An EU-funded project built by and for the astroparticle and the astronomy communities
  
On the 16th and 17th of September was held in Paris the kick-off meeting for the Astrophysics Centre for Multimessenger studies in Europe - ACME. This HORIZON-INFRA-2023-SERV-01 EU-funded project coordinated by Centre national de la recherche scientifique CNRS aims to realize an ambitious coordinated European-wide optimization of the accessibility and cohesion between multiple leading astroparticle and astronomy research infrastructures, offering access to instruments, data and expertise, focused on the new science of multi-messenger astrophysics.
 
With 40 world-class collaborating institutions from 15 countries, ACME brings together the astroparticle and astronomy communities in a joint effort to forge a basis for strengthened long-term collaboration between these research infrastructures irrespective of location and level up access opportunities across Europe and beyond.
 
ACME objectives are to implement the Astroparticle Physics European Consortium’s (APPEC) and the Planning and Advisory Network for European Astronomy’s (ASTRONET) roadmaps’ recommendations and act as a pathfinder to broaden and improve access to the respective research infrastructures services and data, assess and evaluate new models for better coordination and provision of at-scale services, provide harmonized trans-national and virtual access, develop centres of expertise, improve science data products management, improve interoperable systems for rapid identification of astrophysical candidate events and alert distribution to optimize follow-up observations, provide training for a new and broader generation of scientists and engineers, open the astrophysics and astroparticle physics data sets to other disciplines and increase citizen engagement.
The ACME project coordinator Prof. Antoine Kouchner (CNRS/Université Paris Cité), and co-coordinator Paolo D’Avanzo (INAF), represent each community to ensure balance and drive cross-domain collaboration.

Among the various activities, a workpackage is dedicated to "Improved coordination for real-time detection of transient events and low-latency alert management". It will be lead by Marek Kowalski (Humboldt-University Berlin / Desy-Zeuthen) and Fabian Schüssler (IRFU, CEA Paris-Saclay). The goal of the WP is to create a realtime ecosystem, in which researchers obtain virtual access to different, essential and improved alert streams, provide tools to manage and analyse the streams, and visualise the data and organise follow-up observations based on detections made in near real time . 
 
Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or of the European Research Executive Agency (REA). Neither the European Union nor the granting authority can be held responsible for them.
 
Project page: https://cordis.europa.eu/project/id/101131928
 
Contact:
Antoine KOUCHNER, Scientific Coordinator (CNRS/UPCité): [email protected]
Paolo D’AVANZO, Scientific Co-coordinator (INAF): [email protected]
Julie EPAS, Project Manager (CNRS): [email protected]
​WP 5: [email protected]

Tilepy is a cutting-edge platform designed to optimize and facilitate the scheduling of follow-up observations of multi-messenger events. Developed over the last eight years by a dedicated team of researchers at IRFU, led by myself, the team includes former PhD students Monica Seglar-Arroyo and Halim Ashkar, who began their contributions during their doctoral studies, as well as postdoc Mathieu de Bony de Lavergne. Tilepy, now publicly accessible, offers innovative and easy to use solutions for scheduling follow-up observations of events like gravitational waves (GW), gamma-ray bursts (GRB), and high-energy neutrinos. Tilepy has already been adopted by the international H.E.S.S. and CTA/LST-1 observatories as the default scheduling tool for multi-messenger studies.

​Supporting Multi-Messenger Astronomy
Tilepy's capabilities are showcased through its application in various multi-observatory, multi-wavelength campaigns. It is currently the default scheduling tool for searches of high-energy gamma-ray emission with the H.E.S.S. and CTA/LST-1 observatories [2]. Tilepy played a pivotal role in the H.E.S.S. multi-messenger campaign of GW170817, the first detected neutron star merger, scheduling observations that allowed H.E.S.S. to be the first ground-based instrument to observe the merger location and this several hours before the discovery of the optical counterpart [3]. Tilepy has since been extended and can now be used to efficiently schedule observations across the full electromagnetic spectrum from radio waves to the highest energy gamma-rays.

Open Source and Community-Driven
As an open-source project, Tilepy invites contributions from the global research community. The code is available on GitHub, and we welcome feedback and collaboration to continuously improve its functionalities.

Recent Publication
Tilepy has been detailed in a recently published article in the Astrophysical Journal Supplement Series (ApJS 274 (2024) 1). This publication provides an in-depth look at the algorithms and performance of Tilepy, further establishing its significance in the field of multi-messenger astrophysics.

For more information and to access the Tilepy code, visit our GitHub repository (https://github.com/astro-transients/Tilepy) and the official Tilepy website (https://Tilepy.com). In addition, the Astro-COLIBRI platform (https://astro-colibri.science) is providing an integrated experience to Tilepy's scheduling capabilities.


Contact and Support
For questions, support, and further discussions, visit the Astro-COLIBRI forum (https://forum.astro-colibri.science/c/instrumentation-and-tools/Tilepy) or reach out to the Tilepy team at [email protected]. 


References:
[1] M. Seglar-Arroyo et al., “Cross-Observatory Coordination with tilepy: A Novel Tool for Observations of Multi-Messenger Transient Events”, ApJS 274 (2024) 1
[2] H. Ashkar et al., “The H.E.S.S. gravitational wave rapid follow-up program”, JCAP 03 (2021) 045 
[3] Abdalla H. et al. (H.E.S.S. Collaboration), “TeV gamma-ray observations of the binary neutron star merger GW170817 with H.E.S.S.”, Astrophys. J. Lett. 850 L22

We recently launched several new ways of interaction with the Astro-COLIBRI platform. The first is an openn discussion forum. We hope that it will bring together the community of time domain astrophysics and will be interesting for both professional and amateur astronomers. It is also the prime place to get in contact with the Astro-COLIBRI team, ask questions, request new features and report bugs. I will continue to post major results here, but will use the forum for more rapid updates on scientific results, new analyses and observations, etc. Make sure to log-in to the forum (using your Astro-COLIBRI account) and hit the bell icon next to interesting categories.
Check it out at: https://forum.astro-colibri.science 

This blog entry is largely based on a H.E.S.S. "Source of the Month" article mainly written by Jean Damascene Mbarubucyeye (Desy-Zeuthen) and that will be published on the H.E.S.S. website soon.

An extremely energetic flash emitted with a stellar explosion that happend 3 billions years ago in a distant galaxy hit Earth in October 2022 and was recorded as the brightest gamma-ray burst that has ever been detected. It has rapidly be dubbed "The BOAT" (for "Brightest Of All Times") in the GRB community. The flux of gamma rays was overwhelming most detectors and caused changes in the electromagnetic properties of Earths atmosphere. The following figure shows the number of X-ray photons (in millions of counts/second!) detected by the GBM instrument onboard the Fermi satellite for a selected number of GRBs. Many results have been published in a special issue in ApJLetter. See this link for an overview.

Gamma-ray bursts (GRBs) are observed as bright X-ray and gamma-ray flashes from distant sources outside of our Galaxy. They come from the deaths of rapidly rotating massive stars and the mergers of compact objects such as neutron stars. For the first kind of GRB, the core of the star collapses and a fraction of the released gravitational energy is fed into a violent blast wave ploughing through the remnants of the star at nearly the speed of light. The resulting electromagnetic emission can be roughly divided into two phases: the prompt phase, the initial bright flashes that typically last up to a few tens of seconds, and the slowly fading afterglow phase that can be detectable over a large part of the electromagnetic spectrum for days or months. During the afterglow, the blast wave produces relativistic shocks that propagate into the surrounding material and accelerate charged particles such as electrons. These accelerated electrons then interact with the magnetic field in the material, emitting X-ray radiation in the form of synchrotron radiation.

The physics of GRB afterglows is a prime opportunity for the study of relativistic shocks. The synchrotron emission process, which produces the X-ray emission, has been studied extensively and is well understood; however, the details of emission at higher energies, such as the very-high energy (VHE) gamma-ray (> 100 GeV) domain, is still under debate. In the last few years, a number of GRBs have been detected in VHE gamma rays. This VHE component is usually associated with the inverse Compton scattering of either ambient or synchrotron photons, the latter being known as the synchrotron self-Compton (SSC) scenario. However, a mismatch between observations and the usually employed single-zone (i.e., uniform magnetic field) SSC description was noted in GRB 190829A (see [2]  and the June 2021 SOM). Further observations, especially in the VHE gamma-ray domain, are therefore necessary to shed light onto this open question.

​On October 9th 2022, the Fermi Gamma-ray Burst Monitor and later the Neil Gehrels Swift Burst Alert Telescope detected and localized GRB 221009A. H.E.S.S. was not able to observe the burst immediately as, due to the full moon at the time, the night sky background was too high for the operation of our highly sensitive instruments. However, observations become possible two days later, on October 11, and continued over more than a week after the burst was first detected. Unfortunately, the campaign was heavily influenced by rather poor atmospheric conditions including cloudy skies and a high aerosol content. Careful analyses of the data acquired by H.E.S.S did not yield any significant detection of VHE gamma rays at the GRB position in the total dataset nor for individual nights.
Nevertheless, we were able to derive stringent upper limits on the VHE gamma-ray flux. These can be put into the context of the X-ray afterglow (cf. left figure below) and are useful to constrain the various possible theoretical emission scenarios. An example for this is given in the right figure below.

GRB 221009A had many exceptional features in addition to its exceptional brightness. In particular, the long lived X-ray afterglow spectrum remained consistently hard for several nights; i.e., in the X-ray regime, a larger fraction of the total energy was emitted in higher-energy photons compared to lower-energy photons than is often seen for GRBs at these times. A possibility is that the electrons are continuously accelerated with a remarkably hard spectrum. Why this should be the case is a mystery. A firm detection by H.E.S.S. in such cases would greatly advance our understanding of these rare events, but the upper limits equally play an important role in disentangling the various theoretical models.
​Another important question that remains to be answered is why some afterglows are gamma-ray bright with comparable X-ray and gamma-ray fluxes (e.g. GRB 190829A), while for others, like GRB 221009A, the VHE emissions seems evidently much weaker. VHE gamma-ray astronomy has a major role to play in this story.
​

References
​[1] F. Aharonian et al. (H.E.S.S. Collaboration), H.E.S.S. follow-up observations of GRB 221009A, 
ApJLetters 946, L27 (2023)
[2] Abdalla et al. (H.E.S.S. Collaboration). 2021, Revealing x-ray and gamma ray temporal and spectral similarities in the GRB 190829A afterglow, Science, 372, 1081

This blog entry is in large parts a copy of the H.E.S.S. Source of the Month published in 2022/04. The original article can be found here.

In the summer of 2021, the binary star RS Ophiuchi reprised its explosive role as a nova and caused a bright spectacle over the night sky visible to the naked eye. RS Ophiuchi lies in the Galactic neighbourhood of the constellation of Ophiuchus and is a well known recurrent nova, meaning it undergoes an eruption every few years. The latest eruption was marveled at by observers worldwide using everything from binorculars and commercial hobby telescopes all the way up to large collaboration driven instruments. H.E.S.S. was one of these large instruments, which discovered the nova as the first Galactic very-high-energy transient and subsequently captured the evolution of the gamma-ray emission over several weeks [1].

Novae are astrophysical transient phenomena originating from binary systems consisting of a white dwarf (WD) and a less compact companion star. The distance between the stars in such binary systems is typically comparable to the distance between the Earth and Sun, which allows the more compact WD to accrete matter from its companion star throughout most of the orbit. The accretion process forms a dense layer of hydrogen on the WD’s surface. As the hydrogen layer grows, the pressure and temperature at the bottom of the layer keeps rising until a critical limit is reached. A thermonuclear reaction is then triggered, which blows away matter from the WD in the form of a rapidly expanding shell, similarly to a supernova. The nova is then seen as a bright outburst of light in the sky which fades away after a few weeks. The brightest novae are visible with the naked eye and appear as if a new star came and went. Recurrent novae are known to experience multiple such eruptions at semi-regular intervals of a few years. The well known recurrent nova RS Ophiuchi (RS Oph) was first recorded to erupt in 1898 and has erupted every 9 to 26 years [2], with the latest instance occuring in August of 2021. The nova at its peak reached a visual magnitude of about 4.5, far higher than the binary's usual quiescent magnitude of about 12.5. The eruption was then noticed by amateur astronomers and reported to the wide network of astronomers and astrophysicists [3], triggering numerous follow-up observations worldwide. 
As a side note: we hope to increase the connections and exchanges with the the amateur astronomy community via our multi-messenger transients platform Astro-COLIBRI. 

During the rise of the RS Oph 2021 outburst, telescopes across multiple wavebands reported heightened flux levels coincident with the nova. Following the detection of the outburst with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope [4], the H.E.S.S. telescopes rapidly commenced observations of the nova event. The first night of observations by H.E.S.S. on the 9th of August led to a clear detection of the nova, establishing novae as Galactic transients reaching TeV energies for the first time in history. The discovery by H.E.S.S. was reported promptly to the astronomical community, informing everyone of the historical event and prompting further follow-up observations [5]. The MAGIC collaboration independently confirmed the detection of the nova in the very-high-energy (VHE) domain [6]. H.E.S.S. continued to take data of the RS Oph outburst for several weeks and further updated the astronomical community [7]. A pause of observations had to be taken once the night sky became too bright towards full moon, preventing further observations with the H.E.S.S. telescopes.

Analysis of the first five nights resulted in a strong signal on the position of the nova (Fig. 1, left). The data taken after the pause of observations, about 17 days after the initial burst, showed that the outburst became much fainter, but was still detectable (Fig. 1, right). The light curve derived over the observed time frame (Fig. 2) shows an initial increase in flux reaching a plateau about two days after the maximum in the GeV band, before starting to decay. A dedicated Fermi-LAT analysis then shows a consistency in the temporal decay between the Fermi-LAT GeV waveband and H.E.S.S. VHE waveband.

The detection of VHE gamma rays in the RS Oph 2021 outburst by H.E.S.S. has strong implications for the physics in the environment of novae. As the hydrogen layer accreted by the WD detonates and expands, it crashes into the stellar wind of the companion star. The interaction between the expanding shell and the companion wind creates an astrophysical shock. In shocks such as these, the kinetic energy of ejected matter is converted into acceleration of cosmic rays such as protons, electrons and heavier nuclei through a process known as diffusive shock acceleration. Accelerated particles subsequently cool down through interactions with matter and magnetic fields, which in the end leads to emission of photons over several wavebands. The detection of gamma rays in particular gives insight into the efficiency of the particle acceleration. Efficiency of acceleration is key to understanding the origin of the cosmic rays that constantly surround us. By comparing energetics derived from parameters such as the wind speeds and mass loss rates against the measured maximum gamma-ray energies, H.E.S.S. was able to confirm that the efficiency of particle acceleration in the RS Oph 2021 outburst reached the theoretical limit. Additionally, the changes of the nova environment throughout its evolution is reflected in the observed gamma-ray emission (Fig. 3). Over time the spectrum visibly flattens, the GeV emission subsides while the TeV emission increases and reaches higher maximal energies, a behavior well described by an expanding shock. This discovery bodes well for the theoretical origin of Galactic PeV cosmic rays, as these are assumed to originate from supernova remnants that must in turn achieve the theoretical limit with maximal efficiency. An extrapolation of the nova conditions to the related phenomenon of supernovae would be able to support the theory of supernova remnants as the origin of Galactic PeV cosmic rays.

[1] H.E.S.S. Collaboration et al., Science, Vol 376, Issue 6588 , pp. 77-80, DOI: 10.1126/science.abn0567 (2022) and arXiv e-prints 2202.08201 (2022).
[2] E. Brandi, C. Quiroga, J. Mikołajewska, O. E. Ferrer, L. G. García, A&A 497, 815 (2009).
[3] S. Kafka, Observations from the AAVSO International Database, https://www.aavso.org (2021).
[4] C. C. Cheung, S. Ciprini, T. J. Johnson, The Astronomer's Telegram No. 14834 (2021).
[5] S. J. Wagner, H. E. S. S. Collaboration, The Astronomer's Telegram No. 14844 (2021).
[6] MAGIC Collaboration et al., arXiv eprint 2202.07681 (2022).
[7] S. J. Wagner, H. E. S. S. Collaboration, The Astronomer's Telegram No. 14857 (2021).

Thanks to these detailed preparations, H.E.S.S. was the first ground-based instrument to observe the event GW170817 [1-3] on 2017 August 17, the neutron star merger that was accompanied by a gamma-ray burst. These observations resulted in the first stringent upper limits on the VHE emission from a neutron star merger. Details about this amazing event and our observations can be found in previous blog post: the-birth-of-multi-messenger-astrophysics.html
Since 2017, H.E.S.S has also observed four merger events of two black holes. Black hole mergers are not typically expected to produce gamma-ray emission because, due to long merger times, most of the surrounding matter has been accreted before the merger happens. However, a weak gamma-ray transient detected by the gamma-ray telescope Fermi-GBM [4] less than half a second after a merger of two black holes triggered much interest in the astrophysical community and encouraged the search for VHE emission from such events. Moreover, GW signals from black hole mergers are detected much more often than from neutron star mergers and are therefore a good way to test H.E.S.S.'s response to GW events.
The four observations reported in another paper published in 2021 have allowed us to commission the H.E.S.S. GW follow-up program and to improve it for future observations. The paper itself is available here and via the arXiv. The localization regions of GW sources can span several tens to hundred of square degrees in the sky, while the H.E.S.S. field of view is around 20 square degrees. Therefore, follow-up observations of GW alerts are challenging and require special strategies to be implemented. These strategies are detailed in [5]. In a nutshell, the goal of these observation strategies is to identify the regions that are most likely to have hosted the event of the GW event, taking into consideration the visibility and observational constraints of the H.E.S.S. telescopes.

Only alerts of GW events whose localization regions do not exceed a few hundred degrees in the sky are picked for observation by our algorithms; otherwise, the region would be too large to cover in a manageable time. H.E.S.S. was able to schedule observations on several GW events. However, nature is not always on our side, and clouds and rain disturbed two of these observations. In total, and in addition to GW170817, four successful observations were performed on four different black hole mergers: GW170814 [6-7] in 2017 during the second GW observing period, GW190512_180714 and GW190728_064510 in 2019 during the first part of the third observation period [8] and GW200224_222234 in 2020 during the second part of the third observing period [9]. Electromagnetic counterparts of these events were never found by the astronomical community and thus they remain poorly localized. However, H.E.S.S. was able to cover large portions of their localization regions which increases the chance of having covered the true positions of the events. The start of the H.E.S.S. observations ranges from three hours after the detection of the event GW200224_222234 to two days for GW170814.
No significant VHE signal was found but the good quality data obtained permit us to compute upper limits on the VHE gamma-ray emission in the sky regions covered by H.E.S.S. [11-12]. These upper limits constrain for the first time VHE emission from black hole mergers. 
​
In the future the interpretation of upper limits can be eased by having earlier and longer observations. Assuming that the H.E.S.S. observation strategy remains the same, and due to the expected high rate of GW detections with additional and/or more sensitive detectors in the future, H.E.S.S. will have on average the chance to observe several GW events with minimal delay. Moreover, the localization of GW events is expected to improve as more GW detectors join the network, which means that some of these events will be sufficiently well localized that they will be able to be observed with a single pointing. This will allow us to spend more time on one individual position and obtain longer, more sensitive observations, instead of spending time on covering large areas in the sky. In conclusion, this will result in stronger constraints on the VHE emission from black hole mergers. All this will hopefully become a reality during the next observation run of the Ligo/VIRGO/KAGRA interferometers that is scheduled to start end of 2022.

References

• H. Ashkar et al., "The H.E.S.S. Gravitational Wave Rapid Follow-up Program", JCAP 03 (2021) 045; arXiv:2010.1617
  
• H. Abdalla et al. (H.E.S.S. Collaboration), "H.E.S.S. follow-up observations of Binary Black Hole Coalescence events during the second and third Gravitational Waves observing runs of Advanced LIGO and Advanced Virgo", ApJ, 923, 109 (2021); arXiv:2112.08307

A few other articles on the results:

• H.E.S.S. Source of the Month
• Academic Times 
• VICE​
• CEA press release
• IRFU: Fait marquant

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)

This is one of these rare moments: after decades of (quite often rather frustrating) searches we finally did it!

​






​

​What did we do?
We detected very high-energy emission from a gamma-ray burst (GRB). These extremely energetic cosmic explosions typically lasting for only a few tens of seconds. They are the most luminous explosions in the universe. The burst is followed by a longer lasting afterglow mostly in the optical and X-ray spectral regions whose intensity decreases rapidly. The prompt high energy gamma-ray emission is mostly composed of photons several hundred-thousands to millions of times more energetic than visible light, that can only be observed by satellite-based instruments. Whilst these space-borne observatories have detected a few photons with even higher energies, the question if very-high-energy (VHE) gamma radiation (at least 100 billion times more energetic than visible light and only detectable with ground-based telescopes) is emitted, has remained unanswered until now.On 20 July 2018, the Fermi Gamma-Ray Burst Monitor and a few seconds later the Swift Burst Alert Telescope notified the world of a gamma-ray burst, GRB 180720B. Immediately after the alert, several observatories turned to look at this position in the sky.
For H.E.S.S. (High Energy Stereoscopic System), this location became visible only 10 hours later. Nevertheless, the H.E.S.S. team decided to search for a very-high-energy afterglow of the burst. After having looked for a very-high-energy signature of these events for more than a decade, the efforts by the  collaboration now bore fruit.
A signature has now been detected with the large H.E.S.S. telescope that is especially suited for such observations. The data collected during two hours from 10 to 12 hours after the gamma-ray burst showed a new point-like gamma-ray source at the position of the burst. While the detection of GRBs at these very-high-energies had long been anticipated, the discovery many hours after the initial event, deep in the afterglow phase, came as a real surprise. The discovery of the first GRB to be detected at such very-high-photon energies is reported in a publication by the H.E.S.S. collaboration et al., in the journal 'Nature' on November 20, 2019.

Who is "we"?
The results were obtained using the High Energy Stereoscopic System (H.E.S.S.) telescopes in Namibia. This system of four 13 m diameter telescopes surrounding the huge 28 m H.E.S.S. II telescope is the world's most sensitive very high-energy gamma ray detector. The H.E.S.S. telescopes image the faint, short flashes of bluish light emitted when energetic gamma rays interact with the Earth's atmosphere (so-called Cherenkov light), collecting the light with big mirrors and focusing it onto extremely fast reacting sensitive cameras. These Cherenkov images allow H.E.S.S. to reconstruct the properties of the interacting gamma-rays and ultimately detect their sources.
The High Energy Stereoscopic System (H.E.S.S.) team consists of over 200 scientists from Germany, France, the United Kingdom, Namibia, South Africa, Ireland, Armenia, Poland, Australia, Austria, the Netherlands, Japan  and Sweden, supported by their respective funding agencies and institutions.

While I was not personally involved in the data analysis for this particular event, I am responsible for the searches for transient (i.e. rapidly fading) phenomena within the H.E.S.S. collaboration (technically speaking I am the convener of the "Transient" working group). Among the various topics covered in this group, searches for emission from GRBs have always been (and will obviously remain) the highest priority. Other searches include the quest for gamma-ray emission associated to Gravitational Waves, high-energy neutrinos, Fast Radio Bursts, Novae and Supernovae as well as flares from AGN, stars, magnetars, etc.

What does this mean and what is next?
The very-high-energy gamma radiation which has now been detected not only demonstrates the presence of extremely accelerated particles, but also shows that these particles still exist or are created a long time after the explosion. Most probably, the shock wave of the explosion acts here as the cosmic accelerator. Before this H.E.S.S. observation, it had been assumed that such bursts likely are observable only within the first seconds and minutes at these extreme energies.
At the time of the H.E.S.S. measurements the X-ray afterglow had already decayed very considerably. Remarkably, the intensities and spectral shapes are similar in the X-ray and gamma-ray regions. There are several theoretical mechanisms for the generation of very-high-energy gamma light by particles accelerated to very high energies. The H.E.S.S results strongly constrain the possible emission mechanisms, but also present a new puzzle, as they request quite extreme parameters for the GRB as a cosmic accelerator.
Together with the observations of very-high-energy gamma radiation following later GRBs with MAGIC (published in the same edition of Nature on Nov. 20, 2019) and again with H.E.S.S. (GRB190829A), this discovery provides deeper insights into the nature of gamma-ray bursts and opens the window for deeper observations and further studies. 
For more than a decade, Cherenkov telescopes such as H.E.S.S., MAGIC and VERITAS have searched for very-high-energy gamma radiation from GRBs and continuously improved their observation strategies. Now several GRBs have been detected at very high energies within a very short time, and we now know that these bursts are emitting at extreme energies for many hours. This opens entirely new perspectives for further observations with the current instruments and is even more promising for the successor instrument, the Cherenkov Telescope Array, which will enable us to study these stellar explosions in much more detail.

A few highlights of the media coverage:

• A very-high-energy component deep in the γ-ray burst afterglow; H.E.S.S. collaboration; Nature 575, 464-467 (2019); https://www.nature.com/articles/s41586-019-1743-9 also available on the arXiv: https://arxiv.org/abs/1911.08961
• Teraelectronvolt emission from the gamma-ray burst GRB 190114C, Nature 575, 455-458 (2019); https://www.nature.com/articles/s41586-019-1750-x
• Observation of inverse Compton emission from a long gamma-ray burst, Nature 575, 459-463 (2019); https://www.nature.com/articles/s41586-019-1754-6
• Nature editorial by Bing Zhang
• Fait marquant de l'IRFU

For this blog, I reused text prepared within the H.E.S.S. collaboration. See H.E.S.S

High-energy astrophysics has seen quite some amazing revolutions over the last years. To name just a few: we detected Gravitational Waves (GWs) and could link them to Gamma-Ray Bursts (GRBs), we detected high-energy neutrinos and could link (at least one of them) to a flaring blazar, we scanned the Galactic Plane discovering a wealth of new sources emitting VHE gamma-rays and we could finally observe VHE gamma-rays from a GRB. These and many more discoveries are not only great achievements of the past: more importantly they open new windows to the high-energy universe and thus promise even more exciting observations and future discoveries. To fully exploit these possibilities, we obviously need the right instruments and observatories. But fortunately the future is also bright in this respect: the VIRGO and LIGO interferometers are being improved continuously and will path the way towards 3rd generation instruments like the Einstein Telescope and LISA. The IceCube neutrino telescope is continuing operations while IceCube-Gen2, GVD and KM3NeT are being prepared and built. The SVOM and later the ATHENA (and hopefully the THESEUS) X-ray satellites are being constructed, the pathfinders of the Square Kilometer Array (SKA) radio observatory have started operations and the full SKA is approaching fast. In the VHE gamma-ray domain, the current Imaging Air Cherenkov Telescopes (H.E.S.S., MAGIC and VERITAS) are shifting their focus more and more to transient and multi-messenger studies and operations are assured for the next few years when the Cherenkov Telescope Array (CTA), which is currently entering its construction phase, will take over. The HAWC observatory is producing novel and surprising results at an amazing rate, while the next-generation observatory LHAASO is being constructed.
While this global landscape of observatories is certainly extremely broad, there is at least one crucial piece missing: a large field-of-view VHE gamma-ray observatory in the Southern hemisphere. While many arguments​ lead to this conclusion, the main ones are: 

• All current and planned VHE monitoring observatories (e.g. HAWC, LHAASO) are located on the Northern hemisphere thus missing half of the sky
• The Southern sky provides access to the wealth of gamma-ray sources found in Galactic Plane
• Monitoring instruments play a crucial role in the new era of multi-wavelength and multi-messenger time domain astronomy

Following the success of the HAWC air shower array, the idea to build a next-generation observatory in the Southern hemisphere has been floating around the community for several years. Different groups started to develop various design ideas and started building prototypes to valide them. With the aim of structuring these efforts, the Southern Gamma-ray Survey Observatory (SGSO) Alliance was founded about a year ago. Its aim is to form a community of scientists to work together towards the definition and implementation of a next-generation, large field-of-view high-energy gamma-ray observatory in the Southern hemisphere. The Alliance (and yes, the reference to the Star Wars universe is on purpose ;-)) already attracted more than 110 friends and colleagues from 18 countries around the globe (cf. right map above).
Over the last year I helped coordinate an effort to define the science case of such an observatory. Put in simple terms, the aim of this enterprise was to agree on and outline a baseline of what science we want to do with such an instrument. Many people contributed to the discussions and the writing of what is now called the "Science Case for a Wide Field-of-View Very-High-Energy Gamma-Ray Observatory in the Southern Hemisphere".  The paper is available on the arXiv today...

In summary, we believe that the Southern Gamma-ray Survey Observatory, a next-generation high-energy gamma-ray observatory in the Southern hemisphere, will provide unprecedented observations of high-energy phenomena in the universe. These can be roughly divided into four main categories:

• SGSO will allow studying particle acceleration in the most violent sources of our Galaxy. It will provide an unbiased and deep survey of the Milky Way at multi-TeV energies, localize sources able to accelerate particles at least up to the knee of the cosmic rays spectrum and study large scale and extended emission throughout our cosmic neighbourhood.
• Thanks to its large field-of-view and duty-cycle, the SGSO continuous monitoring capability of the Southern sky will be unrivaled. It will collect long-term measurements of variable high-energy emitters ranging from Galactic binaries to active galactic nuclei and will thus provide crucial input to the understanding the underlying phenomena that give rise to gamma-ray emission from these objects. As a truly multi-messenger observatory, SGSO will contribute to our understanding of compact objects via searches for VHE counterparts to gravitational waves and the study of gamma-ray bursts at VHE energies. SGSO will elucidate links between astrophysical sources of high-energy radiation and high-energy neutrinos and play a leading role in searches for VHE emission associated to novel transient phenomena like fast radio bursts.
• SGSO will provide new observational impulses for searches of phenomena beyond the Standard Model of particle physics. It will enable detailed searches for dark matter in a large variety of regions like the center of our Galaxy or newly discovered satellite galaxies. SGSO will also enable searches of Lorentz-invariance violating effects, the evaporation of primordial black holes and axion-like particles. 
• Complementing the searches for the sources of high-energy cosmic rays using gamma rays as tracers, SGSO will record an enormous amount of high-precision data on hadronic cosmic rays. It will therefore be able to study the cosmic ray energy spectrum in the crucial energy range around the knee and provide novel insights into cosmic ray anisotropy at various angular scales and across a large range of energies. SGSO will help study the Sun and fluxes of energetic particles in the heliosphere. 

SGSO will be a key player in the multi-wavelength and especially the multi-messenger community. Its unique monitoring capabilities will allow to alert observers around the world, across the full electromagnetic spectrum and all known messengers of new detections and phenomena. These alerts will be provided in real-time and will thus allow triggering detailed follow-up observations. While broad-band MWL information will be an important ingredient to SGSO science, the expected performance is to a significant extent complementary to and beneficial for the science of the upcoming Cherenkov Telecope Array (CTA). SGSO will for example act as a high-duty cycle, large FoV finder scope for deep and high-resolution CTA observations for many sources, including transient phenomena in various multi-wavelength and multi-messenger contexts. In addition to providing real-time ``triggers" to CTA, SGSO may also provide important input for detailed CTA analysis on extended sources like PWNe, their TeV halos, the Fermi-Bubbles or the diffuse Galactic emission.
The ambitious goals of SGSO will be made possible by important developments and design studies. Various detector and array designs are currently studied with simulations and validated with prototypes. In parallel, several candidates for an optimal site for the future observatory have been identified and are being assessed.

​More details in our extensive paper: https://arxiv.org/abs/1902.08429arxiv.org/abs/1902.08429

Exciting times ahead... Lets see where this adventures takes us...