Circularly polarised radio emission from GRS 1915+105 and other X-ray binaries

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Circularly polarised radio emission from GRS 1915+105and other X-ray binaries Ron Fender Astronomical Institute ‘Anton Pannekoek’,University of Amsterdam,Kruislaan 403,1098SJ Amsterdam,The Netherlands Abstract.I report the detection of circular polarisation,associated with relativistic ejec-tions,from the ‘microquasar’GRS 1915+105.I further compare detections and limits of circular polarisation and circular-to-linear polarisation ratios in other X-ray binaries.Since in at least two cases the dominance of linear over circular polarisa-tion or vice versa is a function of frequency,this seems to indicate that this is a strong function of depolarisation in the source.Furthermore,I note that circular polarisation has only been detected from sources whose jets lie close to the plane of the sky,whereas we have quite stringent limits on the circular polarisation of jets which lie close to the line of sight.Keywords:polarization –ISM:jets and out?ows –radio continuum:stars 1.Introduction Studies of circular polarisation from jets,and indeed of relativistic jets in general,have traditionally focussed on active galactic nuclei (AGN).However,in the past decade or so it has become clear that relativistic jets from X-ray binaries,also known as ‘microquasars’(Mirabel et al.1992),share many of the properties (observationally,and therefore almost certainly physically)of their extragalactic cousins.Further-more,due to the huge mass ratio 105≤M AGN ≤108,we may probe timescales associated with accretion and jet formation by observing X-ray binaries which would be humanly impossible for AGN (e.g.Sams,Eckart &Sunyaev 1996).For recent reviews of jets from X-ray binaries,

see Mirabel &Rodriguez (1999),Fender (2003).In this paper I shall ?rst discuss in detail observations of circularly polarised radio emission from the ‘microquasar’GRS 1915+105(§2),and then compare it to observations of circularly polarised radio emission from other X-ray binaries (§3).

c 2008Kluwer Academic Publishers.Printe

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2Rob Fender

Figure1.Daily radio and X-ray monitoring of GRS1915+105,with the times of our ATCA and MERLIN observations indicated.

2.Circular polarisation from GRS1915+105

In this section we present observations of circular polarisation associ-ated with relativistic ejections from the X-ray binary jet source(‘micro-quasar’)GRS1915+105,as well as some preliminary interpretations. These results have been published in Fender et al.(2002b).

2.1.Observations

In Fig1we show radio and soft X-ray monitoring of GRS1915+105, over a150-day period.The radio monitoring data were obtained with the Ryle Telescope(RT),at a frequency of15GHz;for a more detailed description of this monitoring program see Pooley&Fender(1997). The X-ray data are from the Rossi XTE All-Sky Monitor(ASM)and measure the total?ux in the2-12keV band.The Rossi XTE ASM is described in Levine et al.(1996)and the public data can be obtained at https://www.sodocs.net/doc/8a4191931.html,.

Indicated in the top panels of Fig1are the times of our two ATCA and multiple MERLIN observations of GRS1915+105.

2.1.1.ATCA

The Australia Telescope Compact Array(ATCA;Frater,Brooks& Whiteoak1992)has a number of design features which enable very accurate circular polarization measurements.The low antenna cross-polarization and high polarization stability enable accurate calibration

3 of polarization leakage terms,and the linearly-polarized feed design largely isolates Stokes V from contamination by Stokes I.

ATCA observed GRS1915+105twice,for six hours each,on2001 January17and2001March23.During the January observations,simul-taneous observations at1384MHz and2496MHz were interleaved with observations at4800MHz and8640MHz;for the March observations, only4800MHz and8640MHz were observed.For both epochs the array was in a‘6km’con?guration,for which the lack of short baselines served to reduce confusion from other galactic sources.The observation and calibration procedures were similar to those described in Fender et al. (2000).

As discussed in Fender et al.(2000),calibration of circular polariza-tion data requires the”strongly-polarized”calibration equations(Sault, Killeen&Kesteven,1991),using a point-source with a few percent linear polarization.This is needed to calibrate the leakage of linear po-larization into circular.For the4800MHz and8640MHz observations, the VLA calibrator1923+210was used as a polarization calibrator for both epochs.Calibrator confusion and low linear polarization,however, precluded the use of any of the observed calibrators as polarization calibrators for the January1384MHz and2496MHz observations. As a result,we were forced to use calibration solutions derived using the”weakly-polarized”equations with the ATCA primary calibrator, 1934-638.

The use of the”weakly-polarized”equations will cause a time-varying leakage of linear polarization into circular.In tests,peak leakages of5% of the linear polarization into circular have been observed.For the1384 MHz observations,the low linear polarization of GRS1915+105implies the e?ect of such leakage is negligible.Even for the2496MHz observa-tions,where the linear polarization rises rapidly during the observation, in the worst-case the leakage would be only half the Stokes V error due to thermal noise.The full polarisation ATCA data for both epochs are presented in Fig2.

2.1.2.MERLIN

The Multi Element Radio Linked Interferometer Network(MERLIN) consists of six individual antennae with a typical diameter of25m and a maximum baseline of217km(Thomasson1986).The observations presented here were undertaken in continuum mode at a frequency of 4994MHz with a total bandwidth of16MHz.As MERLIN measures all four correlation products as a matter of course when in this mode, full polarimetric information can be derived from all images.Ongoing work is seeking to establish the reliability of Stokes V measurements with MERLIN;these will not be reported here.

4Rob Fender

Figure2.Dual-frequency full polarisation measurements of GRS1915+105in2001 January(left)and March(right).Circular polarisation is detected at all frequencies observed.Note also the correlated change in linear polarisation strength and position angle at4.8&8.6GHz in2001January,a‘rotator’event.

GRS1915+105was observed eleven times with MERLIN following the?are observed on2001,March22/23.The?rst?ve epochs,corre-sponding to daily observations between2001March24and March27 and again on March29,are presented in this paper(Fig3);further details and analysis of the full set of MERLIN observations will be published in McCormick et al.(in prep).In each case a?ux calibrator, 3C286,a point source,OQ208,and a phase calibrator,1919+086,

5 were included in the observing schedule.The?ux calibrator and point source calibrator were observed at the beginning and end of the run whilst the rest of the observation was devoted to a cycle of1.5minutes on the phase calibrator and5minutes on GRS1915+105.

Initial data editing and calibration were performed using the stan-dard MERLIN d-programs and the data were then transferred to the NRAO Astronomical Image Processing System(AIPS).Within AIPS the data were processed via the MERLIN pipeline,which calibrates and images the phase reference source and then applies these solutions to the target source.This process also derives instrumental polarisation corrections and calibrates the linear polarisation position angle,using 3C286as the calibrator and assuming a position angle of33?for its E vector.The position angles measured by MERLIN and ATCA are consistent with the same value,independently con?rming the position angle calibration of each array.

Further self calibration was then carried out within AIPS and GRS 1915+105imaged in total intensity and stokes Q and U.These maps were then combined using the AIPS task PCNTR to produce the?nal maps with total intensity contours and vectors denoting the direction and strength of linear polarisation.

Note that we can be con?dent both from previous studies(e.g. Mirabel&Rodriguez1994;Fender et al.1999)and these data(Mc-Cormick et al.in prep)that the component(s)to the south east(la-belled in Fig3as‘SE1’)is‘approaching’,and component(s)to the north west are‘receding’(although in fact both sides of the jet have Doppler factorsδ<1).

2.2.V ariable circular polarisation

In both sets of ATCA observations,GRS1915+105is unambiguously detected as a source of circularly polarised radio emission(Stokes V).

2.2.1.2001January17

In2001January(Fig2,left panels),signi?cant CP is measured at all four ATCA frequencies,from1–9GHz.The total?ux density is clearly declining,indicating the decay phase of a major?are,but there is also signi?cant variability superposed on the relatively smooth decline, preferentially at higher frequencies.This is almost certainly indicative of repeated activity in the core,corresponding to fresh ejection events. The spectral indices support this interpretation;between1.4–2.9GHz the spectrum is signi?cantly?atter than expected for optically thin synchrotron emission;between4.8–8.6GHz it is displaying the rapidly

6Rob Fender Epoch 1 : March 24 Peak Flux : 34.6 mJy / Beam Countour Level : 0.37 mJy

Epoch 2 : March 25 Peak Flux : 18.5 mJy / Beam

Countour Level : 0.37 mJy

Epoch 3 : March 26 Peak Flux : 19.5 mJy / Beam Countour Level : 0.49 mJy Epoch 4: March 27

Peak Flux : 26.8 mJy / Beam

Countour Level : 0.65 mJy

Epoch 5 : March 29

Peak Flux : 17.9 mJy / Beam

Countour Level : 0.23 mJy

0 100 50 -50

Milliarcseconds

T i m e (M J D - 51000)992.3993.3994.3995.3996.3

997.3SE1

Figure 3.MERLIN imaging of relativistic (superluminal)ejections from GRS 1915+105.We can con?dently associated most of the ?ux measured by ATCA (Fig 2)with the ejected component to the SE of the core.Contours are at -1,1,2,4,8,16,32,64times the level listed by each image.

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Figure4.Mean total?ux density,linear and circular polarisation spectra for GRS 1915+105in2001January and March,from the data in table1.Note,in2001 January,the inversion in the linear to circular polarisation ratio between the two lowest frequencies.

varying behaviour associated with‘core’ejection events(Fender et al. 2002a).

Inspection of the total?ux and spectral index light curves indicates there were at least four separate ejection events contributing to the light curve at this epoch.Fig1also indicates that this outburst was more prolonged than that in2001March(see below).

The CP?ux is clearly rising to lower frequencies,but the exact fractional spectrum is di?cult to determine as the multiple components contributing to the observed emission are unresolved with ATCA.Table 1lists the mean total,linearly polarised and circularly polarised?ux densities,and Fig4plots these both as total and fractional spectra.We also note that there are measurements when the Stokes V?ux is not signi?cantly non-zero,and even a few points where it appears to have changed sign.However,(i)the mean Stokes V?uxes are signi?cant, and negative(at both epochs),(ii)the apparent Stokes V sign change has a signi?cance<2σand so we do not consider it convincing.

2.2.2.2001March23

In2001March,GRS1915+105was again observed to?are in our15 GHz monitoring program,reaching～160mJy at MJD51990.43.This time we triggered both ATCA and MERLIN–in fact the?rst epoch of

8Rob Fender

Table I.Mean total?ux densities,linear and circular polarisations of GRS1915+105as measured by ATCA,2001January and March.The spectra resulting from these data are plotted in Fig4.

1384425±401.0±0.2?1.57±0.21

2496368±373.7±0.6?0.96±0.22

4800262±268.5±1.1?0.63±0.11173±175.2±0.3?0.56±0.07 8640193±119.2±0.7?0.48±0.10113±113.2±0.2?0.31±0.07

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Figure5.ATCA4800&8640MHz light curves compared with exactly adjacent MERLIN5000MHz total intensity light curves on March23,2001.Also indicated are the levels of the core and SE1(ejecta)components as measured from the MERLIN image at that epoch.From combination of this and the images in Fig3we can be con?dent that ATCA measured mostly?ux from the ejected component.

The smooth rotation in the electric vector position angle seems to be a little at odds with both the MERLIN observations of Fender et al.(1999)in which the electric vector was varying seemingly erratically from day-to-day,and the observations presented here(both ATCA and MERLIN)for2001March,in which the vector remains approximately constant.Such smooth rotation indicates either a genuinely rotating jet or,perhaps more likely,a smooth change in the(projected)position angle of the magnetic?eld in the emitting region–such as a global curved structure in the jet.

Similar behaviour,‘polarisation rotator events’–see Saikia&Salter (1988)and references therein-has also been observed in AGN.While initially interpreted as physical rotation of the magnetic?eld structure (which could directly link a jet to e.g.removal of angular momentum from an accretion?ow)the more favoured interpretation is the forma-tion of a shock inclined at some angle to the line of sight.The lack of the repeat of this phenomenon in2001March would seem to indicate it does not re?ect physical rotation of the jet,which we would assume either always happens or never happens.However,more recently Gomez et al. (2001)have interpreted the steady rotation of the linear polarisation vector of a superluminal component in the jet of the AGN3C120as indicating an underlying twisted(helical)magnetic?eld structure.

10Rob Fender

2.4.Discussion

The origin of a circularly polarised component in the radio emission from AGN and X-ray binaries remains uncertain.However,in both classes of object the bulk of the radio emission can be con?dently assumed to arise from similar physical processes,namely synchrotron emission from relativistic electrons in a magnetised plasma?owing away from the central black hole(or neutron star)in collimated jets.Since we have every reason to believe that the Stokes I,and probably Q and U,?uxes from these objects arise via the same processes,we can hope that by studying the origin of Stokes V in X-ray binaries we may shed light on its origin in AGN,and vice versa.

2.4.1.Association of CP with young ejections

What can we learn from these observations of GRS1915+105?From analysis of the combined ATCA and MERLIN data sets for2001March, we are con?dent that the measured circularly polarised radio emission is associated with the relativistic ejection SE1.Our reasoning is as follows:

1.The decreasing Stokes I?ux measured at both epochs(most obvi-

ously2001March)arises from ejected components whose radio?ux decays steadily,probably due to adiabatic expansion losses,with time.This can be inferred both from past experience and directly from the MERLIN observations which directly image the fading ejecta as they propagate away from the core.In Fig5we show the ATCA light curves,plus the total?ux light curve from MERLIN and the?uxes of the two components(core and ejection SE1).The core stays at roughly the same?ux level over all of the?rst?ve epochs of MERLIN imaging,whereas the?ux of SE1continues to fade–from this we can infer that the bright and decreasing ?ux observed by ATCA is dominated by emission from ejected component SE1.

2.There is a signi?cant correlation between the Stokes I and(-)Stokes

V?uxes,especially at8640MHz.For all epochs,and the combined data sets,there is a signi?cant rank correlation between Stokes

I and V at8640MHz;at4800MHz the correlation is marginal.

Since the Stokes I,as argued above,is associated with SE1,we can therefore be con?dent that the Stokes V?ux also arises primarily in SE1.Note that we cannot rule out a Stokes V?ux of amplitude |V|≤0.2mJy associated with the core,based on Fig6.

3.Furthermore,the MERLIN imaging clearly shows that the linearly

polarised radio emission also arises in component SE1.Therefore

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Figure6.Correlation between Stokes I and V?ux densities.Spearman rank corre-lation coe?cients are listed in table2.The8640MHz Stokes I and V are(rank) correlated at>99%con?dence at both epochs,and as a combined data set.The4800 MHz data are marginally correlated at the≤70%con?dence level in2001January, rising to～90%con?dence in2001March and for both data sets combined.

we are able to accurately measure the fractional linear and circular polarisation(ie.all Stokes parameters)for the synchrotron emission from a single optically thin component

https://www.sodocs.net/doc/8a4191931.html,parison of GRS1915+105with AGN,Sgr A*and M81* Rayner et al.(2000)and Homan et al.(2001)have established that most AGN have ratios of linear to(absolute)circular polarisation≥10, whereas the low-luminosity radio cores Sgr A*and M81*have ratios ≤1(Bower et al.1999;Sault&Macquart1999;Brunthaler et al. 2001).It is interesting that for the two XRBs for which we have so far measured CP,we?nd both situations,depending on the frequency observed.In both SS433(Fender et al.2000)and GRS1915+105, LP

12Rob Fender

power law component,unlike the two X-ray binaries discussed here, which will further reduce the expected LP(Bower et al.1999).

In addition,the majority of AGN are found to maintain the same sign of Stokes V on timescales from months to decades(Komesaro?et al.1984;Rayner et al.2000;Homan et al.2001).Bower et al.(2002) have more recently shown that Sgr A*has maintained the same sign (and level)of CP over20years.In comparison,for X-ray binaries there are two observations of SS433separated by10days(Fender et al. 2000),and two observations of GRS1915+105separated by65days presented here;in both cases the sign remains the same.Of course the X-ray binary sample is extremely small,but should expand rapidly in the near future,with several events per year bright enough to mean CP at the<0.1%level(Fender&Kuulkers2001).It is interesting to note that if accretion timescales scale linearly with mass from X-ray binaries to AGN(e.g.Sams,Eckart&Sunyaev1996)then timescales of tens of days for SS433and GRS1915+105would correspond to timescales of thousands of years for Sgr A*and millions of years for some AGN–ie.we may have already probed longer in‘accretion time’than in all the studies of AGN to date!However,McCormick,Fender&Spencer (these proceedings)report an apparent secular sign-change in Stokes V from SS433over longer timescales than those reported in Fender et al. (2000),possibly indicating some long-term evolution of the magnetic ?eld geometry(see also Ensslin,these proceedings).

2.4.

3.Origins of the CP component

The CP spectrum detected from GRS1915+105is rather similar to that observed from SS433(Fender et al.2000),being observed over a broad range(1–9GHz)and with a decreasing Stokes V(although not necessarily V/I)spectrum.The broadband nature of the CP spectrum suggests that coherent emission mechanisms are unlikely.Furthermore, we do not consider the birefringent scintillation mechanism of Macquart &Melrose(2000)very likely either,since the CP component seems to be associated with a physical event in the source,yet because of the high velocities of the ejecta it is likely to be a large distance from the source during the periods in which we measured CP.

This leads us(once again)to consider one of two mechanisms most viable,an intrinsic CP component to the synchrotron emission or LP→CP conversion(‘repolarisation’).Do we have any evidence in favour of either of these?The intrinsic synchrotron mechanism should,naively, produce a well-de?nedν?0.5V/I spectrum in a homogenous,optically thin,source.Our observations in2001March match these criteria quite closely(supported by direct imaging of a single ejection event with MERLIN),but unfortunately we only have a two-point CP spectrum

13 at this epoch.In addition,the relatively low signal-to-noise ratio of the CP detections only allows us to constrain the(-V)/I spectral index to beα?V/I=?0.3±0.3.In2001January the situation is rather more complex,the mean?ux and polarisation spectra certainly containing the contributions of multiple components with di?erent optical depths. The data may suggest that the CP arises preferentially in‘core’com-ponents with the highest densities and optical depths,similar to the situation in AGN.This interpretation may favour instead the LP→CP ‘repolarisation’mechanism,which will operate most e?ciently at higher optical depths,but currently the data are not su?ciently constraining.

However,both mechanisms require a signi?cant population of low-energy electrons.In principle,a strong probe of this requirement could be obtained by measuring the low-frequency extent of radio emission during outbursts of GRS1915+105and other systems.Prompt obser-vations at MHz frequencies could probe the electron distributions at Lorentz factors of30and lower(based on calculations in Fender et al. 1999)although,depending on the energy and magnetic?eld density in the ejecta,this emission may be self-absorbed.The consequences for the energetics of ejection events would be signi?cant,especially in the case of a neutral baryonic plasma with one proton associated with each electron.

3.CP from other X-ray binaries

In this section I shall present a brief compilation of measurements(in most cases limits)on radio CP from other X-ray binaries,and also compare the CP:LP ratio in X-ray binaries with that observed from blazars and low-luminosity AGN.

3.1.Measurements of other systems

As well as the clear detections of CP from the three X-ray binary jet sources SS433,GRO J1655-40and GRS1915+105(see also Mc-Cormick et al.,and Macquart,these proceedings,and references therein), there are also limits on CP from other X-ray binaries,some of which are stringent.In table3I list the measurements and limits on CP from X-ray binaries.

It is interesting to note that the three sources from which CP has been strongly detected have jets which are close to the plane of the sky (60?<θ<90?).However,the jets from Cir X-1,V4641Sgr and Cyg X-3are believed to be very close to the line of sight(θ<15?).The limits on the CP from Cyg X-3and V4641are particularly stringent.

14

Rob Fender 0.010.1

1

101

10

-C P / L P

Frequency (GHz)1915 epoch 11915 epoch 2SS433 epoch 1SS433 epoch 2

Figure 7.Ratio of circular to linear polarisation (equivalent to the R CL parameter of Brunthaler et al.2001)for two X-ray binaries SS 433and GRS 1915+105as a function of frequency.For both sources the ratio is <1at the lowest frequency (1.4GHz)but >1at the three higher frequencies.

The limits obtained on CP in both cases are between 5–10times below the levels of V/I detected from SS 433,GRS 1915+105and GRO J1655-

40.This indeed seems like a hint that CP,at least from X-ray binaries,is stronger when the jet is viewed approximately ‘side on’.However,it should also be noted that both sources show evidence (either on the speci?c occasion or others)for a strongly self-absorbed outburst,unlike e.g.SS 433which is nearly always optically thin.In addition,the orientation e?ect would be rather contrary to the observations of AGN,in which strong CP is observed from ‘blazars’with approximately face-on jets (e.g.Homan et al.2001).

3.2.The CP:LP ratio and comparison with AGN

Both Homan et al.(2001)and Brunthaler et al.(2001)investigate the ratio of circular to linear polarisation.Speci?cally,Brunthaler et al.(2001)de?ne R CL as the ratio of fractional CP to fractional LP,and ?nd that R CL >1for the two low-luminosity AGN (LLAGN)M81*and Sgr A*between 1–15GHz (in fact no LP is detected at all from

15 Table II.Measurements and limits(3σ)on CP from radio-emitting X-ray binaries.All the systems are believed to host a black hole,except Cir X-1 which probably contains a neutron star,and Cyg X-3for which there is

little evidence either way.

Source Freq.(GHz)%CP Ref

GRS1915+105 1.384～0.3Fender et al.(2002b)

2.496～0.2

4.800～0.2–0.3

8.640～0.2–0.3

GX339-48.64<0.7Corbel et al.2000

Cyg X-3 5.0<0.08de Bruyn(https://www.sodocs.net/doc/8a4191931.html,m.)

(Sept2001)

Cyg X-1 4.86<0.49Brocksopp,Fender,

8.46<0.33Bower&Clarke(2003)

14.94<0.63

16

Rob Fender 00.2

0.4

0.60.8

1

F r a c t i o n a l (a b s o l u t e ) C P (%)Fractional LP (%)Figure 8.Fractional CP vs.fractional LP for the two X-ray binaries SS 433and GRS 1915+105,the two LLAGN Sgr A*and M81*,and the most signi?cant detections from the blazar survey of Homan et al.(2001).Note that for the X-ray binaries and LLAGN we present measurements at multiple frequencies,whereas for the blazars all measurements are at 5GHz.

less than unity depends on the degree of Faraday depolarisation in the emitting plasma.It should be noted that for the X-ray binary GRO J1655-40,which showed a high degree of linear polarisation,R CL <1at all frequencies (Macquart et al.2002).

In Fig 8I plot fractional CP against fractional LP for these two X-ray binaries,plus the two LLAGN and the most signi?cant detections from the blazar sample of Homan et al.(2001).The two groups of AGN are separated by the line corresponding to LP=CP,whereas the X-ray binaries lie either side of the line depending on frequency.It is interesting to note that,since for both LLAGN there are only upper limits on LP,it is only for the low-frequency observations of the X-ray binaries that the exact value of R CL <1,when it is less than unity,has been measured.

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4.Conclusions

Circularly polarised radio emission has been clearly detected from three X-ray binaries,all of which are associated with powerful jets which share many of the characteristics of AGN.In the cases of both GRS 1915+105and GRO J1655-40,strong and variable circular polarisa-tion was associated with clearly resolved ejection https://www.sodocs.net/doc/8a4191931.html,paring the fractional circular polarisation spectrum,and circular to linear polarisation ratio,clear similarities with AGN are noted.In particular, multi-frequency measurements of X-ray binaries,which reveal that the circular to linear polarisation ratio increases with wavelength,support interpretations in which the dominant factor for this ratio is the degree of Faraday depolarisation in the source.

Acknowledgements

The author would like to thank all the participants at the workshop in Amsterdam for many discussions,some useful,some useless but amusing.In addition,he would like to thank Ger de Bruyn and Bob Sault for providing information on unpublished results.

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