Catching a gamma-ray flare with the Event Horizon Telescope & friends

Title: Broadband Multi-wavelength Properties of M87 during the 2018 EHT Campaign including a Very High Energy Flaring Episode

Authors: The Event Horizon Telescope – Multi-wavelength science working group, The Event Horizon Telescope Collaboration, The Fermi Large Area Telescope Collaboration , H.E.S.S. Collaboration, MAGIC Collaboration, VERITAS Collaboration, and EAVN Collaboration

Status: Submitted to Astronomy and Astrophysics

Figure 1: The first image (ever!) of a supermassive black hole, M87*, which is also the subject of today’s paper. This image was taken by the EHT Collaboration during their first campaign on M87 in 2017

The Event Horizon Telescope: more than just donut pictures

The team behind the famous donut-esque pictures of supermassive black holes that have come out over the past few years (see these bites on Sgr A* and M87) – the Event Horizon Telescope (EHT)– is back! This time, they report on a multiwavelength campaign on M87*1, the first supermassive black hole ever imaged, which is located in the M87 galaxy, a known active galactic nucleus (AGN). The campaign discussed in today’s paper is particularly interesting because it coincides with a flaring episode of M87 in very high energy  (VHE) gamma-rays, which are photons with energies higher than 100 GeV

Figure 2: The composite image of M87* and its jet during the 2018 EHT campaign. This image is for relative scale only, and the colour/brightness of each image is not necessarily indicative of any physical differences between images. Figure 10 from the paper.

These VHE gamma-ray flares are thought to originate from clumps of material (often just called “blobs”) falling into the supermassive black hole’s jet, a powerful outflow of particles that’s launched from the black hole’s poles. These blobs contain particles that can be accelerated by the powerful jet up to some of the highest energies we see in the universe, producing bright flares across the electromagnetic spectrum that last from hours to weeks. However, there’s a lot we don’t understand about the origin and structure of jets, let alone how they produce extra blobs of particles and are able to accelerate those particles to such high energies. 

Some jet-setting gamma-rays

Today’s paper seeks to investigate any changes to the jet during the flare in 2018 compared to the first EHT campaign (in 2017), when M87* was in a quiescent (non-flaring) state. This is where the multiwavelength complementarity comes in: gamma-ray instruments like Fermi-LAT, H.E.S.S., MAGIC, and VERITAS, can pick up on rapid changes in gamma-ray brightness to fully capture the time structure of the flare. The radio components of the traditional EHT image (along with Chandra’s excellent X-ray spatial resolution at higher energies) can help resolve if there are any changes to the physical structure of the jet itself by directly examining the images of the jet (see Figures 2 and 3). 

Figure 3: Comparison of the East Asia VLBI Network (EAVN) jet images between the 2017 campaign (top) and this paper’s 2018 campaign (bottom). The colour and contours correspond to the flux (photons/cm2/s; roughly a proxy for brightness) at 43 GHz. The differences are relatively minor, but a keen eye might be able to pick up on a counterclockwise rotation in the jet’s position angle or its orientation. Figure 11 from the paper. 

The gamma-ray flare lasted three days and exhibited the “harder when brighter” phenomenon that’s seen in most AGN flares – where the source not only brightens, but we also detect emission at higher energies than usual. All three major operating ground-based gamma-ray telescopes (called imaging atmospheric Cherenkov telescopes or IACTs) caught at least some of the 2018 flare, and were able to complement each other to provide a full “picture” of the flare’s brightness developing in time (see Figure 4).

Fermi-LAT, a slightly lower energy all-sky gamma-ray instrument, also caught the flare. Fermi-LAT’s detection is important for understanding the full range energies at which the flare was seen. The brightness of the source increased by a factor of over three times the low brightness state of the source measured a few months earlier. Variability in M87’s gamma-ray emission has been known for over two decades, so it is regularly monitored by gamma-ray instruments in hopes of catching flares like this one!

Catching a gamma-ray flare is really valuable, because we get two key pieces of information that help interpret any visual changes we see in the jet images:

  1. The timescale of the flare tells us about the size of the region that’s emitting gamma-rays. The timescale of the flare can’t be faster than the time it takes light to travel across the size of the emitting region – otherwise we wouldn’t see a pulse-like shape and the emission would look fairly continuous.
  2. The spectral energy distribution (SED; the distribution of how bright the source is as a function of energy) gives us information about how powerful the accelerator is, which is determined by the physics going on in the jet, among other places in the vicinity of the supermassive black hole.
Figure 4: The VHE gamma-ray flare of M87 as seen by three IACTs: VERITAS, H.E.S.S., and MAGIC in 2018. The telescopes are all in different geographic locations, so captured different parts of the flare depending on their ability to observe M87. The data are fit with a Gaussian model. The horizontal lines represent the flux of the source compared to the Crab pulsar wind nebula, which is the brightest source in the VHE sky. 

Long [wavelength] story short

In the longer, lower energy radio wavelengths, the authors couldn’t identify any variability – i.e., no flare. However, with the super high resolution images that longer wavelength instruments were able to get of the jet, the authors can tell that it has changed its orientation angle in the year between the 2017 and 2018 campaigns (see Figure 3). EHT also detected some change in the orientation of the accretion disk since the first time it was imaged in 2017 (see the donut-shaped image at the top of Figure 2). 

However, the authors found that it is pretty complicated to model M87’s emission mechanisms and regions from fitting models to its SED. Different components of the multiwavelength emission seem to be coming from different regions of the AGN, or perhaps different ways of accelerating particles. Modelling M87’s emission is further complicated by the evidence that the source is only flaring (so brighter!) in high energies. For now, the authors can’t pinpoint where the gamma-ray emission is coming from and seem to need to venture beyond simple models of the source to figure it out. More complicated models need more data to be constrained, though!

Much more to come!

Since this paper’s 2018 campaign, the EHT multiwavelength team have taken more observations of M87*, with a more sensitive very long baseline interferometry (VLBI) array, and have planned more for the future. The observations from 2021 and 2022 are currently being analyzed, and the team hopes that they will reveal even more information about the physics of the system. In particular, they hope to be able to link the physics in the accretion disk with the jet and finally figure out where the observed gamma-ray emission is coming from. The EHT team is also busy looking at other nearby AGN, including our own Galaxy’s Sgr A*, so stay tuned for more exciting results to come!

1 For clarity, M87* refers to the supermassive black hole, and M87 (without the *) refers to the whole galaxy, which is often what’s referred to in this paper, since high energy instruments typically don’t have the angular resolution to distinguish the black hole from its host galaxy.

Disclaimer: Today’s author is a member of the VERITAS Collaboration and an author on this paper but did not contribute significantly to this project.

Astrobite edited by Maria Vincent

Featured image credit: Event Horizon Telescope (modified by the author)

About Samantha Wong

I'm a graduate student at McGill University, where I study high energy astrophysics. This includes studying all sorts of extreme environments in the universe like active galactic nuclei, pulsars, and supernova remnants with the VERITAS gamma-ray telescope.

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