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تسجيل مستخدم جديدSuper-orbital variability of LS I +61{deg}303 at TeV energies
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The gamma-ray binary LS I +61$^{circ}$303 is a well established source from centimeter radio up to very high energy (VHE; E$>$100 GeV). Its broadband emission shows a periodicity of $sim$26.5 days, coincident with the orbital period. A longer (super-orbital) period of 1667 $pm$ 8 days was discovered in radio and confirmed in optical and high energy (HE; E>100 MeV) gamma-ray observations. We present a four-year campaign performed by MAGIC together with archival data concentrating on a search for a long timescale signature in the VHE emission. We focus on the search for super-orbital modulation of the VHE peak and on the search for correlations between TeV emission and optical determination of the extension of the circumstellar disk. A four-year campaign has been carried out by MAGIC. The source was observed during the orbital phases when the periodic VHE outbursts have occurred ($phi$=0.55-0.75). Additionally, we included archival MAGIC observations and data published by the VERITAS collaboration in these studies. For the correlation studies, LS I +61$^{circ}$303 has also been observed during the orbital phases where sporadic VHE emission had been detected in the past ($phi$=0.75-1.0). These MAGIC observations were simultaneous with optical spectroscopy from the LIVERPOOL telescope. The TeV flux of the periodical outburst in orbital phases $phi$=0.5--0.75 was found to show yearly variability consistent with the $sim$4.5 years long-term modulation found in the radio band. This modulation of the TeV flux can be well described by a sine function with the best fit period of $1610pm 58$ days. The complete dataset span two super-orbital periods. There is no evidence for a correlation between the TeV emission and the mass-loss rate of the Be star but this may be affected by the strong, short timescale (as short as intra-day) variation displayed by the H$alpha$ fluxes.
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The high-mass X-ray binary LS I +61{deg}303 exhibits variability in its radio and X-ray emissions, ranging from minute to hour time-scales. At such short time-scales, not much is known about the possible correlations between these two emissions from
this source, which might offer hints to their origin. Here, we study the relationship between these emissions using simultaneous X-ray and radio monitoring. We present new radio observations using the Arcminute Microkelvin Imager Large Array telescope at two frequency bands, 13-15.5 and 15.5-18 GHz. We also describe new X-ray observations performed using the XMM-Newton telescope. These X-ray and radio observations overlapped for five hours. We find for the first time that the radio and X-ray emission are correlated up to 81 per cent with their few percent variability correlated up to 40 per cent. We discuss possible physical scenarios that produces the observed correlations and variability in the radio and X-ray emission of LS I +61{deg}303.
LS I +61 303 is a gamma-ray binary that exhibits an outburst at GHz frequencies each orbital cycle of $approx$ 26.5 d and a superorbital modulation with a period of $approx$ 4.6 yr. We have performed a detailed study of the low-frequency radio emissi
on of LS I +61 303 by analysing all the archival GMRT data at 150, 235 and 610 MHz, and conducting regular LOFAR observations within the Radio Sky Monitor (RSM) at 150 MHz. We have detected the source for the first time at 150 MHz, which is also the first detection of a gamma-ray binary at such a low frequency. We have obtained the light-curves of the source at 150, 235 and 610 MHz, all of them showing orbital modulation. The light-curves at 235 and 610 MHz also show the existence of superorbital variability. A comparison with contemporaneous 15-GHz data shows remarkable differences with these light-curves. At 15 GHz we see clear outbursts, whereas at low frequencies we see variability with wide maxima. The light-curve at 235 MHz seems to be anticorrelated with the one at 610 MHz, implying a shift of $sim$ 0.5 orbital phases in the maxima. We model the shifts between the maxima at different frequencies as due to changes in the physical parameters of the emitting region assuming either free-free absorption or synchrotron self-absorption, obtaining expansion velocities for this region close to the stellar wind velocity with both mechanisms.
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 e
mission 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.
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).
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 26.49
6 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.
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