Spectral and Timing Properties of the Galactic X-Ray Transient Swift J1658.2–4242 Using Astrosat Observations

Abstract

We present the X-ray timing and spectral analysis of the new Galactic X-ray transient Swift J1658.2-4242 observed with LAXPC and SXT instruments onboard Astrosat. We detect prominent C-type quasi-periodic oscillations (QPOs) of frequencies varying from ∼ 1.5 Hz to ∼ 6.6 Hz along with distinct 2nd harmonics and sub-harmonics. The QPO detected at ∼ 1.56 Hz drifts to a higher centroid frequency of ∼ 1.74 in the course of the observation, while the QPO detected at ∼ 6.6 Hz disappeared during hard flarings. The fractional rms at the QPO and the sub-harmonic frequencies increases with photon energy, while at the 2nd harmonic frequencies the rms seems to be constant. In addition, we have observed soft time lag at QPO and sub-harmonic frequencies up to a time scale of ∼ 35 ms, however, at the 2nd harmonic frequencies there is weak/zero time lag. We attempt spectral modeling of the broadband data in the 0.7-25 keV band using the doubly absorbed disk plus thermal Comptonization model. Based on the spectral and timing properties, we identified the source to be in the hard intermediate state of black hole X-ray binaries. To quantitatively fit the energy and frequency-dependent fractional rms and time lag, we use a single zone fluctuation propagation model and discuss our results in the context of that model.

NICER uncovers the transient nature of the type-B quasi-periodic oscillation in the black hole candidate MAXI J1348−630

“…QPOs are typically transient, appearing or disappearing on timescales of a few tens of seconds. Such transient behaviour has been observed in all type-A (Sriram et al 2021), -B (e.g., Miyamoto et al 1991;Nespoli et al 2003;Casella et al 2004;Huang et al 2018;Sriram et al 2021), and -C (Xu et al 2019;Jithesh et al 2019;Sriram et al 2021) QPOs. In particular, fast transitions between different types of low-frequency QPOs or broadband noise components are sometimes observed with significant changes of the source spectrum (e.g., Miyamoto et al 1991;Takizawa et al 1997;Wijnands et al 2001;Homan et al 2001;Casella et al 2004;Motta et al 2011;Sriram et al 2012Sriram et al , 2013Sriram et al , 2016Sriram et al , 2021.…”

The obscured X-ray binaries V404 Cyg, Cyg X–3, V4641 Sgr, and GRS 1915+105

“…However, GRS 1915+105 recently entered a new accretion state that presents lower fluxes throughout its spectral energy distribution than ever before during its 27 year-long outburst. In this state, sporadic X-ray flares have been observed (e.g., Iwakiri et al 2019;Neilsen et al 2019;Jithesh et al 2019) in addition to the X-ray spectra, indicating heavy obscuration . Strong radio flares were also observed in the flaring period (Motta et al 2019;Trushkin et al 2019;Koljonen et al 2019), indicating episodic jet emission that is also similar to the multiwavelength evolution of V404 Cyg and Cyg X-3.…”

NICER uncovers the transient nature of the type-B quasi-periodic oscillation in the black hole candidate MAXI J1348-630

“…QPOs are typically transient, appearing or disappearing on timescales of a few tens of seconds. Such transient behaviour has been observed in all type-A (Sriram et al 2021), -B (e.g., Miyamoto et al 1991;Nespoli et al 2003;Casella et al 2004;Huang et al 2018;Sriram et al 2021), and -C (Xu et al 2019;Jithesh et al 2019;Sriram et al 2021) QPOs. In particular, fast transitions between different types of low-frequency QPOs or broadband noise components are sometimes observed with significant changes of the source spectrum (e.g., Miyamoto et al 1991;Takizawa et al 1997;Wijnands et al 2001;Homan et al 2001;Casella et al 2004;Motta et al 2011;Sriram et al 2012Sriram et al , 2013Sriram et al , 2016Sriram et al , 2021.…”