Constraints on X-ray polarization of synchrotron jets from stellar-mass BHs Dave Russell niversity of Amsterdam In collaboration with: Dipankar Maitra, Robert Dunn, Sera Markoff, Rob Fender, Fraser Lewis, Piergiorgio Casella, Peter Curran... and some results from a larger collaboration 28 th June 2011

Let’s start with the conclusions Future spaceborne X-ray polarimeters will* detect variable X-ray polarization from synchrotron emission from the jets released from accreting Galactic black holes X-ray binary – microquasar system I will show: Sometimes, synchrotron emission from jets can dominate the X-ray flux of a black hole X-ray binary This synchrotron emission is polarized at a low, and variable level The polarization properties can help us understand jet physics *probably

Optical & infrared data published in Jain et al. 2001; radio in Corbel et al X-ray analysis as in Dunn et al Introducing the 2000 outburst of XTE J Well monitored in X-ray, optical and near-infrared (NIR) We can separate disc and jet emission Assumes continuation of the exponential decay of disc flux Jet has optically thin spectrum

Russell, Maitra, Dunn & Fender 2011, MNRAS, in press Hardness-intensity vs colour-magnitude diagrams

Could it be a synchrotron jet dominating X-ray? NIR jet flux is proportional to X-ray flux variability amplitude increases here (Kalemci et al. 2001) Russell, Maitra, Dunn & Markoff 2010, MNRAS, 405, 1759 α (NIR  optical) ~ -0.7 α (optical  X-ray) = -0.7 α (X-ray power law) = -0.7 (photon index = 1.7) α (X-ray power law before) = -0.6 A single power law decreasing in flux by a factor of ten Jet origin of X-ray emission predicted by Markoff, Falcke & Fender 2001 (modelling of multiwavelength SED of XTE J )

Fender, Gallo & Jonker 2003: Energetics are jet dominated at low luminosities in the hard state A possible revised picture for BH outbursts Jet could dominate X-ray flux in the hard state between

News flash: XTE J late X-ray + optical flare outburst of this new BH transient: Faulkes Telescope monitoring in 4 optical filters α (optical excess) = No change in X-ray PL properties Russell et al. in prep.

Polarization of optically thin synchrotron emission Shahbaz et al In NIR, the observed emission can be highly polarized Depends on magnetic field configuration Ordered field  up to 70% polarized Russell & Fender 2008 Tangled field  ~ no net polarization

We infer a predominantly tangled, variable magnetic field near the jet base  The PA of polarization is ~ perpendicular to the PA of the resolved radio jet  The magnetic field is approximately parallel to the jet axis (Russell et al. in prep.)  We observed GX in September 2008 during a hard state with VLT+ISAAC  We detect significant, variable linear polarization in the near-infrared (when the jet dominated) Resolved radio jet of GX (Gallo et al. 2004) New VLT observations of GX in 2008 News flash: Polarized γ-ray emission from Cygnus X-1 might be from the jet (Laurent et al. 2011, Science) Polarization strength is very high: % !! (0.4-2 MeV) Derived from 58 days of exposure time This would imply a very highly ordered, constant B field at the base of the jet of Cyg X-1 PA is offset from jet axis

Conclusions revisited NIR synchrotron emission from jets in BH X-ray binaries is polarized The results so far suggest: Near the jet base of GX 339-4, the magnetic field is:  generally turbulent (only partially ordered) and rapidly changing  parallel to the jet axis Near the jet base of Cygnus X-1 (Laurent et al. 2011), the magnetic field is possibly:  extremely ordered and not rapidly changing  offset from the jet axis These magnetic geometry/variability properties are useful for jet models This polarized synchrotron emission can occasionally dominate the X-ray flux (This is probably true XTE J , and possibly XTE J ) Regular X-ray & optical/NIR monitoring is beneficial, such as SMARTS & Faulkes Future spaceborne X-ray polarimeters like GEMS and NHXM may be able to detect variable X-ray polarization from synchrotron emission from the jets released from accreting Galactic black holes