The many-faced quasar PKS 1510-089

January 2020

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Among all classes of Active Galactic Nuclei (AGN), the so-called BL Lac objects had been prime candidates for Very High Energy (VHE) gamma-ray emission (Note 1) from the start, as they posses a feature that sets them apart from the 'Flat-Spectrum Radio Quasars' (FSRQs) which dominated the catalogs of astronomical GeV sources detected with satellites before the advent of TeV astronomy (Note 2): BL Lac objects lack thermal radiation fields, in particular the luminous 'Big Blue Bump' indicative of a hot accretion disk. Thermal radiation from the accretion disk surrounding the supermassive Black Hole had been considered as a blessing and a curse for studies in the GeV and TeV bands, respectively. On the one hand, the primary emission from the disk may be reprocessed in the vicinity of the Black Hole and a dense radiation field resulting from this reprocessing will provide photons for inverse Compton scattering and copious amounts of GeV photons. On the other hand the primary radiation of the disk, peaking in the range of 1 to 10 eV effectively absorbs gamma emission in the range of 100 GeV to 1 TeV. Quasars with strong thermal radiation were hence expected to be faint emitters of very high energy (VHE) gamma-rays (Note 3).

Partly resulting from this anticipation, FSRQs were observed rather late with Cherenkov Telescopes but eventually proved no less fascinating than BL Lac objects. The first FSRQ to be detected with the H.E.S.S. telescopes was PKS 1510-089 ([1], [2], [3]), a bright quasar at a cosmological redshift of z = 0.361. In early 2009 this quasar was flaring in the optical and GeV bands, as identified with the ATOM telescope supporting the H.E.S.S. array and with the Fermi LAT instrument. This triggered observations with H.E.S.S. and led to the discovery of PKS 1510-089 in the VHE regime ([4]). This suggested that the absorption paradigm preventing the escape of VHE radiation was too simplistic. The observations in 2009 also revealed that flares of optical synchrotron emission are sometimes, but not always accompanied by increasing flux levels in the gamma-ray band, dominated by inverse-Compton scattering. The changes in the scaling relations between synchrotron and Compton emission suggested that the energy densities of the magnetic field and/or the radiation fields during different flares changed in time or that flares emerged from different environments. In order to further explore these possible options, the H.E.S.S. telescopes were used for a five-year monitoring project of PKS 1510-089 throughout 2015-2019. The resulting data set reveals a wide range of correlations between different bands, and two particularly bright flares detected in 2016 and 2019.

Figure 1 shows the lightcurve in the TeV- and GeV gamma-ray and the optical (eV) bands for the 2015-2016 period. Right from the start of the monitoring program PKS 1510-089 exhibited prominent flares. Throughout the first half of 2015 a number of bright outbursts were observed in the optical and GeV bands, but not in the TeV regime. On May 29 2016, however, H.E.S.S. observed the brightest VHE flux so far and alerted the community about this event. In the next night, May 30, the VHE flux increased even more, reaching fluxes more than 20 times brighter than the average value.The flare was significant in the GeV and optical bands as well, but the TeV emission was much more pronounced. Figure 2 shows the TeV-, GeV and the optical lightcurves of the extraordinary flare in 2016. MAGIC followed up the H.E.S.S. alert detecting the flare. During this prominent outburst intranight variability was observed in the TeV band which provides challenging constraints on the source model. In the same night. The combined H.E.S.S. and MAGIC lightcurve provides important clues about the mechanism that led to the event ([5]).

Fig. 1: TeV, GeV, and optical light curves of PKS 1510-089 obtained in 2015 and 2016 with (top-to-bottom) the H.E.S.S. telescopes, the Fermi-LAT instrument and ATOM, the optical monitor of the HESS array. In 2015 pronounced flares on the GeV and optical bands are not mirrored in the TeV band. In 2016 a very bright flare is detected with H.E.S.S. while the variations at lower energies are inconspicuous. In all panels data points represent averages over 24h periods Historically, Quasars were assumed not to vary significantly on shorter time-scales. During the bright flare in June 2016, however, the flux varied significantly during a 24h period in all of the energy bands recorded during this event (Figure 2).

The changes in the scaling relations between the different flux bands helps to reveal one of the puzzles emerging from the detection of Quasars at very high energies: How do the gamma-ray photons escape the bright vicinity of the supermassive black hole? In fact, the emission site is a critical issue. As mentioned above, the presence of strong optical photon fields in FSRQs is a blessing and a curse, as it allows for efficient gamma-ray production through inverse-Compton scattering, and for a similarly efficient absorption of VHE gamma rays if they are produced too close to the central region. However, the detection of VHE emission directly implies that the absorption cannot be too strong. As the absorption strength depends on the path length of the gamma-ray photon through the absorber, a lower limit of the VHE emission region from the central black hole can be derived. For the 2016 flare, the lower limit is beyond the so-called broad-line region, which reprocesses accretion disk emission to provide a quasi-isotropic optical photon field within the region. The broad-line region has been typically assumed as the main source of soft photons for the inverse-Compton process to produce gamma-rays.

Fig. 2: A zoom into the 10 day period including the bright TeV flare observed in mid 2016 illustrates the rapid variability and the changes in the scaling relations between the three flux bands. While the lightcurves in the GeV and optical bands clearly reveal a flare matching the outburst detected with the H.E.S.S. telescopes, the amplitudes were much less pronounced.

For the 2015 flares, the lack of VHE activity in April could imply that the emission region is within the broad-line region, and that the VHE emission is effectively absorbed. On the other hand, the detection of significant fluxes in May and July places constrains on the absorption and implies that the emission region must be close to edge of or beyond the broad-line region. In turn, this suggests good circumstantial evidence that flares take place at different sites within the jet. If this is indeed the case, the different environments of the regions producing flares lead to different flux variations and spectral shapes. In PKS 1510-089 interferometric radio observations suggest that the jet is oriented almost perfectly along the line-of-sight (Figure 3), leading to a rare geometric orientation of the thermal radiation field and the motion of the gamma-ray emitting jet. While this orientation would apply to any potential flaring region within the jet of PKS 1510-089, sites at different distances would encounter different ambient radiation- and different intrinsic magnetic fields. The very rapid variations confirm the assumption that PKS 1510-089 is subject to high relativistic beaming. Concluding that the VHE emission site is outside of the broad-line region during several flares, implies that the broad-line region cannot be responsible for the gamma-ray production during these events. Whether or not other soft photon fields can explain the observations, or if fundamentally different models are required (such as models including highly energetic protons as in ([6][7]) requires extensive modeling and further data.

In any case, the described observations reveal that PKS 1510-089 is, indeed, many-faced, and only dense monitoring can unmask it step-by-step. Remarkably, the continuation of the monitoring through 2019 has revealed another bright VHE flare in July 2019 ([8]), similar to the one in 2016. This is one more face to uncover.

Fig. 3: Radio observations of the jet of PKS 1510-089 ([9]) reveal the jet extending to the north close to the nucleus (right panel), bending over by almost 180 degrees 0.3 arcsec north of the core (central panel) and propagating southwards at larger distances (left panel). Such a strong bend suggests that the jet is seen nearly end-on such that a very slight curvature is magnified in projection. This geometry allows an unhindered escape of the very high-energy gamma-ray emission form the dense photons fields of the quasar along our line-of-sight.


(Note 1) The term "Very-High Energy (VHE) gamma-rays" refers to photon energies in excess of 100 GeV. For comparison, an optical photon, i.e. one that is visible to the human eye, has a typical energy of 2 eV.
(Note 2) Gamma-ray astronomers have always been interested in AGN dominated by non-thermal emission, i.e. so-called 'Blazars'. These objects are bright in every energy band of the electromagnetic spectrum, and they exhibit multiple patterns of multi-wavelength variability. In Blazars, the relativistic jet emanating from the central supermassive black hole and the surrounding accretion disk of a distant galaxy is pointed close to our line-of-sight. This enhances the (non-thermal) radiation produced within the jet in the observer's frame through relativistic beaming giving Blazars the ability to even outshine their host galaxies. Blazars are generally separated into two categories, BL Lac objects and flat-spectrum radio quasars (FSRQs) due to their spectral appearance.
Note (3) The absence of thermal radiation fields in BL Lac objects benefits the escape of TeV photons emitted in the center of the active galaxy and made BL Lac objects the key targets for early searches of VHE emitters in the Universe.


[1] Wagner, S., for the HESS collaboration, 2010, HEAD, 11, 27.06
[2] H.E.S.S. Collaboration, Abramowski, A., et al., 2013, A&A, 554, A107
[3] Barnacka, A., Moderski, R., Behera, B., Brun, P., Wagner, S., 2014, A&A, 567, A113
[4] H.E.S.S. Source of the Month 2012-06
[5] Zacharias, M., et al., for the H.E.S.S. and MAGIC Collaborations, 2019, Galaxies, 7, 41
[6] H.E.S.S. Collaboration, Abdalla, H., et al., 2019, A&A, 627, A159
[7] H.E.S.S. Source of the Month 2017-07
[8] Mathieu de Naurois for the H.E.S.S. Collaboration, 2019, ATel #12965,
[9] Homan, D.C., et al., 2002, The Astrophysical Journal, 580, 742-748