Magic Constraints on γ-ray Emission from Cygnus X-3
Cygnus X-3 is a microquasar consisting of an accreting compact object orbiting around a Wolf-Rayet star. It has been detected at radio frequencies and up to high-energy γ rays (above 100 MeV). However, many models also predict a very high energy (VHE) emission (above hundreds of GeV) when the source displays relativistic persistent jets or transient ejections. Therefore, detecting such emission would improve the understanding of the jet physics. The imaging atmospheric Cherenkov telescope MAGIC observed Cygnus X-3 for about 70 hr between 2006 March and 2009 August in different X-ray/radio spectral states and also during a period of enhanced γ-ray emission. MAGIC found no evidence for a VHE signal from the direction of the microquasar. An upper limit to the integral flux for energies higher than 250 GeV has been set to 2.2 × 10^(–12) photons cm^(-2) s^(-1) (95% confidence level). This is the best limit so far to the VHE emission from this source. The non-detection of a VHE signal during the period of activity in the high-energy band sheds light on the location of the possible VHE radiation favoring the emission from the innermost region of the jets, where absorption is significant. The current and future generations of Cherenkov telescopes may detect a signal under precise spectral conditions.
Additional Information© 2010 The American Astronomical Society. Received 2010 May 3; accepted 2010 July 18; published 2010 September 1. We thank the Instituto de Astrofisica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The support of the German BMBF and MPG, the Italian INFN, the Swiss National Fund SNF, and the Spanish MICINN are gratefully acknowledged. This work was also supported by the Polish MNiSzW Grant N N203 390834, by the YIP of the Helmholtz Gemeinschaft, and by grant DO02-353 of the Bulgarian National Science Fund. S.T. is grateful for the support of the RATAN observations and the Russian Foundation for Basic Research, grant N08-02-00504-a. We credit the Swift/BAT and RXTE/ASM teams for making public their transient monitor and quick-look results, respectively. We also thank Stéphane Corbel for providing useful comments and information on the OVRO and Fermi/LAT light curves.
Published - Aleksic2010p11591Astrophys_J.pdf