Gamma-ray quasars from the bright to the faint end: PKS 0736+017

March 2020

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About two percent of the hundreds of billions of galaxies in the universe stand out as 'active galaxies'. A very small nuclear region radiates enormous amounts of light, making Quasars (the most luminous class among these 'active galactic nuclei (AGN)') the brightest objects in the universe. Not all Quasars are alike and many of them are considerably fainter than the most luminous ones. The mechanisms that enable the high luminosities and define their characteristics (such as the fraction of energy that is emitted in form of gamma-rays) are still puzzling. A promising way to disentangle the complex physical processes are searches for correlations between different properties of Quasars. One option is to explore whether Quasars that differ in luminosity also systematically differ in other ways. The H.E.S.S. collaboration has now published a study of a nearby rather dim Quasar that allows a comparison to the much more luminous Quasars studied earlier. The particular object, PKS 0736+017, has been discovered more than 50 years ago among the first Quasars to be found in systematic searches among radio sources ([1]).

All Quasars and other AGN are thought to host a super-massive black hole at the core, fueled by an accretion disk (have you watched the movie 'Interstellar' ?). In a few cases, they expel beamed, relativistic jets well beyond their host galaxy. AGN are complex and can differ in many ways (e.g. presence and orientation of jets, properties of accretion disks, etc.). Cherenkov telescopes, such as H.E.S.S., preferably detect the so-called 'blazars', that is, AGN whose relativistic jet is closely aligned to our line of sight, thus benefiting from an enhancement of their flux, due to the relativistic Doppler effect. So-called 'flat spectrum radio quasars (FSRQs)' are a subclass among blazars that differ in several ways from the rest. They show prominent emission lines in their spectra, and their broadband, multi-wavelength throughput peaks at lower energies. FSRQ are thus less likely to be detected by telescopes observing very-high-energy gamma-rays.

Fig. 1: Optical spectra of PKS 0736+017 obtained during the period when the quasar was detected for the first time in the very high energy gamma-ray band (colored spectra). The spectrum has very typical emission lines with (being a Quasar) rather moderate line-widths. The different features are labeled above the top spectrum. The three colored spectra on top show clear differences in the absolute flux level of the continuum emission. The line emission does not show any significant differences. The lower, black spectrum was obtained during an epoch of low flux in 2016 is shown for reference (taken from [2]).

The emission lines observed in Quasars originate from dense clouds orbiting close to and fast around the core. These clouds are heated by the accretion disk and hence radiate thermal emission with characteristic spectral lines. The line emission is broadened due to the high velocity of the clouds, giving rise to the term 'broad line region (BLR)' for the central parts of Quasars. Gamma-rays such as those detectable with H.E.S.S. tend to interact with the light that is emitted by the BLR. As a result of this interaction, the very energetic gamma-rays are absorbed. The very fact that such energetic gamma-rays can still be detected demonstrates that the gamma-ray emission region is be located outside the BLR (e.g. ([3]), and is thus further away from the very nucleus then previously assumed.

Fig. 2: Sketch illustrating the central parts of a Quasar. While it is generally assumed that the gamma-ray emission originates from the relativistic jet, the distance of this region from the Black Hole is under debate. Luminous broad line regions prevent gamma-rays from escaping the nucleus. This, in turn, allows to derive a minimum distance of the emission region. Comparing Quasars of different luminosities further allows to test this model in a quantitative way (Credit: Heike Prokoph and DESY/Science Communication Lab).

The blazar PKS 0736+017 was observed with the H.E.S.S. telescopes during a flare detected with the Fermi-LAT instrument. During the H.E.S.S. observation campaign, this FSRQ briefly switched on and off at very high energies, and H.E.S.S. detected its gamma-ray signal only during the night of February 19, 2015, see also [4]. This flaring behavior was also observed in other wavelength regimes. The simultaneous observations allow of the continuum emission and the flux of the Broad Line region clouds enable the derivation of the absorption effect of the thermal emission on the gamma-ray radiation, and hence the derivation of the minimum distance of the emission region from the black hole.

Because light, though traveling fast, has a finite speed, the observation of such variability pattern encodes also information on the size of the emitting region. Taking relativistic aberration into account, the time scale of ~5h recorded on the night of February 19, 2015, implies a diameter of about 100 thousand million kilometers. This is much smaller than the size of the broad line region, and hence much smaller than the distance of this emitting region from the black hole. Other quasars detected with Cherenkov telescopes, like 3C 279 and PKS 1510-089 (e.g. [5]), are much more luminous than PKS 0736+017, which lies at the faint end of the quasar population. In fact, PKS 0736+017 is barely more luminous than so-called Seyfert 1 galaxies, a category of AGN which normally does not display jet emission. Despite the low overall luminosity, the density of the radiation field is still high enough to prevent gamma-rays from escaping. The low-luminosity quasar PKS 0736+017 is hence very similar to its brighter cousins, suggesting that gamma-ray properties are not closely correlated with the thermal luminosity of the quasar.


[1] Bolton J.G. and Kinman T.D., "Radio and optical data on twelve quasi-stellar objects", Astrophys. J., 145, 951-953 (1966)
[2] H.E.S.S. collaboration (Abdalla H. et al.), "H.E.S.S. detection of very-high-energy gamma-ray emission from the quasar PKS 0736+017", Astronomy & Astrophysics, 633, 162, (2020)
[3] Meyer M., Scargle J.D. & Blandford R.D., "Characterizing the Gamma-Ray Variability of the Brightest Flat Spectrum Radio Quasars Observed with the Fermi LAT", Astrophys. J., 877, 39-76 (2019)
[4] HESS Source of the Month 2016/07
[5] HESS Source of the Month 2020/01