A weak compact jet in a soft state of Cygnus X-1

Abstract

We present evidence for the presence of a weak compact jet during a soft X-ray state of Cygnus X-1. Very-high-resolution radio observations were taken with the VLBA, EVN and MERLIN during a hard-to-soft spectral state change, showing the hard state jet to be suppressed by a factor of about 3-5 in radio flux and unresolved to direct imaging observations (i.e. < 1 mas at 4 cm). High time-resolution X-ray observations with the RXTE-PCA were also taken during the radio monitoring period, showing the source to make the transition from the hard state to a softer state (via an intermediate state), although the source may never have reached the canonical soft state. Using astrometric VLBI analysis and removing proper motion, parallax and orbital motion signatures, the residual positions show a scatter of ~0.2 mas (at 4 cm) and ~3 mas (at 13 cm) along the position angle of the known jet axis; these residuals suggest there is a weak unresolved outflow, with varying size or opacity, during intermediate and soft X-ray states. Furthermore, no evidence was found for extended knots or shocks forming within the jet during the state transition, suggesting the change in outflow rate may not be sufficiently high to produce superluminal knots.Comment: Accepted in MNRAS; 4 figures and 1 tabl

Gamma rays detected from Cygnus X-1 with likely jet origin

“…its spectrum is never fully disk-dominated. Whereas in the HS it shows a mildly relativistic (v ∼ 0.6 c) radio jet (Stirling et al 2001;Gallo et al 2003) which carries a significant fraction of the system X-ray luminosity (Gallo et al 2005), in the SS there is evidence for a factor 3-5 weaker unresolved compact jet (Rushton et al 2012). The constant mean level of the radio emission is of ∼10-15 mJy, with a flat spectrum and no evidence for a cutoff (Fender & Hendry 2000) up to IR frequencies, where the emission is totally dominated by the supergiant, making impossible the measurement of the spectral break.…”

“…These 1-2 d long events occurred on 2009, October 16 (0.38-0.56 orbital phase) with an integral flux of (2.32±0.66) ×10 −6 ph cm −2 s −1 between 0.1 and 3 GeV (Sabatini et al 2010); on 2010, March 24 (Bulgarelli et al 2010) with a flux above 2.50 ×10 −6 ph cm −2 s −1 above 100 MeV; and on 2010, June 30 with a (1.45±0.78) ×10 −6 ph cm −2 s −1 average flux in the same energy range (Sabatini et al 2013). Whereas the first two episodes happened when the source was in the HS, the last one occurred during a hard-to-soft state transition, but coincident with the source entering in the SS and a couple of days before of an anomalous radio flare (Negoro et al 2010;Rushton et al 2010;Wilson-Hodge & Case 2010). An independent analysis of 3.6 yr of Fermi-LAT data confirmed evidence of flaring activity on 1-2 d timescales contemporaneous, but not coincident, with AGILE at 3-4σ level (Bodaghee et al 2013).…”

Formation and Destruction of Jets in X-Ray Binaries

“…Exactly the opposite seems to occur in Cyg X-1 (Rushton et al 2011). This source is known to be in the hard state for long periods of time (Di Salvo et al 2001), although it makes repeated attempts ('failed' state transitions; Pottschmidt et al 2003) to reach the soft state, where it did exist at least once .…”

Astrometric Observations of X-ray Binaries Using Very Long Baseline Interferometry

“…The offset between frequencies is caused by frequency-dependent structure in the phase reference calibrator. Adapted with permission from Figure 2 in Rushton et al (2012). orbital signature is given by…”