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The Binary Be2-Pulsar PSR B1259-63/ SS 2883 System: A VHE γ-Ray Candidate

27/12/2011

PSRB1259−63 / SS 2883 is a binary system consisting of a ∼48ms pulsar in orbit around a massive B2e companion star (Johnston et al. 1992a, 1992b). Due to the highly eccentric orbit of the pulsar, approximately every 3 and a half years it is just 10m (some 6210000 miles) apart from its Be-class* companion during their periastron period. Be stars are known to have non-isotropic stellar winds forming an equatorial disk with enhanced mass outflow (e.g.Waters et al. 1988). In the case of PSR B1259−63, timing measurements suggest that the disk is inclined with respect to the orbital plane (Wex et al. 1998), probably because the neutron star received a substantial birth kick after the core-collapse of its progenitor, causing the pulsar to cross the disk two times near periastron. Such unique properties make the binary system PSRB1259−63 an excellent cosmic laboratory for the study of pulsar winds interacting with a changing environment in the presence of an extremely intense photon field.

N.B. Be stars are typically variable and can either be classified as Gamma-Cassiopeiae variables due to the transient nature of the disk and the scattering processes, or as Lambda-Eridani variables on account of their rapid pulsational nature and subsequent rotational distortion.

The intense photon field provided by the Be-type companion star not only plays an important role in the cooling of relativistic electrons but also serves as perfect target for the production of high energy (HE) γ-rays through inverse Compton scattering (Tavani et al. 1996; Kirk et al. 1999; Ball & Kirk 2000; Ball & Dodd 2001; Murata et al. 2004). Some of these emission models predict wind powered shock acceleration of electrons to multi-TeV energies, radiating predominantly through the synchrotron and inverse Compton channels, with the main energy release in the X- and high energy γ-ray bands, respectively.

The unpulsed non-thermal X-ray emission detected from PSRB1259−63 throughout its orbital phase in 1992 to 1996 by  the ROSAT and ASCA satellites (Cominsky et al. 1994; Kaspi et al. 1995; Hirayama et al. 1996) generally supports the synchrotron origin of X-rays. The spectrum of the synchrotron radiation seems to extend to hard X-rays/low energy γ-rays as shown by OSSE (Grove et al. 1995) and recently confirmed by observations with the INTEGRAL satellite (Shaw et al. 2004).

Since the companion star provides the dominant source of photons for inverse Compton scattering, the target photon density is well known throughout the entire orbit. Therefore, the ratio of X-ray flux to high energy γ-ray flux depends only on the strength of the ambient magnetic field (Aharonian et al. 2005). Although the latter can be estimated within a general magneto-hydrodynamic treatment of the problem, it contains large uncertainties.

Kirk et al. (1999) studied the light curves of very-high-energy (VHE) γ-rays under the assumption of a  dependence of the magnetic field which implies that the ratio of the energy density of the photon field to that in the magnetic field, , is independent of orbital phase, .

They also assumed that the position of the termination shock as well as the strength of the magnetic field is not affected by the disk of the B2e star. Under such assumptions, they predicted an asymmetric γ-ray light curve with respect to periastron (because of the inclination of the orbit with respect to the line of sight and the dependence of the inverse Compton γ-ray emissivity on the scattering angle), with an increase towards periastron and monotonic decrease after the passage of periastron. However, one might possibly expect significant deviation from such a simplified picture given the apparent strong impact of the disk on the pulsar wind termination as seen in the X-ray light curve (Tavani & Arons 1997).

Moreover, during the time periods of interaction of the pulsar wind with the equatorial disk one may expect, in addition to the inverse Compton γ-rays, a new component of γ-radiation associated with interactions of accelerated electrons and possibly also protons with the dense ambient gas (Kawachi et al. 2004). Up to now, the theoretical understanding of the properties of this complex ystem, involving pulsar and stellar winds interacting with each other, is quite limited because of the lack of constraining observations.

Nevertheless, the fortunate combination of: (1) the high spin-down luminosity of the pulsar, , which is partially converted into populations of ultra-relativistic particles, (2) the presence of the intense target photon field provided by the companion star; and (3) the relatively small distance to the source () makes this object a very attractive candidate for VHE γ-ray emission.

Journal References:

  • Kirk, J. G.; Ball, L.; Skjaeraasen, O. (1999) Inverse Compton Emission Of TeV Gamma-rays From PSR B1259-63Astroparticle Physics, 10 (1): pp. 31-45.
  • Abdo, A.A. et al. (2010) Fermi LAT Detection Of GeV Gamma-ray Emission From The Binary System PSRB1259-63. The Astronomer’s Telegram, #3085.
  • Abdo, A.A. et al. (2011) Discovery Of High-Energy Gamma-ray Emission From The Binary System PSR B1259-63/LS 2883 Around PeriastronThe Astrophysical Journal: Letters, 736 (1): Article I.D.: L11.
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