A glimpse into the Very (High Energy) bright future

Paper Title: Detecting VHE prompt emission from binary neutron-star mergers: ET and CTA synergies

Authors: Biswajit Banerjee, Gor Oganesyan, Marica Branchesi, Ulyana Dupletsa, Felix Aharonian, Francesco Brighenti, Boris Goncharov, Jan Harms, Michela Mapelli, Samuele Ronchini, and Filippo Santoliquido

Corresponding Author Affiliation: Gran Sasso Science Institute, INFN – Laboratori Nazionali del Gran Sasso, and INAF – Osservatorio Astronomico d’Abruzzo, Italy

Status: ArXiv open access

Multi-messenger astronomy is a powerful tool allowing us to study astrophysical sources by looking at them in different “messengers” – different signals that could come from the same source, which include photons, neutrinos, gravitational waves, and cosmic rays. By studying these different messengers, we can get new insight into the astrophysical sources that produce these signals.

One of the first multi-messenger success stories was a binary neutron star merger observed on August 17th, 2017 by the LIGO and Virgo gravitational wave detectors, which was named GW170817. 1.7 seconds after the merger, the Fermi and INTEGRAL satellites observed a short gamma-ray burst, GRB 170817A, which was associated with the gravitational wave signal. (For more information about GRBs, check out the Astrobites Guide to Transient Astronomy!) This discovery was hugely exciting in the multi-messenger astronomy community – it cemented neutron star mergers as the source of short gamma-ray bursts (GRBs), and paved the way for further investigation of neutron star mergers in both gravitational waves and light.

Since then, more investigation into short GRBs has taken place, but there are still open questions to solve. Next generation very high energy (VHE) telescopes and next generation gravitational wave detectors may give us more insight into the properties of neutron star mergers, which is the topic of today’s paper.

Promptly searching for prompt emissions

We separate gamma-ray bursts (GRBs) into two types based on the length of time they last, called short (lasting less than 2 seconds) and long (lasting more than 2 seconds). Short GRBs are those associated with binary neutron star mergers, which is the focus of today’s paper. 

There are often two phases of GRB emission: the prompt or initial burst phase, and an afterglow or later emission. In this paper, the authors target the prompt phase of the emission, which is believed to be powered by relativistic jets, a high energy beam of photons that is produced when the neutron stars merge. By looking at this initial burst of light, astronomers can gain insight into what is happening inside the jet, and what is powering the high energy photons we see.

Schematic showing the gravitational wave event and the gamma-ray burst. The gravitational wave signal increases in frequency, until the merge (at time 0) and then a short delay (0.01 to 10 seconds) happens before a sharp peak illustrating the gamma ray burst, which falls off rapidly over time.
Figure 1: A schematic of the gravitational waves observed from a binary neutron star merger, and the prompt emission of photons after the merger. The light blue line shows the gravitational wave, which increases rapidly in frequency and amplitude before the merge. After a short delay, the short gamma-ray burst can be seen (dark red color) which is powered by relativistic jets after the merge of the two neutron stars. (Figure 2 in the paper).

Although we’ve seen very high energy (energies greater than 30 GeV) emission from the afterglow phase of GRBs before, we have not yet observed VHE emission from the prompt phase. Observing this emission is crucial to our understanding of the relativistic jet which produces the GRB, but observation of the prompt emission has its challenges. 

Today’s paper analyses the prospects for detection of VHE gamma-rays from binary neutron star mergers with the next generation of gravitational wave detectors in the Einstein Telescope (ET) and Cosmic Explorer (CE) and the next generation of VHE gamma-ray telescopes using the Cherenkov Telescope Array (CTA)

The challenges, and the solutions

Two of the main challenges to observing a short GRB from a gravitational wave event are the localizations of the events and the time it takes to reposition the telescope after an event occurs. 

Gravitational wave event locations are typically difficult to pinpoint, leading to large localization areas on the sky (often up to 100s or 1000s of square degrees) while most VHE telescopes have much smaller fields of view (10s of degrees). Both the ET and CE gravitational wave detectors are expected to improve the localization of the gravitational wave event, even before the merge happens. 

These telescopes will also be able to detect the inspiral (the period right before the merge, when the neutron stars get very close together) of the two neutron stars, which will give early warning to telescopes up to 15 minutes before the merge happens.

This early warning will be a huge advantage to telescopes trying to observe the burst of light expected from the merge. They will be able to reposition and be ready for the burst before it happens, which is critical since a short GRB lasts only up to 2 seconds. 

The Cherenkov Telescope Array (CTA) also brings large improvements to these issues on the VHE photon side. CTA’s Medium Size Telescopes have a field of view of 44 square degrees and 90 second slew time (the slew time is the time it takes to repoint the telescopes), and its Large Size Telescopes have a field of view of 13 square degrees and 20 second slew time. Using this, and the early warning from the gravitational wave detectors, CTA could possibly provide the first detection of this VHE prompt photon emission from a binary neutron star merger. 

The authors use a simulation of both the gravitational wave detectors and the CTA medium size telescopes to calculate the number of binary neutron star mergers that CTA could observe VHE photons from the prompt phase of a GRB. With only 5 minutes of early warning from ET and CE, CTA would be able to detect VHE prompt photons from many GRBs, even those with large (more than 1000 square degrees) localizations!

two-panel plot of anticipated detections per year of gravitational wave events with different sky localization sizes. There are many more detections expected with large localization, but there are some with a localization that is fairly small around 10 degrees.
Figure 2: Number of gravitational wave events expected to be detected per year using the next generation of gravitational wave detectors, with an early warning (time before the merger) of 15 minutes (left panel) or 5 minutes (right panel). These are shown versus their sky localization on the x-axis in degrees squared. The θv<10 degrees in the y-axis means that we are positioned to see the relativistic jets from these mergers, which means that we could observe a short GRB from these events. (Figure 5 bottom left two panels in the paper).

Two panel plot showing expected detections with CTA. As the sky localization increases, CTA is able to detect about 1 event for each localization with 15 minutes of warning or 10 events with 5 minutes of warning.
Figure 3: Number of CTA VHE prompt photon detections expected from these binary neutron star mergers from Figure 2 (above), assuming either 15 minutes (left) or 5 minutes (right) early warning time. There are more gravitational wave events with a 5 minute early warning time than 15 minutes, leading to more detections, but as the sky localization in either case gets large the field of view of the telescopes can no longer cover the entire region where the gravitational wave event may be. (Figure 7 first column in the paper).

The takeaways

With the next generation of both gravitational wave detectors and VHE gamma-ray detectors, searches for VHE photon emission from binary neutron star mergers will be within reach for the first time. This will allow astronomers and astrophysicists to probe the fundamental properties of relativistic jets formed during the merger of the two neutron stars.

Astrobite edited by: Lina Kimmig

Featured image credit: ESO/L. Calçada/M. Kornmesser, CC BY 4.0, via Wikimedia Commons

About Jessie Thwaites

Jessie is a PhD student at the Wisconsin IceCube Particle Astrophysics Center at the University of Wisconsin-Madison. They study possible astrophysical sources for high energy neutrinos through multimessenger astrophysics. Outside of physics, they play horn and enjoy spending time outdoors, especially skiing and biking.

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1 Comment

  1. As one of the corresponding authors of this paper, I am glad to see such a compact and nice summary of our work. I liked the title so much that I might use it in my talks on this topic from now on.
    Thanks and regards!


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