A low-luminosity soft state in the short-period black hole X-ray binary Swift J1753.5-0127

Abstract

We present results from the spectral fitting of the candidate black hole X-ray binary Swift J1753.5-0127 in an accretion state previously unseen in this source. We fit the 0.7-78 keV spectrum with a number of models, however the preferred model is one of a multi-temperature disk with an inner disk temperature kT in = 0.252±0.003 keV scattered into a steep power-law with photon index Γ = 6.39 +0.08 −0.02 and an additional hard power law tail (Γ = 1.79 ± 0.02). We report on the emergence of a strong disk-dominated component in the X-ray spectrum and we conclude that the source has entered the soft state for the first time in its ∼10 year prolonged outburst. Using reasonable estimates for the distance to the source (3 kpc) and black hole mass (5M ⊙ ), we find the unabsorbed luminosity (0.1-100 keV) to be ≈ 0.60% of the Eddington luminosity, making this one of the lowest luminosity soft states recorded in X-ray binaries. We also find that the accretion disk extended towards the compact object during its transition from hard to soft, with the inner radius estimated to be R in = 28.0 +0.7 −0.4 R g or ∼ 12R g , dependent on the boundary condition chosen, assuming the above distance and mass, a spectral hardening factor f = 1.7 and a binary inclination i = 55• .

Disc–jet quenching of the galactic black hole Swift J1753.5−0127

“…The source remained in this state until January 2015 when the flux suddenly quenched below the 3σ monitoring detection limit (< 150 µJy bm −1 ). This corresponded to a soft-state transition seen in the X-rays; the hard X-ray emission dropped by about an order of magnitude and the soft X-rays slightly increased (see Shaw et al 2016, for a detailed description of this low luminosity soft-state). Swift J1753.5 remained in the radio-quenched soft-state for ∼ 170 days before returning to the hard-state in July of 2015.…”

“…To help constraint this we use an XMM-Newton/NuSTAR measurement of the source studied in this paper during a different epoch. Shaw et al (2016) fitted a hard power law component of Γ = 1.79 with a flux of 8.98…”

“…Estimates for the unabsorbed luminosity in the 0.7-78 keV band was found to be only ≈ 0.6% of the Eddington luminosity, even though the source clearly transitioned to the soft-state (Shaw et al 2016). In this paper we analyse all the radio monitoring observations of this source since its initial discovery and report deep limits on the radio activity in the soft-state.…”

“…It is important to note that Swift J1753.5 is a more 'canonical' transient XRB than 4U 1957+11 as the latter is a persistent soft X-ray emitter whilst the former a typical XRB transient that has transitioned across different X-ray states (although the soft-state for Swift J1753.5 is unusually low -see Shaw et al 2016). Also the radio emission from Swift J1753.5 is clearly under-luminous compared to typical hard-state XRBs (see section 3.2) and the quenching factor with respect to the upper-branch of the empirical relationship is similar to 4U 1957+11.…”

“…The durations of these 'failed state transitions' have typically been short (∼ 25 d), but in early 2015, the source appeared to undergo another state transition when the Swift-BAT flux appeared to drop to its lowest levels since the source's discovery (Onodera et al 2015). Subsequent follow-up with the Swift X-ray Telescope (XRT; Burrows et al 2005), XMM-Newton (Jansen et al 2001) and the Nuclear Spectroscopic Telescope Array (NuSTAR; Harrison et al 2013) revealed that Swift J1753.5 had transitioned to one of the lowest luminosity soft-states recorded in LMXBs (Shaw et al 2015(Shaw et al , 2016.…”