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Register a new userThe hour-timescale GeV flares of PSR B1259-63 in 2017
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GeV flares from PSR B1259-63/LS 2883 were seen starting around 30 days after the two periastron passages in 2010 and 2014. The flares are clearly delayed compared to the occurrence of the X-ray and TeV flux peaks during the post-periastron disk crossing. While several attempts have been put forward to explain this phenomenon, the origin of these GeV flares remains a puzzle. Here we present a detailed analysis of the observational data taken by the Fermi and Swift observatories over the 2017 September periastron passage. For the first time, we find short-lived but powerful GeV flares on time scales of down to three hours. The onset of the GeV flaring period in 2017 is also delayed compared to those seen in 2011 and 2014. Supplemented by a re-analysis of previous data, we compare the Fermi/LAT, Swift/XRT and Swift/UVOT light curves in 2017 with those taken over the 2010 and 2014 periastrons, and difference in UVOT light curves are noted.
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PSR B1259-63 is a gamma-ray binary system composed of a high spindown pulsar and a massive star. Non-thermal emission up to TeV energies is observed near periastron passage, attributed to emission from high energy e+e- pairs accelerated at the shock with the circumstellar material from the companion star, resulting in a small-scale pulsar wind nebula. Weak gamma-ray emission was detected by the Fermi/LAT at the last periastron passage, unexpectedly followed 30 days later by a strong flare, limited to the GeV band, during which the luminosity nearly reached the spindown power of the pulsar. The origin of this GeV flare remains mysterious. We investigate whether the flare could have been caused by pairs, located in the vicinity of the pulsar, up-scattering X-ray photons from the surrounding pulsar wind nebula rather than UV stellar photons, as usually assumed. Such a model is suggested by the geometry of the interaction region at the time of the flare. We compute the gamma-ray lightcurve for this scenario, based on a simplified description of the interaction region, and compare it to the observations. The GeV lightcurve peaks well after periastron with this geometry. The pairs are inferred to have a Lorentz factor ~500. They also produce an MeV flare with a luminosity ~1e34 erg/s prior to periastron passage. A significant drawback is the very high energy density of target photons required for efficient GeV emission. We propose to associate the GeV-emitting pairs with the Maxwellian expected at shock locations corresponding to high pulsar latitudes, while the rest of the non-thermal emission arises from pairs accelerated in the equatorial region of the pulsar wind termination shock.
PSR B1259-63/LS2883 is a binary system composed of a pulsar and a Be star. The Be star has an equatorial circumstellar disk (CD). The {it Fermi} satellite discovered unexpected gamma-ray flares around 30 days after the last two periastron passages. The origin of the flares remain puzzling. In this work, we explore the possibility that, the GeV flares are consequences of inverse Compton-scattering of soft photons by the pulsar wind. The soft photons are from an accretion disk around the pulsar, which is composed by the matter from CD captured by the pulsars gravity at disk-crossing before the periastron. At the other disk-crossing after the periastron, the density of the CD is not high enough so that accretion is prevented by the pulsar wind shock. This model can reproduce the observed SEDs and light curves satisfactorily.
PSR B1259-63 is a gamma-ray binary system hosting a radio pulsar orbiting around a O9.5Ve star, LS 2883, with a period of ~3.4 years. The interaction of the pulsar wind with the LS 2883 outflow leads to unpulsed broad band emission in the radio, X-rays, GeV and TeV domains. While the radio, X-ray and TeV light curves show rather similar behaviour, the GeV light curve appears very different with a huge outburst about a month after a periastron. The energy release during this outburst seems to significantly exceed the spin down luminosity of the pulsar and the GeV light curve and energy release varies from one orbit to the next. In this paper we present for the first time the results of optical observations of the system in 2017, and also reanalyze the available X-ray and GeV data. We present a new model in which the GeV data are explained as a combination of the bremsstrahlung and inverse Compton emission from the unshocked and weakly shocked electrons of the pulsar wind. The X-ray and TeV emission is produced by synchrotron and inverse Compton emission of energetic electrons accelerated on a strong shock arising due to stellar/pulsar winds collision. The brightness of the GeV flare is explained in our model as a beaming effect of the energy released in a cone oriented, during the time of flare, in the direction of the observer.
The pulsar/massive star binary system PSR B1259-63 / LS 2883 is one of the best-studied gamma-ray binaries, a class of systems whose bright gamma-ray flaring can provide important insights into high-energy physics. Using the Australian Long Baseline Array we have conducted very long baseline interferometric observations of PSR B1259-63 over 4.4 years, fully sampling the 3.4-year orbital period. From our measured parallax of $0.38pm0.05$ mas we use a Bayesian approach to infer a distance of $2.6^{+0.4}_{-0.3}$ kpc. We find that the binary orbit is viewed at an angle of $154pm3$ degrees to the line of sight, implying that the pulsar moves clockwise around its orbit as viewed on the sky. Taking our findings together with previous results from pulsar timing observations, all seven orbital elements for the system are now fully determined. We use our measurement of the inclination angle to constrain the mass of the stellar companion to lie in the range 15-31$M_{odot}$. Our measured distance and proper motion are consistent with the system having originated in the Cen OB1 association and receiving a modest natal kick, causing it to have moved $sim$8 pc from its birthplace over the past $sim3times10^5$ years. The orientation of the orbit on the plane of the sky matches the direction of motion of the X-ray synchrotron-emitting knot observed by the Chandra X-ray Observatory to be moving away from the system.
Three periastron passages of the PSR B1259$-$63/LS 2883 binary system, consisting of a 48 ms rotation-powered pulsar and a $sim30$ M$_{odot}$ Be star, have been observed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope, in 2010, 2014, and 2017. During the most-recent periastron passage, sustained low-level gamma-ray emission was observed over a $sim3$-week long interval immediately after periastron, which was followed by an interval of no emission. Sporadic flares were detected starting 40 days post-periastron and lasted approximately 50 days, during which the emission displayed significant spectral curvature, variability on timescales as short as 1.5 minutes, and peak flux levels well in excess of the pulsar spin-down power. By contrast, during the 2010 and 2014 periastron passages, significant gamma-ray emission was not observed with the LAT until 30 and 32 days post-periastron, respectively. The previous flares did not exhibit spectral curvature, showed no short term variability, and did not exceed the pulsar spin-down power. The high flux and short timescales observed in 2017 suggest significant beaming of the emission is required and constrain the size of the emission region. The flares occur long enough after periastron that the neutron star should already have passed through the extended disk-like outflow, thus constraining options for target material and seed photon sources for inverse Compton models.
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