July 2016
Active Galactic Nuclei (AGNs) dominate the extragalactic sky in gamma-rays. But not all AGNs look the same. An AGN is the result of accretion of matter onto the super-massive black-hole (with a mass of the order of 108-9 solar masses) that dwells in the center of the host galaxy. In some cases, the accretion is associated with an outflow in the form of a pair of relativistic jets. When one of the jets is closely aligned with the line of sight, the AGN is called a blazar, and thanks to the relativistic boosting blazars are indeed bright gamma-ray emitters. But again, not all blazars look the same. In a subclass of blazars, called BL Lac objects, the emission is completely dominated by non-thermal emission from the relativistic jet, produced by relativistic electrons through the synchrotron and inverse Compton radiation mechanisms. On the other hand, in the subclass of Flat-Spectrum Radio Quasars (FSRQs), thermal emission from the accretion disk is visible in the ultra-violet band, together with reprocessed emission produced in the Broad Line Region (BLR), predominantly in the form of Lyman-alpha photons. As a matter of fact, in the very-high-energy regime (VHE, E > 100 GeV) accessible with Cherenkov telescopes such as the High Energy Stereoscopic System (H.E.S.S.), the extragalactic sky is largely dominated by BL Lac objects, and FSRQs are much rarer: only 5 quasars are known to emit VHE photons, as compared to 56 BL Lac objects. The only FSRQ previously seen by H.E.S.S. from the Southern Hemisphere is PKS 1510-089 [1].
In February 2015, the FSRQ PKS 0736+017 (located at a redshift z=0.189) underwent a bright flaring episode at high energy (MeV to GeV) gamma rays, as observed with the LAT instrument onboard the Fermi satellite. The gamma-ray flux as a function of time, measured with Fermi-LAT, is shown in Fig. 2. This information triggered target-of-opportunity observations with H.E.S.S. During the night of February 18, 2015, H.E.S.S. observed PKS 0736+017 for 1.8 hours, resulting in no significant detection (Fig. 1 top). But on the following night (February 19, 2015) a significant excess of VHE emission, at a level of more than 7 standard deviations above the background, was detected from a position coincident with PKS 0736+017, in 1.8 hours of H.E.S.S. observations (Fig. 1 middle). The VHE flux is estimated to be of the order of 10% of the Crab nebula flux above an energy of 100 GeV. The detection was achieved using the H.E.S.S. II instrument in the monoscopic configuration and represents the first H.E.S.S. II discovery of a new extragalactic VHE gamma-ray source. Additional H.E.S.S observations were taken on February 21, 2015, for 2.7 hours, resulting again in no detection (Fig. 1 bottom). Thus, the VHE observations with H.E.S.S. indicate that the source shows (at least) night-by-night variability in the VHE band.
But why are FSRQs rare VHE emitters, and what can we learn from this detection? The answer to the first question is based both on the intrinsic properties of the emission at the source, and from propagation effects. All blazars show gamma-ray emission, the spectral energy distribution of which can peak from the MeV band to the TeV band. While BL Lac object can have the peak of their gamma-ray emission in the VHE band, where Cherenkov telescopes can observe them, FSRQs are characterized by a peak in the MeV band, and VHE observations sample only the rapidly decaying part of the spectrum.
In addition, VHE astrophysics is characterized by an important process: the pair production of an electron and positron from the interaction of a VHE photon with an infrared/optical photon. This process efficiently absorbs VHE photons. As we discussed before, in FSRQs the external photon field from the accretion disk and the BLR is important and can efficiently absorb the VHE emission. In particular, if the gamma-ray emitting region is located at the basis of the relativistic jets, well within the BLR, the VHE emission should have been largely attenuated, and no emission should have been detected. On the other hand, the very detection of VHE photons from PKS 0736+017, tells that, at least during the VHE flare observed by H.E.S.S., the gamma-ray emission was produced downstream in the jet and outside the BLR. For a super-massive black-hole with a mass of ~ 300 million solar masses (as estimated for PKS 0736+017 [2]), this means a distance of at least 0.2 light years away from the black hole powering the quasar.
This new VHE discovery, together with other recent results from the H.E.S.S. collaboration, will be presented at the Gamma 2016 conference held in Heidelberg, Germany during July 11-15, 2016.
References:
[1] H.E.S.S. Collaboration, A. Abramowski et al.: H.E.S.S. discovery of VHE gamma-rays from the quasar PKS 1510-089, Astronomy & Astrophysics 554 (2013) 107.
[2] R.J. McLure & J.S. Dunlop: The black hole masses of Seyfert galaxies and quasars, Mon. Not. Roy. Astron. Soc. 327 (2001) 199.
