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 ( catalog 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 ( catalog 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 .