSTAR exposure. Our analysis reveals that Swift J1658.2-4242 displays characteristics typical for a black hole binary that is viewed at a high inclination angle, making it a good system for studying the accretion geometry in black hole binaries.