Rapid mid-infrared variability in an X-ray binary
Abstract
We present optical to mid-infrared (mid-IR) spectral-timing observations of the new, bright, black hole candidate transient X-ray binary, MAXI J1535-571. Mid-IR (5-12 micron) observations were acquired with VISIR on the VLT, and optical-near-IR monitoring was taken with the Faulkes, Las Cumbres Observatory (LCO) and Rapid Eye Mount (REM) telescopes. We detect bright (up to ∼ 100 mJy), highly variable mid-IR emission in September 2017 as the source made a transition from the hard state towards the soft state. We use X-ray data to track how the optical-IR changes as the source evolves in the hardness-intensity diagram.The mid-IR 9-12 micron flux density faded dramatically by more than an order of magnitude over one week in September 2017. This is contemporaneous with a near-IR and optical fade of a factor of ∼ 10 and ∼ 2, respectively. Before the fade, the IR de-reddened spectrum can be described by an optically thin synchrotron power law, with a spectral break at the longer wavelengths to a flatter spectrum. This spectrum, and the wavelength dependence of the fade can be explained by a synchrotron jet which is quenching over the state transition, and a less variable disc component in the optical.We also present the first (to our knowledge) mid-IR variability study of an X-ray binary on short (minute) timescales. On some dates the mid-IR flux of MAXI J1535-571 varied by a factor of two in less than 15 minutes, and on one date there is a sudden decrease of the flux to undetectable levels. The near-IR flux is also variable on short timescales. The mid-IR fractional rms variability on minute timescales is ∼ 15%, which is comparable to the optical fractional rms of GX 339-4 at similar time resolution. We compare these results to the expected fractional rms from the jet internal shock model of Malzac et al., and find that the variability amplitude is consistent with this model.We also combine our optical-IR spectral energy distributions with radio, mm and X-ray data to build a coherent picture of the evolution of the broadband spectrum. We find that contrary to other sources, the jet does not simply fade over the transition, with the jet spectral break shifting to lower frequencies, but instead there are jet flares and rapid evolution of the jet spectrum that correlate with X-ray hardness deviations. These results represent an excellent case of multiwavelength jet spectral-timing and demonstrate how rich, multiwavelength time-resolved data of X-ray binaries over state transitions can help refine models of the disc-jet connection and jet launching in these systems.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E2918R