Smoothed significance map of 3C 66A. The location of 3C 66A is shown as an open star and 3C66B as the closed star. See Figure 4 below for more details.

3C66A: Observations of a VHE γ-ray flare

Reference:  V. A. Acciari et al. (The VERITAS Collaboration), The Astrophysical Journal, 693: L104-L108, 2009

Full text version here , Erratum here

ArXiv version: ArXiV:0901.4527 (v2: 2010-12-13)

Contact person: Jeremy Perkins

One of the most highly studied blazars, 3C 66A, was detected in a flaring state using the VERITAS array of Cherenkov telescopes during 2007 - 2008.  3C 66A is classified as an intermediate-frequency peaked BL Lac (IBL) due to the location of its synchrotron peak between 10¹⁵ and 10¹⁶ Hz.  This makes it an unusual object at VHE (very-high-energies; E > 100 GeV) because that places its high energy peak below the peak sensitivity of Cherenkov telescopes.  In addition to this, 3C 66A has an uncertain redshift; it could be as high as 0.444 or as low as 0.096.  If the redshift is in fact above 0.4, this would be one of the most distant blazars detected at VHE to date and the detection of 3C 66A would have enormous implications of the strength of the extragalactic background light.  3C 66A was detected by EGRET in the HE band and Fermi has also detected this IBL at a higher flux than previously reported.  In the VHE band, the Crimean Astrophysical Observatory reported a 5.1 sigma detection above 900 GeV but contemporaneous experiments have failed to detect it.

VERITAS detected 3C 66A at very high significance (over 20 standard deviations) in ~33 hours of live time.  This corresponds to an integral flux of 6% of the Crab Nebula's flux above 200 GeV.  Additionally, a very strong flare from 3C 66A was seen in early October with 80% of the excess seen during this flaring period.  The spectrum is well described by a smooth power-law with a very soft index of 4.1 ± 0.4 (stat) ± 0.6 (sys). Interestingly, no evidence for variability is seen during the non-flaring nights.

One of the difficulties in observing 3C 66A at VHE energies is the proximity (in the field-of-view) of the radio galaxy 3C 66B which could also be a VHE source.  Since 3C66B only lies 0.12 degrees away from 3C 66A, care must be taken to ensure where the VHE emission is originating.  In fact, the MAGIC experiment recently reported a detection of VHE emission from the 3C 66A/B region and excluded 3C 66A at an 85% confidence level as the source of the observed emission.  Since the detection reported by VERITAS has greater statistics, these observations rule out 3C 66B as the source of the emission at the 4.3 sigma level.  This does not rule out that MAGIC detected 3C 66B during the time their data were taken but it must have been considerably brighter in 2007 than in 2008 due to the small period of overlap between the MAGIC and VERITAS observations.

Figures from paper (click to get full size image):

Figure 1: The distribution of theta^2 for on-source events (points) and normalized off-source events (shaded region) from observations of 3C 66A. The dashed line represents the cut on theta^2 applied to the data.

Figure 2 (corrected): The energy spectrum of 3C 66A shown as solid points. The spectrum is well fitted by a power law with photon index of 4.1 +/- 0.4 (stat) +/- 0.6 (sys) (solid line). The shaded area outlines the systematic error in the spectral index. Using the models of Franceschini et al. (2008) and assuming a redshift of z = 0.444, the de-absorbed spectral index is calculated to be 1.1 +/- 0.4 showing that the very steep measured spectrum could be due to the distance of 3C 66A. This de-absorbed spectrum is shown as a dashed-dotted line and points. The MAGIC spectrum with photon index of 3.1 from Aliu et al. (2009) is shown as a dotted line. The Crab Nebula spectrum divided by 10 is also shown for comparison (dashed line).

Figure 3: Light curve binned by dark period (the time between two full moons) for the full data set from 3C66A. These data indicate night-by-night variability for the second dark period (MJD 54734 through 54749) but not within any of the other dark periods. The inset details this dark period binned by night when the flare occurred.

Figure 4: Smoothed significance map of 3C 66A. The location of 3C 66A is shown as an open star and 3C66B as the closed star. The cross is the fit to the excess VHE emission resulting in a localization of 2h 22m 41.6s +/- 1.7s +/- 6.0s, 43 deg 02' 35.5