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VERITAS observations of Cygnus X-3 during a major radio flare

ATel #10252; R. Mukherjee on behalf of the VERITAS Collaboration, M. L. McCollough (SAO/CFA, U. S.A.), M. A. Gurwell (SAO/CFA, U. S.A), G. Petitpas(SAO/CFA, U. S.A), S. A. Trushkin (SAO RAS, Russia), G. Pooley (MRAO, UK), K. I.I. Koljonen (FINCA, Finland)
on 8 Apr 2017; 15:12 UT
Credential Certification: Reshmi Mukherjee (muk@astro.columbia.edu)

Subjects: Radio, Millimeter, X-ray, Gamma Ray, >GeV, TeV, VHE, Binary, Transient

Referred to by ATel #: 10257, 10274

VERITAS observed Cygnus X-3 during a recent radio and high energy gamma ray flare. The high-mass X-ray binary Cygnus X-3 has recently been observed transitioning to a soft X-ray state, which is a spectral state that has historically been associated with enhanced gamma-ray emission in the high energy band (e.g. Tavani et al. 2009, Nature, 462, 620 and The Fermi LAT Collaboration 2009, Science, 326, 1512). Detection of gamma-ray emission from Cygnus X-3 was reported by Fermi/LAT, at the onset of this radio flare, at a significance near 5 sigma with a flux (E > 100 MeV) of (1.4 +/- 0.4) x 10^-6 cm^-2 s^-1 (ATel# 10243).This flaring activity triggered VERITAS observations during the nights of April 5-6 (UT), 2017. VERITAS took a combined 2.5 hours of data over the course of two nights, and preliminary analysis shows a non detection of Cygnus X-3 at VHE gamma rays. VERITAS reports preliminary upper limits on the flux ~ 3.11 x 10^-12 cm^-2 s^-1 (5.2% of the Crab Nebula flux) above 0.5 TeV.

The VERITAS measurements were right at the peak of the major radio flare (see figure 1 below for the RATAN-600 observations). During the time periods of the VERITAS measurements the AMI-LA measured fluxes of 13 Jy (April 5th) and 10 Jy (April 6th). Metsahovi radio observatory observed a preliminary flux of 7 Jy at 37 GHz on MJD 57848). Ongoing radio monitoring of Cyg X-3 with the RATAN-600 radio telescope has been reported earlier (ATel# 10126).

During this period an observing campaign of Cyg X-3 using the Submillimeter Array was being performed. Some preliminary results are:

(Times are midpoints of the observations.) 
Time                  Freq.      Flux              Comments 
2017-Apr-03 20.86 UT   248 GHz   0.60 +/- 0.03 Jy  SNR~100  (increases by  
                                                   30% over ~20 min)  
2017-Apr-04 17.63 UT  344.0 GHz  2.97 +/- 0.15 Jy  SNR~2000 (varies by 10  
                                                   over ~6 hours,indication  
						   of ~5% polarization) 
2017-Apr-05 18.60 UT  220.1 GHz  2.614 +/- 0.13 Jy SNR~1900 (steady over ~ 2  
2017-Apr-06 17.05 UT  220.1 GHz  3.092 +/- 0.15 Jy SNR~2300 (varies by  
                                                   ~10-15% over 3 hours,  
					           indication of ~10%  

This also corresponded to a rapid rise in the 15-50 keV count rate observed by Swift/BAT (see figure 2 below for the March 31st to April 7th light curve). On April 6th there was a 600 sec Swift/XRT observation which had a mean count rate of 88 counts/sec. It can be fit with an absorbed black-body which yields an NH of 5 x 10^22 cm^-2 with a T of 1.7 keV. The estimated absorbed and unabsorbed 0.5-10 keV fluxes were 1.1 x 10^-8 erg/s/cm^2 and 1.3 x 10^-8 erg/s/cm^2 respectively. The corresponding BAT Survey data was fit with a powerlaw with a photon index of 2.2 and yielded a 15-100 keV flux of 2.7 x 10-9 erg/s/cm^2.

Continued observations from VERITAS will likely be hindered due to bright moonlight.

Figures are available at: http://veritas.astro.columbia.edu/CygX3_April2017

Questions regarding the VERITAS observations should be directed to Reshmi Mukherjee (muk@astro.columbia.edu). VERITAS (Very Energetic Radiation Imaging Telescope Array System) is located at the Fred Lawrence Whipple Observatory in southern Arizona, USA, and is most sensitive to gamma rays between ~85 GeV and ~30 TeV (http://veritas.sao.arizona.edu).