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Register a new userRadio/X-ray correlations and variability in the X-ray binary LS I +61{deg}303
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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.
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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 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.
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.
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 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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