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Register a new userCharacterizing the VHE emission of LS I +61 303 using VERITAS observations
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The TeV gamma-ray binary LS I +61 303, approximately 2 kpc from Earth, consists of a low mass compact object in an eccentric orbit around a massive Be star. LS I +61 303 exhibits modulated VHE gamma-ray emission around its 26.5 days orbit, with strongest TeV emission during its apastron passage (orbital phases {phi}=0.55-0.65). Multiple flaring episodes with nightly flux variability at TeV energies have been observed since its detection in 2006. GeV, X-ray, and radio emission have been detected along the entire orbit, enabling detailed study of the orbital modulation pattern and its super-orbital period. Previously reported TeV baseline emission and spectral variations may indicate a neutron star flip-flop scenario, in which the binary system switches between accretor and propeller phases at different phases of the orbit. Since September 2007, VERITAS has observed LS I +61 303 over three additional seasons, accruing 220+ hours of data during different parts of its orbit. In this work, we present a summary of recent and long-term VERITAS observations of LS I +61 303. This analysis includes a discussion of the observed variation of TeV emission during different phases of the orbit, and during different superorbital phases.
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We present results from a long-term monitoring campaign on the TeV binary LSI +61 303 with VERITAS at energies above 500 GeV, and in the 2-10 keV hard X-ray bands with RXTE and Swift, sampling nine 26.5 day orbital cycles between September 2006 and February 2008. The binary was observed by VERITAS to be variable, with all integrated observations resulting in a detection at the 8.8 sigma (2006/2007) and 7.3 sigma (2007/2008) significance level for emission above 500 GeV. The source was detected during active periods with flux values ranging from 5 to 20% of the Crab Nebula, varying over the course of a single orbital cycle. Additionally, the observations conducted in the 2007-2008 observing season show marginal evidence (at the 3.6 sigma significance level) for TeV emission outside of the apastron passage of the compact object around the Be star. Contemporaneous hard X-ray observations with RXTE and Swift show large variability with flux values typically varying between 0.5 and 3.0*10^-11 ergs cm^-2 s^-1 over a single orbital cycle. The contemporaneous X-ray and TeV data are examined and it is shown that the TeV sampling is not dense enough to detect a correlation between the two bands.
We present the results of observations of the TeV binary LS I +61 303 with the VERITAS telescope array between 2008 and 2010, at energies above 300 GeV. In the past, both ground-based gamma-ray telescopes VERITAS and MAGIC have reported detections of TeV emission near the apastron phases of the binary orbit. The observations presented here show no strong evidence for TeV emission during these orbital phases; however, during observations taken in late 2010, significant emission was detected from the source close to the phase of superior conjunction (much closer to periastron passage) at a 5.6 standard deviation (5.6 sigma) post-trials significance. In total, between October 2008 and December 2010 a total exposure of 64.5 hours was accumulated with VERITAS on LS I +61 303, resulting in an excess at the 3.3 sigma significance level for constant emission over the entire integrated dataset. The flux upper limits derived for emission during the previously reliably active TeV phases (i.e. close to apastron) are less than 5% of the Crab Nebula flux in the same energy range. This result stands in apparent contrast to previous observations by both MAGIC and VERITAS which detected the source during these phases at >10% of the Crab Nebula flux. During the two year span of observations, a large amount of X-ray data were also accrued on LS I +61 303 by the Swift X-ray Telescope (XRT) and the Rossi X-ray Timing Explorer Timing (RXTE) Proportional Counter Array (PCA). We find no evidence for a correlation between emission in the X-ray and TeV regimes during 20 directly overlapping observations. We also comment on data obtained contemporaneously by the Fermi Large Area Telescope (LAT).
The MAGIC collaboration has recently reported correlated X-ray and very high-energy gamma-ray emission from the gamma-ray binary LS I +61 303 during ~60% of one orbit. These observations suggest that the emission in these two bands has its origin in a single particle population. We aim at improving our understanding of the source behaviour by explaining the simultaneous X-ray and VHE data through a radiation model. We use a model based on a one zone population of relativistic leptonic particles assuming dominant adiabatic losses located at the position of the compact object. The adiabatic cooling timescale is inferred from the X-ray fluxes. The model can reproduce the spectra and lightcurves in the X-ray and VHE bands. Adiabatic losses could be the key ingredient to explain the X-ray and partially the VHE lightcurves. From the best fit result, we obtain a magnetic field of B=0.2 G, a minimum luminosity budget of ~2x10^35 erg/s and a relatively high acceleration efficiency. In addition, our results seem to confirm that the GeV emission detected by Fermi does not come from the same parent particle population as the X-ray and VHE emission and the Fermi spectrum poses a constraint on the hardness of the particle spectrum at lower energies. In the context of our scenario, more sensitive observations would allow to constrain the inclination angle, which could determine the nature of the compact object.
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.
LS I +61 303 and LS 5039 are exceptionally rare examples of HMXBs with MeV-TeV emission, making them two of only five known or proposed gamma-ray binaries. There has been disagreement within the literature over whether these systems are microquasars, with stellar winds accreting onto a compact object to produce high energy emission and relativistic jets, or whether their emission properties might be better explained by a relativistic pulsar wind colliding with the stellar wind. Here we present an attempt to detect radio pulsars in both systems with the Green Bank Telescope. The upper limits of flux density are between 4.1-14.5 uJy, and we discuss the null results of the search. Our spherically symmetric model of the wind of LS 5039 demonstrates that any pulsar emission will be strongly absorbed by the dense wind unless there is an evacuated region formed by a relativistic colliding wind shock. LS I +61 303 contains a rapidly rotating Be star whose wind is concentrated near the stellar equator. As long as the pulsar is not eclipsed by the circumstellar disk or viewed through the densest wind regions, detecting pulsed emission may be possible during part of the orbit.
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