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Ben Gompertz

About:

I am a postdoctoral research associate in the extra-galactic astrophysics group of the physics department. I joined the department at Warwick in September 2017 to work with Professor Andrew Levan (now at Radboud University, Nijmegen). My research focuses on explosive transients and gravitational wave sources - primarily short Gamma-Ray Bursts (GRBs), which are explosive events powered by the mergers of compact objects like neutron stars or black holes. I am a current member of the GOTO team, the LSST consortium, and the ENGRAVE consortium.

I received my PhD from the University of Leicester in 2015. The title of my PhD thesis was "The Progenitors of Extended Emission Gamma-Ray Bursts", which focused on the emission mechanisms and environments of a specific sub-class of short GRB that exhibits several hundred seconds of gamma-ray emission that is not seen in 'normal' events. My supervisors were Professor Paul O'Brien and Professor Graham Wynn. I also worked as a member of the X-ray telescope team of the Neil Gehrels Swift Observatory during this time.

After receiving my PhD, I spent two years between June 2015 and August 2017 as a postdoctoral research fellow at Space Telescope Science Institute in Baltimore, USA. I worked with Dr Andy Fruchter on long GRBs - a separate class of astrophysical transients to short GRBs, powered by the collapse of some of the most massive stars in the universe.


Research:

Gamma-Ray Bursts (GRBs) are extremely high energy transients, releasing upwards of 1052 ergs in just a few tens of seconds, which is roughly equivalent to the total energy output of the entire Milky Way galaxy over a two or three year period. They come in three varieties, primarily delineated by the duration of their gamma-ray emission: short, long, and the relatively new ultra-long class. In all three cases, the gamma-rays come from a narrow jet of highly relativistic material that is launched from the central object. As this jet ploughs through the gas and dust in the ambient environment, it forms shocks that radiate right across the electromagnetic spectrum - from X-rays through to radio waves. This is known as the 'afterglow'. As the GRB afterglow fades in the aftermath of the event, we sometimes observe a late plateau in the emission (usually detected at X-ray frequencies due to the excellent coverage from the Swift satellite), suggesting continuing energy injection, perhaps from a long-lived central engine. The nature of this central engine is an important outstanding question in GRB research.

Short GRBs (SGRBs) have durations of less than 2 seconds, and are now known to be caused by the merger of two neutron stars thanks to the recent detection of GW170817 by the LIGO and Virgo gravitational wave interferometers. These mergers are likely to be responsible for the synthesis of most of the elements heavier than iron in the universe. My recent paper (Gompertz et al. 2018) showed that the amount of these elements synthesised in each event may vary by quite a lot. Sometimes, SGRBs show an additional soft gamma-ray emission feature that lasts around a hundred seconds. During my PhD, I showed that this feature is consistent with the energy budget of a magnetar: a highly magnetised neutron star that spins at a rate of close to a thousand times a second. The energy injection plateaus that are seen at X-ray frequencies in some GRBs can also be explained by a magnetar; modelling them as such yields spin periods and magnetic fields that are consistent with magnetar theory (Gompertz et al. 2013). Following on from this, I developed a model capable of powering the emission (Gompertz et al. 2014), invoking a mechanism by which infalling material is accelerated to relativistic speeds by the fast rotating, intense magnetic field. This 'magnetic propeller' is only active if material falls back towards the neutron star after the merger, which may represent the main difference between normal SGRBs and those with extended emission. The presence of a magnetar may be confirmed by well-timed radio observations (Gompertz et al. 2015).

Long GRBs (LGRBs) are caused by the deaths of the most massive stars in the universe; once they have exhausted their fuel, their cores collapse under their own gravity, and this energy release drives the GRB jet. We know this because we see them very close to star forming regions (more massive stars evolve faster, and so die closer to where they were born than less massive stars), and because we always see supernovae alongside LGRBs whenever possible. Their local environments appear to be evenly split between a constant density interstellar medium, and an environment that was strongly influenced by the stellar wind during the life of the star (Gompertz et al. 2018b). The segregation of LGRB gamma-ray energies between these two environment types may hint at two distinct progenitors, though this is currently inconclusive.

Ultra-long GRBs (ULGRBs) are a relatively new class, with only a handful of examples to date. They are characterised by gamma-ray emission that lasts several thousands of seconds (in constract to the several hundred second durations of LGRBs). Their durations are statistically distinct to those of LGRBs, but how the progenitors differ between the two classes is not yet well understood. Clues emerged in the special case of ULGRB 111209A, which had a brighter-than-usual supernova that was consistent with being powered by a magnetar central engine. For this model to be self-consistent, the GRB itself must also be powered by a magnetar, and while this is possible, it is close to the limit of the energy that a magnetar can provide (Gompertz & Fruchter, 2017).

My full publication history can be found by following this NASA ADS search link.


Presentations:

PDF versions of my slides from each presentation are available to download via the links.

UPCOMING: The Diversity of Kilonova Emission in Short Gamma-Ray Bursts (Ioffe Workshop on GRBs and Other Transient Sources: 25 Years of the Konus-Wind Experiment, St Petersburg, Russia in September 2019) - Invited Talk

UPCOMING: The Diversity of Kilonova Emission in Short Gamma-Ray Bursts (National Astronomy Meeting, Lancaster, UK in July 2019)

A Search for NS-BH Binary Mergers in the SGRB Population (RAS Specialist Discussion Meeting: Neutron Star and Black Hole Binary Mergers: The First Results of the LIGO-Virgo Era, London, UK in May 2019)

The Diversity of Kilonova Emission in Short Gamma-Ray Bursts (STScI Spring Symposium: The Deaths and Afterlives of Stars, Baltimore, USA in April 2019)

The Diversity of Kilonova Emission in Short Gamma-Ray Bursts (European Week of Astronomy and Space Science, Liverpool, UK in April 2018)

Magnetars in SGRBs (Capitol Chats, Washington DC, USA in July 2016) - Invited Talk

The Role of Magnetars in Gamma-Ray Bursts (George Washington University, Washington DC, USA in April 2016) - Invited Talk

The Radio Signatures of Magnetar-Driven Short Gamma-Ray Bursts, and their Detectability with the SKA (RAS Specialist Discussion Meeting: Building an Open UK SKA-Science Consortium, London, UK in March 2015)

The Magnetar Central Engine of Extended Emission GRBs and their Link to Short Bursts (Swift: 10 Years of Discovery, Rome, Italy in December 2014)

Magnetars in Extended Emission Gamma-Ray Bursts (New Results in X-ray Astronomy, Cambridge, UK in September 2014)

Gamma-Ray Burst Science with LSST (National Astronomy Meeting, Portsmouth, UK in June 2014)

Magnetars in Extended Emission GRBs (Explosive Transients: Lighthouses of the Universe, Santorini, Greece in September 2013)

Can Magnetar Spin-down Power Extended Emission in Some Short GRBs? (The 7th Huntsville Gamma-Ray Burst Symposium, Nashville, USA in April 2013)

Office: PS.004

Email: b.gompertz@warwick.ac.uk