LS I +61 303 Sky Map
Significance map of the region around LS I +61 303. Top: Observations during orbital phases 0.8 to 0.5. Bottom: Observations around apastron (orbital phases 0.5 to 0.8).

 The X-Ray Binary LS I +61 303


Gamma-ray binaries are a rare class of galactic very-high-energy (VHE) gamma-ray emitter.  LS I + 61 303 (distance 2 kpc, orbital period 26.5 days) is one of only a few high-mass X-ray binaries currently detected in VHE gamma-rays. This binary consists of a massive  Be-type star surrounded by a dense circumstellar disk and a stellar size compact object. The system is relatively small, extending over just a few astronomical units. This fact, combined with the high density of low-energy photons from the stellar companion and an orbital phase dependent geometry, results in a complex environment for the acceleration of high-energy charged particles and the production and absorption of gamma rays. The unknown nature of the compact object (neutron star or black hole) allows two very different scenarios to provide the necessary power for the production of VHE gamma rays. In the microquasar model, charged particles are accelerated in an accretion-driven relativistic jet, similar to the mechanism known from extragalactic blazars . The second scenario assumes that the compact object is a pulsar . Particles are accelerated in the shock created by the collision of the expanding pulsar wind with the equatorial disk or wind of the companion star.

LS I +61 303 was observed over several orbital cycles with VERITAS during the construction phase with a 2-telescope array. The binary has been detected as a source of VHE gamma-rays with energies above 300 GeV at high significance (8.4 sigma). The detected flux is measured to be strongly variable; interestingly, the maximum flux is found during most orbital cycles at apastron (see Figures, periastron takes place at phase 0.23, apastron is at phase 0.73). This indicates a strong dependence of particle acceleration and energy loss mechanism on the relative position. The production, variability and spectral energy distribution of the VHE gamma-rays can be explained by inverse Compton scattering of low-energy photons by electrons with energies in the TeV range. Future observations with the much more sensitive full VERITAS array together with contemporaneous measurements in other wavelengths will tackle the open question whether the high-energy electron are acceleration in accretion-driven jet or in a pulsar wind shock.

 For more information on the science of this object please see V.A.Acciari et al, The Astrophysical Journal, Volume 679, Issue 2, pp. 1427-1432 (arXiv:0802.2363).  You may also contact Gernot Maier .