Publication: Search for very-high-energy gamma-ray emission from the microquasar Cygnus X-1 with the MAGIC telescopes
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The microquasar Cygnus X-1 displays the two typical soft and hard X-ray states of a black hole transient. During the latter, Cygnus X-1 shows a one-sided relativistic radio-jet. Recent detection of the system in the high energy (HE; E greater than or similar to 60 MeV) gamma-ray range with FermiLAT associates this emission with the outflow. Former MAGIC observations revealed a hint of flaring activity in the very high-energy (VHE; E greater than or similar to 100 GeV) regime during this X-ray state. We analyse ~ to 97 h of Cygnus X-1 data taken with the MAGIC telescopes between July 2007 and October 2014. To shed light on the correlation between hard X-ray and VHE gamma rays as previously suggested, we study each main X-ray state separately. We perform an orbital phase-folded analysis to look for variability in the VHE band. Additionally, to place this variability behaviour in a multiwavelength context, we compare our results with Fermi-LAT, AGILE, Swift-BAT, MAXI, RXTE-ASM, AMI and RATAN-600 data. We do not detect Cygnus X-1 in the VHE regime. We establish upper limits for each X-ray state, assuming a power-law distribution with photon index Γ = 3.2. For steady emission in the hard and soft X-ray states, we set integral upper limits at 95 %per cent confidence level for energies above 200 GeV at 2.6 x 10⁻¹² photons cm⁻² s⁻¹ and 1.0 x 10⁻¹¹ photons cm⁻² s ⁻¹, respectively. We rule out steady VHE gamma-ray emission above this energy range, at the level of theMAGIC sensitivity, originating in the interaction between the relativistic jet and the surrounding medium, while the emission above this flux level produced inside the binary still remains a valid possibility.
© 2017 The Authors. Artículo firmado por mas de diez autores. We would like to thank the Instituto de Astrof´ısica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The financial support of the German BMBF and MPG, the Italian INFN and INAF, the Swiss National Fund SNF, the ERDF under the Spanish MINECO (FPA2015- 69818-P, FPA2012-36668, FPA2015-68278-P, FPA2015- 69210-C6-2-R, FPA2015-69210-C6-4-R, FPA2015-69210-C6- 6-R, AYA2013-47447-C3-1-P, AYA2015-71042-P, ESP2015- 71662-C2-2-P, CSD2009-00064), and the Japanese JSPS and MEXT is gratefully acknowledged. This work was also supported by the Spanish Centro de Excelencia “Severo Ochoa” SEV-2012-0234 and SEV-2015-0548, and Unidad de Excelencia “Mar´ıa de Maeztu” MDM-2014-0369, by the Croatian Science Foundation (HrZZ) Project 09/176 and the University of Rijeka Project 188.8.131.52.02, by the DFG Collaborative Research Centers SFB823/C4 and SFB876/C3, and by the 1 as shown in https://www.cta-observatory.org/science/ctaperformance/ Polish MNiSzW grant 745/N-HESS-MAGIC/2010/0. The AMI arrays are supported by the United Kingdom STFC and by the University of Cambridge. R.Z. acknowledges the Alexander von Humboldt Foundation for the financial support and the Max-Planck Institut fur Kernphysik as hosting institution. This research has made use of MAXI data provided by RIKEN, JAXA and the MAXI team.