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Register a new userVery High-energy Gamma-ray Emission from LS I +61$^circ$ ~303 Binary
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D. B. Kieda
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LS I +61$^circ$ ~303 is one of around ten gamma-ray binaries detected so far which has a spectral energy distribution dominated by MeV-GeV photons. It is located at a distance of 2 kpc and consists of a compact object (black hole or neutron star) in an eccentric orbit around a 10-15 $M_{odot}$ Be star, with an orbital period of 26.496 days. The binary orbit modulates the emission ranging from radio to TeV energies. A second, longer, modulation period of 1667 days (the super-orbital period) has also been detected from radio to TeV observations. The VERITAS imaging atmospheric Cherenkov telescope array has been observing LS I +61$^circ$ ~303 since 2006, and has accumulated a dataset that fully covers the entire orbit. Increased coverage of the source in the very-high-energy band is currently underway to provide more results on the modulation pattern, super-orbital period, and orbit-to-orbit variability at the highest energies. The spectral measurements at the highest energies will reveal more information about gamma-ray production/absorption mechanisms, the nature of the compact object, and the particle acceleration mechanism. Using >150 hrs of VERITAS data, we present a detailed study of the spectral energy distribution and periodic behavior of this rare gamma-ray source type at very-high energy.
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The discovery of very high energy (VHE) gamma-ray emitting X-ray binaries has triggered an intense effort to better understand the particle acceleration, absorption, and emission mechanisms in compact binary systems, which provide variable conditions along eccentric orbits. Despite this, the nature of some of these systems, and of the accelerated particles producing the VHE emission, is unclear. To answer some of these open questions, we conducted a multiwavelength campaign of the VHE gamma-ray emitting X-ray binary LS I +61 303 including the MAGIC telescope, XMM-Newton, and Swift during 60% of an orbit in 2007 September. We detect a simultaneous outburst at X-ray and VHE bands, with the peak at phase 0.62 and a similar shape at both wavelengths. A linear fit to the simultaneous X-ray/VHE pairs obtained during the outburst yields a correlation coefficient of r=0.97, while a linear fit to all simultaneous pairs provides r=0.81. Since a variable absorption of the VHE emission towards the observer is not expected for the data reported here, the correlation found indicates a simultaneity in the emission processes. Assuming that they are dominated by a single particle population, either hadronic or leptonic, the X-ray/VHE flux ratio favors leptonic models. This fact, together with the detected photon indices, suggests that in LS I +61 303 the X-rays are the result of synchrotron radiation of the same electrons that produce VHE emission as a result of inverse Compton scattering of stellar photons.
The TeV binary system LS I +61$^circ$ 303 is known for its regular, non-thermal emission pattern which traces the orbital period of the compact object in its 26.5 day orbit around its B0 Ve star companion. The system typically presents elevated TeV emission around apastron passage with flux levels between 5% and 15% of the steady flux from the Crab Nebula (> 300 GeV). In this article, VERITAS observations of LS I +61$^circ$ 303 taken in late 2014 are presented, during which bright TeV flares around apastron at flux levels peaking above 30% of the Crab Nebula flux were detected. This is the brightest such activity from this source ever seen in the TeV regime. The strong outbursts have rise and fall times of less than a day. The short timescale of the flares, in conjunction with the observation of 10 TeV photons from LS I +61$^circ$ 303 during the flares, provides constraints on the properties of the accelerator in the source.
The discovery of emission of TeV gamma rays from X-ray binaries has triggered an intense effort to better understand the particle acceleration, absorption, and emission mechanisms in compact binary systems. Here we present the pioneering effort of the MAGIC collaboration to understand the very high energy emission of the prototype system LS I +61 303. We report on the variable nature of the emission from LS I +61 303 and show that this emission is indeed periodic. The system shows regular outburst at TeV energies in phase phi=0.6-0.7 and detect no signal at periastron (phi~ 0.275). Furthermore we find no indication of spectral variation along the orbit of the compact object and the spectral energy distribution is compatible with a simple power law with index Gamma=2.6+-0.2_(stat)+-0.2_(sys). To answer some of the open questions concerning the emission process of the TeV radiation we conducted a multiwavelength campaign with the MAGIC telescope, XMM-Newton, and Swift in September 2007. We detect a simultaneous outburst at X-ray and TeV energies, with the peak at phase 0.62 and a similar shape at both wavelengths. A linear fit to the strictly simultaneous X-ray/TeV flux pairs provides r=0.81 -0.21 +0.06. Here we present the observations and discuss the implications of the obtained results to the emission processes in the system.
We study the characteristics of the TeV binary LS I +61$^circ$ 303 in radio, soft X-ray, hard X-ray, and gamma-ray (GeV and TeV) energies. The long term variability characteristics are examined as a function of the phase of the binary period of 26.496 days as well as the phase of the super orbital period of 1626 days, dividing the observations into a matrix of 10$times$10 phases of these two periods. It is found that the long term variability can be described by a sine function of the super orbital period, with the phase and amplitude systematically varying with the binary period phase. We also find a definite wavelength dependent change in this variability description. To understand the radiation mechanism, we define three states in the orbital/ super orbital phase matrix and examine the wide band spectral energy distribution. The derived source parameters indicate that the emission geometry is dominated by a jet structure showing a systematic variation with the orbital/ super orbital period. We suggest that LS I +61$^circ$ 303 is likely to be a micro-quasar with a steady jet.
LS I +61 303 is one of only a few high-mass X-ray binaries currently detected at high significance in very high energy gamma-rays. The system was observed over several orbital cycles (between September 2006 and February 2007) with the VERITAS array of imaging air-Cherenkov telescopes. A signal of gamma-rays with energies above 300 GeV is found with a statistical significance of 8.4 standard deviations. The detected flux is measured to be strongly variable; the maximum flux is found during most orbital cycles at apastron. The energy spectrum for the period of maximum emission can be characterized by a power law with a photon index of Gamma=2.40+-0.16_stat+-0.2_sys and a flux above 300 GeV corresponding to 15-20% of the flux from the Crab Nebula.
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