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Post Info TOPIC: Microquasar LS I +61 303


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LSI +61 303
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Title: Study of orbital and superorbital variability of LSI +61 303 with X-ray data
Author: M. Chernyakova, Iu. Babyk, D. Malyshev, Ie. Vovk, S. Tsygankov, H. Takahashi, Ya. Fukazawa

LSI +61 303 is one of the few X-ray binaries with a Be star companion from which radio, X-rays and high-energy gamma-ray (GeV and TeV) emission have been observed. The nature of the high energy activity of the system is not yet fully understood, but it is widely believed that it is generated due to the interaction of the relativistic electrons leaving the compact object with the photons and non-relativistic wind of the Be star. The superorbital variability of the system has been observed in the radio, optical and X-ray domains and could be due to the cyclic change of the Be star disk size. In this paper we systematically review all publicly available data from Suzaku, XMM-Newton, Chandra and Swift observatories in order to measure the absorption profile of the circumstellar Be disk as a function of orbital and superorbital phases. We also discuss short-term variability of the system, found during the analysis and its implications for the understanding of the physical processes in this system.

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RE: Microquasar LS I +61 303
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Title: Unveiling the super-orbital modulation of LSI +61 303 in X-rays
Authors: Jian Li, Diego F. Torres, Shu Zhang, Daniela Hadasch, Nanda Rea, G. Andrea Caliandro, Yupeng Chen, Jianmin Wang

We found evidence for the super-orbital modulation in the X-ray emission of LS I +61 303 from the longest monitoring date by the RXTE. The time evolution of the modulated fraction in the orbital light curves can be well fitted with a sinusoidal function having a super-orbital period of 1667 days. However, we have found a 281.8±44.6 day shift between the super-orbital variability found at radio frequencies and our X-ray data. We also find a super-orbital modulation in the maximum count rate of the orbital light curves, compatible with the former results, including the shift.

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Title: Superorbital modulation of X-ray emission from gamma-ray binary LSI +61 303
Authors: M. Chernyakova, A. Neronov, S. Molkov, D. Malyshev, A. Lutovinov, G. Pooley

We report the discovery of a systematic constant time lag between the X-ray and radio flares of the gamma-ray binary LSI +61 303, persistent over long, multi-year, time scale. Using the data of monitoring of the system by RXTE we show that the orbital phase of X-ray flares from the source varies from \phi_X\simeq 0.35 to \phi_X\simeq 0.75 on the superorbital 4.6 yr time scale. Simultaneous radio observations show that periodic radio flares always lag the X-ray flare by \Delta\phi_{X-R}\simeq 0.2. We propose that the constant phase lag corresponds to the time of flight of the high-energy particle filled plasma blobs from inside the binary to the radio emission region at the distance ~10 times the binary separation distance. We put forward a hypothesis that the X-ray bursts correspond to the moments of formation of plasma blobs inside the binary system.

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Title: Deep Chandra observations of TeV binaries I: LSI +61 303
Authors: N. Rea (CSIC-IEEC), D. F. Torres (CSIC-IEEC, ICREA), M. van der Klis (Amsterdam), P. G. Jonker (SRON), M. Mendez (Groningen), A. Sierpowska-Bartosik (Lodz)
(Version v3)

We report on a 95ks Chandra observation of the TeV emitting High Mass X-ray Binary LSI +61 303, using the ACIS-S camera in Continuos Clocking mode to search for a possible X-ray pulsar in this system. The observation was performed while the compact object was passing from phase 0.94 to 0.98 in its orbit around the Be companion star (hence close to the apastron passage). We did not find any periodic or quasi-periodic signal (at this orbital phase) in a frequency range of 0.005-175 Hz. We derived an average pulsed fraction 3 sigma upper limit for the presence of a periodic signal of ~10% (although this limit is strongly dependent on the frequency and the energy band), the deepest limit ever reached for this object. Furthermore, the source appears highly variable in flux and spectrum even in this very small orbital phase range, in particular we detect two flares, lasting thousands of seconds, with a very hard X-ray spectrum with respect to the average source spectral distribution. The X-ray pulsed fraction limits we derived are lower than the pulsed fraction of any isolated rotational-powered pulsar, in particular having a TeV counterpart. In this scenario most of the X-ray emission of LSI +61 303 should necessarily come from the interwind or inner-pulsar wind zone shock rather than from the magnetosphere of the putative pulsar. Furthermore, we did not find evidence for the previously suggested extended X-ray emission.

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RE: Microquasar LS I +61 303
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Title: Periodic very high energy gamma-ray emission from LS I +61 303 observed with the MAGIC telescope
Authors: MAGIC Collaboration: J. Albert, et al

The MAGIC collaboration has recently reported the discovery of gamma-ray emission from the binary system LS I +61 303 in the TeV energy region. Here we present new observational results on this source in the energy range between 300 GeV and 3 TeV. In total 112 hours of data were taken between September and December 2006 covering 4 orbital cycles of this object. This large amount of data allowed us to produce an integral flux light curve covering for the first time all orbital phases of LS I +61 303. In addition, we also obtained a differential energy spectrum for two orbital phase bins covering the phase range 0.5<phi<0.6 and 0.6<phi<0.7. The photon index in the two phase bins is consistent within the errors with an average index Gamma=2.6±0.2_{stat}±0.2_{sys}. LS I +61 303 was found to be variable at TeV energies on timescales of days. These new MAGIC measurements allowed us to search for intra-night variability of the VHE emission; however, no evidence for flux variability on timescales down to 30 minutes was found. To test for possible periodic structures in the light curve, we apply the formalism developed by Lomb and Scargle to the LS I +61 303 data taken in 2005 and 2006. We found the LS I +61 303 data set to be periodic with a period of (26.8±0.2) days (with a post-trial chance probability of 10^{-7}), close to the orbital period.

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Title: VERITAS Observations of the gamma-Ray Binary LS I +61 303
Authors: V.A. Acciari, M. Beilicke, G. Blaylock, S.M. Bradbury, J.H. Buckley, V. Bugaev, Y. Butt, K.L. Byrum, O. Celik, A. Cesarini, L. Ciupik, Y.C.K. Chow, P. Cogan, P. Colin, W. Cui, M.K. Daniel, C. Duke, T. Ergin, A.D. Falcone, S.J. Fegan, J.P. Finley, P. Fortin, L.F. Fortson, D. Gall, K. Gibbs, G.H. Gillanders, J. Grube R. Guenette, D. Hanna, E. Hays, J. Holder, D. Horan, S.B. Hughes, C.M. Hui, T.B. Humensky, P. Kaaret, D.B. Kieda, J. Kildea, A. Konopelko, H. Krawczynski, F. Krennrich, M.J. Lang, S. LeBohec, K. Lee, G. Maier, A. McCann, M. McCutcheon, J. Millis, P. Moriarty, R. Mukherjee, T. Nagai, R.A. Ong, D. Pandel, J.S. Perkins, F. Pizlo, M. Pohl, J. Quinn, K. Ragan, P.T. Reynolds, H.J. Rose, M. Schroedter, G.H. Sembroski, A.W. Smith, D. Steele, S.P. Swordy, J.A. Toner, L. Valcarcel, V.V. Vassiliev, et al (9 additional authors not shown)

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 characterised 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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LS I +61 303.kmz

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Title: LS I+61 303: microquasar or not microquasar?
Authors: G.E. Romero, M. Orellana, A.T. Okazaki, S.P. Owocki

LS I +61 303 is a puzzling object detected from radio up to high-energy gamma-rays. Variability has recently been observed in its high-energy emission. The object is a binary system, with a compact object and a Be star as primary. The nature of the secondary and the origin of the gamma-ray emission are not clearly established at present. Recent VLBA radio data have been used to claim that the system is a Be/neutron star colliding wind binary, instead of a microquasar. We review the main views on the nature of LS I +61 303 and present results of 3D SPH simulations that can shed some light on the nature of the system. Our results support an accretion powered source, compatible with a microquasar interpretation.

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Title:  Accretion vs. colliding wind models for the gamma-ray binary LS I +61 303: an assessment
Authors: G. E. Romero1,  A. T. Okazaki, M. Orellana1,  and S. P. Owocki

Context. LS I +61 303 is a puzzling Be/X-ray binary with variable gamma-ray emission up to TeV energies. The nature of the compact object and the origin of the high-energy emission are unclear. One family of models invokes particle acceleration in shocks from the collision between the B-star wind and a relativistic pulsar wind, whereas another centres on a relativistic jet powered by accretion from the Be star decretion disc onto a black hole. Recent high-resolution radio observations showing a putative "cometary tail" pointing away from the Be star near periastron have been cited as support for the pulsar-wind model.
Aims. We wish to carry out a quantitative assessment of these competing models.
Methods. We apply a "Smoothed Particle Hydrodynamics" (SPH) code in 3D dynamical simulations for both the pulsar-wind-interaction and accretion-jet models. The former yields a dynamical description of the shape of the wind-wind interaction surface. The latter provides a dynamical estimation of the accretion rate under a variety of conditions, and how this varies with orbital phase.
Results. The results allow critical evaluation of how the two distinct models confront the data in various wavebands. When one accounts for the 3D dynamical wind interaction under realistic constraints for the relative strength of the B-star and pulsar winds, the resulting form of the interaction front does not match the putative "cometary tail" claimed from radio observations. On the other hand, dynamical simulations of the accretion-jet model indicate that the orbital phase variation of accretion power includes a secondary broad peak well away from periastron, thus providing a plausible way to explain the observed TeV gamma ray emission toward apastron.
Conclusions. Contrary to previous claims, the colliding-wind model is not clearly established for LS I +61 303, whereas the accretion-jet model can reproduce many key characteristics, such as required energy budget, lightcurve, and spectrum of the observed TeV gamma-ray emission.

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Title: Observation of LS I +61 303 with VERITAS
Authors: Gernot Maier (for the VERITAS Collaboration)

The high mass X-ray binary LS I +61 303 has been observed over several months in 2006 and 2007 with the VERITAS array of imaging air-Cherenkov telescopes. A signal of high energy gamma rays with energies above 350 GeV is detected in several orbital cycles of the binary system. The detected flux of gamma rays is strongly variable with the orbital period of 26.5 days, while the maximum flux (corresponding to about 10% of the flux of the Crab Nebula)is always found at approximately apastron, suggesting a strong dependence of particle acceleration and/or propagation on the relative position of the two objects in the system.

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