constraints on x-ray polarization of synchrotron jets from stellar-mass bhs

Constraints on X-ray polarization of synchrotron jets from stellar-mass

BHsDave Russell

Instituto de Astrofísica de Canarias

In collaboration with:Dipankar Maitra, Sera Markoff, Tariq Shahbaz, Paolo Soleri, Rob

Fender, Fraser Lewis, Piergiorgio Casella, Peter Curran, James-Miller-Jones... and some results from a larger collaboration

10th July 2012

Let’s start with the conclusionsFuture 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 systemI 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

*probably

Optical & infrared data published in Jain et al. 2001; radio in Corbel et al. 2001

X-ray analysis as in Dunn et al. 2010

Introducing the 2000 outburst of XTE J1550-564Well monitored in X-ray, optical and near-infrared (NIR)

We can separate disc and jet emission

Disc flux is approximated by an exponential decay

Jet has optically thin spectrum

Could it be a synchrotron jet dominating X-ray?

NIR jet flux is proportional to X-ray flux

Russell, Maitra, Dunn & Markoff 2010, MNRAS, 405, 1759

α (NIR optical) ~ -0.7α (optical X-ray) = -0.7α (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 tenJet origin of X-ray emission predicted by Markoff, Falcke & Fender 2001

(modelling of multiwavelength SED of XTE J1118+480)

The black hole XRB XTE J1752-223: late X-ray + optical flare

2009-2010 outburst of this new BH transient: Russell et al. 2012, MNRAS, 419, 1740

Faulkes Telescope monitoring in 4 optical filtersα (optical excess) = -0.16 +- 0.29

Polarization of optically thin synchrotron emission

Shahbaz et al. 2008

• In NIR, the observed emission can be highly polarized• Depends on magnetic field configuration• Ordered field up to ~80% polarized

• 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

We observed GX 339-4 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 339-4 (Gallo et al. 2004)

New VLT observations of GX 339-4 in 2008

Polarized γ-ray emission from Cygnus X-1 might be from the jet (Laurent et al. 2011, Science)

Polarization strength is very high: 67 +- 30 % !! (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

Sco X-1: SAAO 1.9m + HIPPO in 2009 We observed Sco X-1 with HIPPO on the SAAO 1.9m simultaneously with RXTE

Sco X-1 is a Z-source

First night:“flaring branch”X-ray state

Second night:“normal branch”X-ray statey-axis: 2-60 keV fluxx-axis:4.4-6.2keV / 2.0-4.4 keV

Clustered around PA ~ 100 degNot much evidence for clustering of PAMigliari &Fender 2006

Sco X-1: SAAO 1.9m + HIPPO in 2009

We are developing a simple model to reproduce the broadband polarization given the input SED (Russell & Shahbaz, in prep.)

Data of GX 339-4 from Gandhi et al. 2011Components:• Self-absorbed synchrotron (radio to IR)

Maximum polarization is (e.g. Blandford et al. 2002)

(<11%)• Optically thin synchrotron (IR to X-ray) with cut-off in X-ray

Maximum polarization is (<82%)• Irradiated disc (a few per cent due to scattering)• Comptonized corona, assumed here to be unpolarized (chaotic geometry, no net aligned field?)

Data of Cyg X-1 from Rahoui et al. 2011, Laurent et al. 2011, Fender et al. 2000, Long et al. 1980

Components:• Self-absorbed synchrotron (radio to IR)

Maximum polarization is (e.g. Blandford et al. 2002)

(<11%)• Optically thin synchrotron (IR to X-ray) with cut-off in X-ray

Maximum polarization is (<82%)• Irradiated disc (a few per cent due to scattering)• Comptonized corona, assumed here to be unpolarized (chaotic geometry, no net aligned field?)

Conclusions revisited• NIR synchrotron emission from jets in BH X-ray binaries is polarized

• The results so far suggest:

• Near the jet base the magnetic field is probably: generally turbulent (only partially ordered) and rapidly changing parallel to 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 J1550-564, and possibly XTE J1752-223)

• Regular X-ray & optical/NIR monitoring is beneficial, such as SMARTS & Faulkes

Future spaceborne X-ray polarimeters may be able to detect variable X-ray polarization from synchrotron emission from the

jets released from accreting Galactic black holes