The astrometric/morphological variability and the birth place of LS 5039
aa r X i v : . [ a s t r o - ph . H E ] J a n The astrometric/morphological variability andthe birth place of LS 5039
J. Mold´on, M. Rib´o and J. M. Paredes
Abstract
LS 5039 is one of the few X-ray binaries detected at VHE, and potentiallycontains a young non-accreting pulsar. The outflow of accelerated particles emittingsynchrotron emission can be directly mapped with high resolution radio observa-tions. The morphology of the radio emission strongly depends on the properties ofthe compact object and on the orbital parameters of the binary system. We presentVLBA observations of LS 5039 covering an orbital cycle, which show morpholog-ical and astrometric variability at mas scales. On the other hand, we discuss thepossible association of LS 5039 with the supernova remnant SNR G016.8 − LS 5039 is a high mass X-ray binary containing a compact object of unknown mass(1.5–10 M ⊙ ) that orbits an O6.5 V((f)) star every 3.9 days. The system has an eccen-tric orbit ( e = . E >
100 GeV) with a clearorbital modulation (Aharonian et al., 2006; Abdo et al., 2009). Several theoreticalmodels have been developed to explain the multiwavelength orbital behavior ofLS 5039. The HE/VHE emission is basically interpreted as the result of inverseCompton upscattering of stellar UV photons by relativistic electrons. The acceler-ation of electrons seems to be produced by shocks between the relativistic windof a young non-accreting pulsar and the wind of the stellar companion (Dubus,2006; Sierpowska-Bartosik & Torres, 2008). No radio pulses have been detected at1.4 GHz Morris et al. (2002), although the strong free-free absorption with the stel-
J. Mold´on · M. Rib´o · J. M. ParedesDepartament d’Astronomia i Meteorologia and Institut de Ci`encies del Cosmos (ICC), Universitatde Barcelona (UB/IEEC), Mart´ı i Franqu`es 1, 08028, Barcelona, Spain,e-mail: [email protected],[email protected],[email protected] lar wind may prevent any detection of pulsed radio emission. Therefore, the natureof the compact object (black hole or pulsar) is unknown.In the non-accreting pulsar scenario the shocked material is contained by the stel-lar wind behind the pulsar, producing a ’bow’ shaped nebula extending away fromthe stellar companion. The high energy emission is produced in the region where thewind pressures are balanced, while a tail of accelerated particles forms behind thepulsar. The cooling processes of these accelerated particles along the adiabaticallyexpanding flow produce the non-thermal broadband emission from radio to X-rays.The morphology is similar to the one expected in isolated pulsars moving throughthe ISM but, as a consequence of the orbital motion of the binary system, the tail ofthe flow bends following an elliptical path during the orbital cycle.
Free-free absorption on the stellar wind becomes optically thin at distances ∼ The first VLBA observation of LS 5039 at 5 GHz, conducted in 1999, showed a ∼
20 mJy central core and bipolar extended emission of a few mJy extending over6 mas on the plane of the sky (Paredes et al., 2000). Similar observations at twodifferent orbital phases (Rib´o et al., 2008) showed a changing morphology at masscales: the core component had a constant flux density, and the Position Angle (P.A.)of the direction of the elongated emission changed by 12 ± ◦ between both runs,with a remarkable symmetry change. Therefore, morphological changes at 5 GHzoccur on timescales of the order of the orbital period.To further study the morphological variability, we observed the source in 2007during five consecutive days, to cover a full orbit of 3.9 days, and an extra day todisentangle between orbital or secular variability. The observations were centered atthe orbital phases 0.98, 0.23, 0.49, 0.75, and 0.00, each one spanning 6 hours (0.07in phase), computed using the ephemerides of Aragona et al. (2009). The observa- he astrometric/morphological variability and the birth place of LS 5039 3 tions were phase-referenced to the phase calibrator J1825 − ∼ −
50 and − ◦ for phases in the range 0.5–0.0, andP.A. 120 ◦ at phase 0.23. Therefore, a subtle change in morphology happens in lessthan 20 hours after periastron.The resolution of the phase-referenced images is limited by the scatter broaden-ing of the calibrator. The astrometric accuracy, extrapolated from the mean disper-sion of the astrometric check source, is 0.23 and 0.27 mas in right ascension anddeclination, respectively (Mold´on et al., in preparation). The positions of the peakof the emission for phases 0.5–0.0 are compatible within errors. The peak of theemission at phase 0.23 shows a displacement of 2 . ± . ◦ , nearlyopposite to the extended emission sense. This indicates that the variations of theextended emission with respect to the core are a combination of intrinsic variabilityof this extended emission and an absolute displacement of the core component. As a by-product, these phase-referenced observations provide one average preciseposition of the source in the sky. Rib´o et al. (2002) computed the trajectory of LS5039 for the last 10 yr using optical/radio astrometry from 1905 to 2002. Theirresult marginally suggests an association with SNR G016.8 − The emission at 5 GHz traces the short-lived electrons of the outflow up to 15 AU.The morphological changes along the orbit allow us to model the velocity, the en-ergy, and the cooling time-scale of the flow of particles that originates the extendedemission. The morphological and astrometric information constrains the inclinationof the orbit ( i ), a key parameter of the system. The mass function of the system and i yield the mass of the compact object. On the other hand, it should be possible toclearly trace the peak position along the orbit with observations at higher frequen- J. Mold´on, M. Rib´o and J. M. Paredes
Fig. 1
Wide field radiomap of LS 5039, showingthe position of the nearbySNR G016.8 − yr ago. D E C L I NA T I O N ( J2000 ) RIGHT ASCENSION (J2000)18 26 30 15 00 25 45 30 15 00 24 45 30-14 303540455055-15 00 o b = - . SNR G016.8-01.1 RCW 164LS 5039
PSR J1825−1446B1822−14 cies (where the phase calibrator is much more compact), yielding direct informationon the absorption around the system.The VLBA images from 2007 at 5 GHz contain relevant information to test themodels (Mold´on et al. in prep.), but the lack of continous astrometric informationfollowing the peak of the emission at all orbital phases is a key point to better con-strain the physical properties of the system. If the peak position is expected to beshifted between 1–2 mas, it is not possible to unambiguously distinguish the dis-placement of the peak from intrinsic variations of the extended emission. The flowvelocity and cooling times strongly depend on this ambiguity, which can be disen-tangled with accurate astrometry, to be obtained in the future.