Magnetars are very highly magnetised neutron stars with a surface magnetic field reaching 10^15 Gauss, about 1000x stronger than that derived for 'normal' neutron stars, and among the strongest magnetic fields found in the universe.. They power the so-called soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs), which are well-known for their irregular bursts in X-rays and soft-gamma-rays. During the short (faster than minutes) bursts, the brightness can increase by a factor of 1000 or more.  There are only 29 SGRs and AXPs catalogued so far (1).
Following its discovery in 2016, SGR J1935+2154 has become probably the most burst-active SGR, emitting dozens of X-ray bursts over the past few years (2) . SGR J1935+2154 has been associated with the middle-aged  SNR G57.2+0.8 at a distance of about 6.6 kpc (3) as shown in Fig. 1.

While this high energy photon activity from SGR J1935+2154 was certainly interesting in its own right, the situation became considerably more notable with the detection of short, intense radio bursts from the direction of  SGR J1935+2154. To-date, two millisecond-duration radio bursts have been detected - the first burst by CHIME and STARE2 (6,7), and the second burst by FAST (8). When removing the dispersion delay, the timing of the first radio burst appears to line up very well with one of the bright X-ray bursts seen by INTEGRAL and the Insight-HXMT (9, 10). This overlap is now seen as the first convincing evidence that magnetars are linked to FRBs, or at least, to repeating FRBs. Fig. 3 below shows that CHIME measured two components within the first radio burst seen from SGR J1935+2154. The energy of the burst is about 10^34-10^35 ergs, just below the low end of the extragalactic FRBs distribution observed so far. This discrepancy is likely due to the sensitivity limits of current radio telescopes, thereby biasing towards detection of energetic extragalactic FRBs (6).

Fast radio bursts (FRBs) have been one of astronomy's major mysteries since the first accepted example was revealed in 2007 (11) from a burst that actually had occurred in 2001 (radio bursts from M87 reported by (12) in 1980 may in fact be the first examples). Now, over 150 FRBs have been discovered (13), with a small fraction exhibiting repeating radio bursts or even periodicity. Some observational progress in recent years was made via radio interferometric observations, locating FRB predominately towards the outer regions of galaxies within the spiral arms or beyond the central bulge (14). In parallel, radio data analysis pipelines have been tailored to searches for short bursts in almost real-time thus allowing to emit alerts and subsequent searches for emission in other wavelength bands.
Although the time-compact nature (milliseconds duration) and burst statistics of FRBs has pointed to a physical link with stellar remnant compact objects such as magnetars (e.g. 15) , the lack of non-radio counterparts to any FRB hampered progress in unambiguously linking them to specific objects. This all changed on 28 April 2020 with the observations of a simultaneous X-ray and radio burst from magnetar SGR J1935+2154. 

​H.E.S.S. searches for VHE emission from FRBs
Since the discovery of FRBs, the global astrophysics community is pursuing significant efforts to pin-point the origins of these enigmatic radio pulses. The H.E.S.S. collaboration has a long history in these searches. Since 2014 several partnerships with some of the most sensitive radio observatories like Parkes or UTMOST have allowed to conduct searches for delayed or afterglow emission of FRBs in the VHE domain. The first such search was conducted following the detection of FRB 150418 by the SUPERB team at Parkes and has been published by the H.E.S.S. collaboration in (16).
Another, complementary, way to search for MWL emission from various transients and in particular from FRBs, is through coordinated campaigns of various observatories scanning the same part of the sky simultaneously. The most extensive of these endeavors are the Deeper-Wider-Faster campaigns (17) in which H.E.S.S. participates. Since the detection of repeating FRBs like FRB121102 and more recently FRB171019, which is conveniently located in the Southern hemisphere, coordinated MWL campaigns of several instruments can also be tailored to cover these special targets, another attempt which the H.E.S.S. collaboration is actively pursuing.
In parallel, H.E.S.S. has  been running a target of opportunity (ToO) program tailored to searches for VHE emission from SGRs for many years, using Swift-BAT to provide prompt and afterglow triggers for H.E.S.S. Since 2019, we have extended the program to include a dedicated, automatic analysis of data from Fermi-LAT (18) to guide the H.E.S.S. observations. Alerts on activity of several SGRs and AXPs have been received since 2018, but H.E.S.S. observation conditions had not been favorable. The Swift-BAT alerts from SGR 1935+2154 received on 27 April 2020 was the first time H.E.S.S. was able to observe an SGR under this ToO programme. As shown in Fig. 2 right panel, almost all of the  X-ray and radio bursts occurred outside the H.E.S.S. observation window which lasted 2h, however one of the INTEGRAL bursts, burst 'A' as denoted by (9), just overlaps the end of the H.E.S.S. observations. A first analysis of the H.E.S.S. data searching for VHE gamma-ray emission at various timescales has been performed by my PhD student Halim Ashkar. Analyzing the data  as a whole (i.e. the total 2h) and searching for variability or short spikes down to timescales of a few minutes, no significant signal could be found. Further analyses related to the INTEGRAL burst 'A' and covering shorter timescales have started.
The H.E.S.S. observations at VHE energies of SGR J1935+2154 will be important to constrain the particle acceleration scenarios of magnetars. Relying on very basic assumptions, the X-ray bursts could be indicators for surges in particle acceleration. Should the accelerated particles reach TeV or higher energies during the burst, VHE gamma-ray emission may arise from inverse-Compton scattering of electrons, or from protons colliding with surrounding plasma. On the other hand, pair production and photon splitting could result in significant energy losses for the VHE gamma-rays and typically lead to strong cutoffs in the MeV to GeV energy range. The flux suppression could be avoided in scenarios where the gamma rays are generated well away from the magnetar's intense magnetic field (cf. 19). The H.E.S.S. observations can therefore also probe the particle transport aspects (such as outflows) in the vicinity of SGR 1935+2154 during the recent flaring episode.

References
(1) McGill Magentar Catalogue: http://www.physics.mcgill.ca/~pulsar/magnetar/main.html
(2) Lin L. et al  893, 156 (2020)
(3) Zhou P. et al (2020) https://arxiv.org/abs/2005.03517
(4) ATel 13675 : http://www.astronomerstelegram.org/?read=13675
(5) ATel 13758 : http://www.astronomerstelegram.org/?read=13758
(6) CHIME/FRB collab. (Science submitted) arXiv:2005.10324 (2020); ATel 16381 :  http://www.astronomerstelegram.org/?read=13681
(7) ATel 13684 :  http://www.astronomerstelegram.org/?read=13684
(8) ATel 13699 : http://www.astronomerstelegram.org/?read=13699
(9) Mereghetti S. et al. ApJ Lett (submitted) arXiv:2005.06335 (2020)
(10) HXMT SGR J1935+2154 burst list : http://enghxmt.ihep.ac.cn/bfy/331.jhtml; ATel : 13692 http://www.astronomerstelegram.org/?read=13692
(11) Lorimer D. et al.  Science 318, 777 (2007).
(12) Linscott I., Erkes J. 1980 ApJ Lett 236, L109 (1980)
(13) FRBCat: http://frbcat.org/
(14) Bannister K. et al. Science 365, 565 (2019)
(15) Wadiasingh Z. et al. ApJ 879, 4 (2019)
(16) H. Abdalla et al. (H.E.S.S. Collaboration), A&A 597, id.A115
(17) I. Andreoni and J. Cooke, Proc. IAU Symposium, Volume 339, pp. 135-138
(18) J.-P. Lenain, Astronomy and Computing, Volume 22, p. 9-15.
(19) Kun H., et al MNRAS 486, 3327 (2019)

On August 17, 2017, the gravitational wave interferometers Advanced Ligo and Advanced Virgo recorded a signal from the merger of a binary neutron star system, a type of signal that had never been seen before. Complementing this exciting discovery, a large variety of electromagnetic observations were able to record signals from the same event. They range from the detection of a gamma-ray burst about 2 seconds after the gravitational wave event, over near-infrared, optical and UV emission from decay of radioactive nuclei created in the resulting kilonova to X-ray and radio emissions detected several days and weeks after the event. This first and extremely successful observation campaign is marking the beginning of multi-messenger astrophysics.

The gravitational wave event was localized within a region of about 30 square degrees, well beyond the H.E.S.S. field of view and requiring multiple pointings to cover the area. We (Monica, my PhD student and myself) rushed to apply the target selection algorithms that we had prepared and identified regions of high probability to find a counterpart of the gravitational wave event. We were able to request H.E.S.S. observations in record time (only 5minutes after the publication of the localisation of the gravitational wave event by Virgo/Ligo!!). Our observations thus started only 5.3h after the gravitational wave event GW170817. The covered regions already contained the counterpart SSS17a that has later been identified in the optical domain, several hours after our observations (see Figure below). As a result, H.E.S.S. was the first ground-based pointing instrument to obtain data on this object. A subsequent monitoring campaign with the H.E.S.S. telescopes extended over several days, covering timescales from 0.22 to 5.2 days and energy ranges between 270 GeV to 8.55 TeV. No significant gamma-ray emission has been found within this time interval. The derived upper limits on the very-high-energy gamma-ray flux for the first time constrain non-thermal, high-energy emission following the merger of a confirmed binary neutron star system, and further observations of this source will allow to check whether TeV energies are reached on a larger time scale. 

Following the observations, we rapidly transferred the data from Namibia to Europe for analysis. After some troubles due to hard-drives that were crushed during the shipping, we managed to carefully analyse the dataset in record time. In parallel we prepared a publication that (again in record time) has been submitted to ApJLetters right in time for the announcement of the gravitational wave event and the extensive follow-up campaign. The paper can be found on the arXiv tonight. For the very impatient ones it is already available on the H.E.S.S. website (incl. a short news article I wrote).

We also were able to contribute to a paper that has been prepared jointly by all observatories and collaborations contributing to these unprecedented observations. This paper can be found here: "Multi-messenger observations of a Binary Neutron Star Merger". It is definitely a milestone in astrophysics and will be regarded as the birth of multi-messenger astrophysics...

After spending several years on the preparation of  the H.E.S.S. multi-messenger program (see Multimessenger searches for more details), this event together with the multi-messenger and multi-wavelength observation campaign and the successful H.E.S.S. participation is extremely exiting and rewarding for my colleagues, friends and me personally... The dreams we were chasing for the last years (and that have been treated as such by many within the more main-stream astrophysics community) have suddenly materialized and become a reality.

Exciting times ahead: the Virgo and Ligo interferometers are currently not taking data but are undergoing upgrades and further improvements. They should be restarting physics observations in summer 2018 with the prospect to detect many more events of this kind. After the frenzy of the weeks since the detection of GW170817, this break is obviously highly appreciated ;-)
​We'll also use it to further improve the response of high-energy gamma-ray instruments like H.E.S.S., CTA and HAWC to these events. You can even become part of this endeavor: see Team / open positions  for details and don't hesitate to contact me (Contact). Exciting times ahead...

The Australian Square Kilometre Array Pathfinder (ASKAP) has started operations and immediately detected three Fast Radio Bursts (FRBs). With only ~30 FRBs detected so far and over several years, detecting 3 new ones within a few days promises a wealth of observations in the future. For the moment the bursts have 'only' been detected in the primary beam of the telescopes (see figure below), but once interferometry between several telescopes pointing in the same direction is used, the pointing accuracy will improve to arcsecond levels. The paper describing one of the detected FRBs can be found here. A  summary is also given in this nice article.
Hopefully ASKAP (and later SKA) will be able to detect the burst during observations and emit online alerts to follow-up instruments. I am already looking forward to extending the H.E.S.S. FRB program from Parkes to SKA/ASKAP alerts. See FRB follow-up with H.E.S.S. for details about my recent work on this.