Striking a Jet -
and Pinpointing the Flame

February 2021

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When striking a match - rubbing the flammable head [of the match] past a rough surface - it ignites in a flash and lights up. In very general terms something similar happens when a relativistic plasma jet hits an obstacle on its way outward from its launching site in the vicinity of the supermassive black hole. In many quasars that expel relativistic jets, the outflow is inhomogeneous and forms 'knots' or 'blobs' moving with different speeds and occasionally hitting each other. It has long been speculated that such interactions might be the cause of strong flares - rapid increases in brightness - observed in the VHE regime that are observed with HESS. In a study published by the H.E.S.S. collaboration and other groups this month ([1],*), it is now possible to directly observe this phenomenon - a plasma blob 'rubbing' against another structure resting in its path and thus striking - not a match, but a powerful, relativist jet to exhibit a bright flare of Very High Energy (VHE) gamma-ray radiation with an enormous amount of energy. This first occasion of detailed simultaneous observations of a VHE flare and monitoring the motions of the same jet with very high angular resolution provided by Very Long Baseline Interferometry (VLBI) sheds new light on participle acceleration mechanisms in jets and pinpoints the sites of the luminous outburst.

The new study deals with quasar PKS 1510-089, whose many faces have been described in an earlier chapter of the Source of the Month collection. It is a flat-spectrum radio quasar, which implies that it hosts a luminous broad-line region (BLR) – a region of plasma rapidly orbiting the central black hole that emits optical emission lines. This quasar was discovered at VHE gamma-rays with H.E.S.S. in 2009 ([2], [3] and H.E.S.S. SOM 2/2012). Since 2015 H.E.S.S. has conducted a monitoring campaign lasting for hundreds of hours of observation time. This resulted in the detection of an unexpected flare in May 2016, which displayed a record VHE gamma-ray brightness and very fast variability timescales. Triggered by the alert, our colleagues of the MAGIC collaboration obtained additional data which allowed the two teams to assemble a combined lightcurve covering most of the brightest flare. The rapid flux variability (as short as about 20 minutes) implies that the emission region within the jet is very compact (see Note 1).

The small size does not tell us, however, where this emission region is located within the jet. The standard assumption has been for a long time that such fast flares originate relatively close to the black hole, where the jet is still very focused and narrow. In case of PKS 1510-089, this would imply that the emission region would be buried within the BLR, which results in a curious problem: BLR photons actually absorb VHE gamma-rays by a process called pair-creation. One would not expect VHE gamma-rays that might be emitted within the BLR to escape and be detectable ([4]. This observation can be used to derive an estimate for the location of the emission: As the absorption of gamma-rays gets more efficient for higher gamma-ray energies, the degree of absorption can be extracted by comparing the observed gamma-ray spectra to models predicting various amounts of absorption. Results of this analysis are shown in Figure 1

Fig. 1: Contemporaneous gamma-ray spectra with model curves for the H.E.S.S. and MAGIC observation windows, see [1].

All plausible models shown in Figure 1 reveal that the amount of absorption is negligible and thus argue that the VHE gamma-rays are indeed emitted outside the BLR and hence at considerable distance from the supermassive black hole. But is this the true location of the emission region? The described method is only an indirect determination and rests upon the assumption that the BLR is well understood. It would be much better to obtain a direct image of the emission region.

This requires high angular resolution – which is available at radio frequencies with VLBI observations. A VLBI monitoring program is conducted by the Blazar group at Boston University. They observe PKS 1510-089 with a monthly cadence. Despite the fact that PKS 1510-089 is located 6 billion light-years from Earth, the VLBI observations can resolve the jet on a scale of about 6 light-years. While this is not enough to image the much more compact emission region directly, it is enough to disentangle regions of enhanced flux within the larger jet structure and localize them with unprecedented accuracy. Figure 2 shows a sequence of VLBI images produced by our colleagues from Boston University. It shows the evolution of the jet of PKS 1510-089 around and after the time of the VHE flare. One can clearly see the radio core – the region of highest radio flux – and the jet emerging in a top-right (north-west) direction. Four additional features in the jet are noted: A1 and A2, which are stationary regions and the outward moving regions K15 and K16 that may pass through or by the stationary features at some time.

Fig. 2: A sequence of radio VLBI images. Visible jet features are marked by the colored circles. The position of the radio core is marked by the line, see [1].

The new publication deals in particular with the motion of K16. It turned out to be a very fast component moving very close to our line-of-sight. As one can see in Figure 3 (bottom right panel), it passed the stationary feature A1 right around the VHE gamma-ray flare in May 2016. If these events are indeed linked, it would imply that the flare occurred at the position of this feature A1, about 150 light-years from the black hole (Note 2). While VHE gamma-ray emission in jets at large distances from the black hole had been discovered previously (e.g. [5], H.E.S.S. SOM 7/2020), the precise localization of a flaring region is a significant achievement.

An animation of the jet behavior around the time of the flare is shown in Figure 3 (top).

Fig. 3: Animation of the passage of K16 through A1 (top), along with the VHE gamma-ray and optical lightcurves (bottom left), and the separation of radio components from the core (bottom right). (c) Credit: Heike Prokoph, DESY, adopting diagrams from [1].

Given the large distance of the emission region from the black hole, the conventional model for gamma-ray radiation from Quasars (which is inverse Compton scattering of photons from the BLR and the so-called 'dusty torus') is not applicable. While we have witnessed the striking of a jet and the resulting flare, the specific physical processes are not yet solved. As a single swallow does not make a summer, the implications of this work need to be confirmed. This raises the question 'How frequent are such events?'. Another seemingly similar flare was discovered with H.E.S.S. in PKS 1510-089 in July 2019 ([6]). A potential connection with another moving radio component is yet to be established. Furthermore, the search for such a connection in other sources continues. Monitoring campaigns, as well as Target-of-Opportunity observations of multi-wavelength flares of blazars with H.E.S.S. are mandatory in order to link VHE gamma-ray flares with moving components in the jets – and thus to pinpoint the location of the emission region.

Note 1: VLBI observations such as those described in Ref. 1 have shown that the jet moves with a velocity close to the speed of light. According to Einstein's Special Relativity, the true physical time-scale appear to be shorter (faster) when observed from an observer at rest. The observed 20 min time-scale hence relates to a true time-scale of nine hours. This is still very fast and implies that the diameter of this region is 9 light-hours, about as large as the orbit of planet Neptune.

Note 2: As the jet is directed close to the line-of-sight from Earth, we can only see it in projection onto the plane of the sky. 150 light-years is the true distance of A1 from the core, while the observed (projected) distance is a lot less.


[1] H.E.S.S. collaboration, et al. ArXiv 2012.10254
* In collaboration with the MAGIC collaboration and the Blazar group at Boston University
[2] Wagner, S., for the H.E.S.S. collaboration, 2010, HEAD, 11, 27.06
[3] H.E.S.S. Collaboration, Abramowski, A., et al., 2013, A&A, 554, A107
[4] Barnacka, A., et al., 2014, A&A, 567, A113
[5] The H.E.S.S. Collaboration: Nature, 582, 356–359 (2020)
[6] Mathieu de Naurois for the H.E.S.S. Collaboration: The Astronomer's Telegram, #12965