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Broad-band spectral energy distribution (SED) of RGB J0710+591. The VERITAS data is represented as open circles near 10^26 Hz. See Figure 3 below for more details.

Reference:  V. A. Acciari et al. (The VERITAS Collaboration), Astrophysical Journal 715: L49-L55, 2010

Full text version here

ArXiv version: ArXiV:1005.0041

Contact person: Jeremy Perkins

RGB J0710+591 is a blazar located at a redshift of z = 0.125, originally discovered by HEAO A-1. Blazars are characterized by spectral energy distributions (SED) consisting of two peaks, the low energy peak arising from synchrotron emission and the high energy peak from either leptonic or hadronic interactions.  Depending on the energy of the lower peak, blazars can be detected in the VHE ( E > 100 GeV) band.  Previous X-ray measurements of this blazar found that it has one of the hardest X-ray spectra seen in the catalogs and suggested that the high energy peak in the SED is at an ideal location for HE ( 20 MeV to 300 GeV) and VHE observations.  In this case, this turned out to be true and VERITAS discovered VHE emission from RGB J0710+591 in ~22 hours of observations in early 2009.  After this announcement, an intense multiwavelength campaign was undertaken including collaborators from Fermi in HE gamma rays, Swift in X-rays and optical and MDM in the optical.  This detailed multiwavelength coverage is very important to classify blazars and to model the underlying physical processes.

The resulting multiwavelength SED spans thirteen decades in energy; starting in the optical band and moving all the way through TeV energies.  In VHE, the emission from RGB J0710+591 is a smooth power law with spectral index 2.69 ± 0.26 (stat) ± 0.20 (sys) and an integral flux above 300 GeV of  3.9 ± 0.8 × 10^-12 cm^-2 s^-1 (3% of the Crab Nebula flux).  VERITAS detected a signal from this blazar at a statistical significance of 5.5 standard deviations.  In the HE band, RGB J0710+591 is detected with high significance (greater than 8 standard deviations) and the emission can be described by a power law with spectral index  1.46 ± 0.17 (stat) ± 0.05 (sys).  In the X-ray band, the Swift results confirm the previous hard spectrum (photon index 1.86 ± 0.01) and indicates that the synchrotron peak is above 10 keV.  UV and optical measurements were also obtained with the Swift UVOT detector and the MDM Observatory 1.3 m telescope.  No variability was seen in any of the bands and the localizations from all the experiments are highly consistent.

The SED was modeled with a purely leptonic synchrotron self-Compton (SSC) model.  The modeling indicates that the synchrotron portion of the SED always turns over far below the X-ray regime which leads to a very hard injection index.  Also, a very low magnetic field needs to be used to fit the SED properly.  Another interesting point is that an external-Compton component does not improve the fitt; this effect is also seen in other HBL (high-frequency peaked BL Lacs) of this type.  From a population standpoint, RGB J0710+591 seems to be a normal blazar with a moderately large redshift and a very hard X-ray spectrum.

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

Figure 1: The distribution of theta^2 for on-source VHE events from observations of RGBJ0710+591. Theta is the distance from the source location to the event location. The dashed line represents the location of the cut on theta^2 applied to the data. The theta^2 cut retains events which lie to the left of the line.

Figure 2: Smoothed VHE significance map of RGBJ0710+591 from the VERITAS observations. The VLA location of the blazar is shown as a star. The solid circle is centered at the best fit location of the excess emission in the VERITAS data; the radius of the circle is the sum of the statistical and systematic uncertainties. The dashed circle is the best fit plus a 95% containment radius of 2'.7 to the Fermi LAT data. Both locations are consistent with the VLA position of RGB J0710+591. The VERITAS point spread function is shown in the top right corner.

Figure 3: Broad-band SED of RGB J0710+591. The closed squares are R- and B-band data from the MDM 1.3m telescope, the open triangles are from the Swift UVOT, closed circles are from the Swift XRT, the shaded area is the Fermi LAT measurement and the open circles are the measured VERITAS data. The broad-band spectral model is the equilibrium SSC version from Boettcher & Chiang (2002) with a cooling break energy occurring around 1.3 x 10^(16) Hz. Note that the optical measurements (MDM and Swift u, b and v bands) are affected by emission from the host galaxy and are thus not used in the SED fit. The SED model curve includes EBL absorption using the model of Franceschini et al. (2008) and the dashed line is the de-absorbed VHE spectrum.