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

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!

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​

​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...

In the last entry to this blog I talked about the birth of gravitational wave multi-messenger astrophysics and how the first joint detection of gravitational waves and electromagnetic waves in August 2017 opened a totally new window to the universe. But the summer of 2017 produced even more surprises!
On September 19, 2017: at 20:54:30.43 Coordinated Universal Time (UTC), a high-energy neutrino of about 290TeV (IceCube-170922A) was detected in an automated analysis that is part of IceCube’s real-time alert system. An alert was distributed to observers 43 seconds later. The direction of the neutrino (reconstructed to an area of about 1 square degree and consistent with the location of a known gamma-ray blazar TXS 0506+056) became visible at the location of the H.E.S.S. observatory in Namibia around 4 hours later. The event perfectly fulfilled the criteria I had outlined in the follow-up program of high-energy neutrinos (cf. Multimessenger searches). We could thus start H.E.S.S. observations and acquired an initial dataset of 1.3h during that night. After a first look at this data and without detecting significant gamma-ray emission I announced this in ATEL #10787. Additional observations were obtained on subsequent nights but still no gamma-ray emission could be detected from the region. Upper limits at 7.5*10^-12 erg / cm^2 /s (95% C.L.) on the gamma-ray flux level were subsequently derived. Too bad for us, but the story fortunately does not stop there...

Observations of lower energy gamma-rays obtained with the LAT instrument onboard the Fermi satellite showed the blazar TXS 0506+056 to be in a flaring state since April 2017. Strong flux variations by almost an order of magnitude with respect to the long-term average had been observed over several weeks. This first observation of a neutrino in spatial coincidence with the gamma-ray emitting blazar during an active phase suggests sparked the interest of the wider astronomical community and triggered an extensive multi-wavelength campaign with observations ranging from radio frequencies to high-energy gamma-rays. During this campaign high-energy gamma rays with energies up to 400 GeV were detected by the MAGIC instrument located at the Roque de los Muchachos Observatory on the Canary Island of La Palma. The emission in X-rays showed clear evidence for spectral variability, the flux in the optical V band was the highest ever observed in recent years and polarization has been detected at the level of 7% in the R band. A summary of these observations can be found in a joint publication in Science 361. Finally, the redshift of TXS 0506+056 was recently determined to be z = 0.3365 +- 0.0010 using the largest single mirror optical telescope, the Gran Telescopio Canarias (S. Paiano et al. ApJL 854), providing crucial input into our understanding on the level of attenuation of the TeV flux following its propagation through extragalactic space.

Is this the first source of high-energy neutrinos?
If true, this would mean that blazars may indeed be a source of high-energy cosmic rays and thus provide a crucial step towards resolving a century old puzzle. Whoa... But I believe it is important to keep a cool head, and note that there remains a roughly 0.1% probability that the coincidence of the neutrino event with the flare of TXS 0506+056 is purely a random chance coincidence. Think of it like rolling a dice 4 times continuously getting the exact same number: not totally out of the ordinary, right? There is also an ongoing discussion about an additional 44% probability that the neutrino was induced by a CR hitting the Earths atmosphere and would thus not point back to any astrophysical origin in the first place.
One thing is clear: this correlation is extremely interesting and promises a significant breakthrough. Confirmation can only come with further observations. Thanks to our preparations, H.E.S.S. and the upcoming Cherenkov Telescope Array are prime instruments to do just this... Looking forward to the next neutrino alert. It may just be around the corner...

A large amount of very interesting articles describing our findings have been published. Here I only a very personal (and thus biased) selection:

• Fait marquant de l'IRFU
• Le Temps (Lausanne)
• Desy-Zeuthen: https://multimessenger.desy.de
• Le Monde
• Der Spiegel
• New York Times
• ...

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 Advanced Ligo and Advanced Virgo observatories announced the first detection of a gravitational waves that has been recorded by all three interferometers simultaneously. The paper describing the event has been published by PRL. The merger event itself, a binary black hole composed of a black hole with ~30 solar masses and another one with ~25 solar masses at a redshift of about 0.1, are matching nicely the previous detections:

The crucial part of this detection is that is has been seen by all three instruments. It is thus confirming the performance of the Virgo instrument that had been switched on only two weeks before this observation. It also illustrates the power of combining the data from the different instruments to significantly improve the localisation uncertainty of the origin of the gravitational wave. Using only data from the two Ligo detectors, the 90% uncertainty maps spans 1160deg^2 across the sky, impossible to scan with typical telescope searching for a counterpart/afterglow/etc. Adding the data from Virgo this region shrinks to less then 100deg^2!! This improvement is clearly visible in the skymaps below.
Regions of this size become accessible for pointing telescopes. As example: we managed to cover almost the complete region with H.E.S.S. observations searching for high-energy gamma rays emitted by the black hole merger. As everybody else we didn't see any... See GCN #21673 for a summary of our observations.
The importance of this event is thus primarily its illustration of the achieved precision of the GW localisation allowing for follow-up observations across the electromagnetic spectrum. Astronomy with gravitational waves is thus becoming a reality... Stay tuned for more... 

Over the last week the 35th International Cosmic Ray Conference has been held in Busan (South Korea). It is the largest and best-known meeting on astroparticle physics and is being held every two years. This year about 850 participants from more than 50 countries made the trip to Busan, making it a perfect place to meet many colleagues and friends and learn about the most recent results from all major astroparticle observatories. This year I was invited to give a plenary talk summarizing searches for transient phenomena with high-energy gamma-ray observatories. My talk is here...

Among the most striking new results is an indication for a small scale anisotropy in the arrival directions of ultra-high energy cosmic rays (UHECR) detected by the Pierre Auger Observatory. In addition to the confirmation of the previously announced dipole structure, they found a correlation with nearby starburst galaxies (i.e. galaxies with a high level of star formation) at angular scales around 13deg. The significance reached about 4sigma (i.e. the probability for a background fluctuation is less then one in 25.000). Details are given here. Somehow everybody fears that this hint might have a similar fate as the previous claim (see Cosmic Rays for more on this). Hopefully these doubts will be proven unsubstantiated by future observations and we will look back at Busan as the place where UHECR astronomy started. ;-)

The H.E.S.S. collaboration reported an updated measurement of the energy spectrum of cosmic electrons with unprecedented precision. It seems like the shape of this spectrum and especially the pronounced and very sharp break around 950GeV will give theorists some sleepless nights. It might be related to the set in of cooling effects in the interstellar magnetic fields, but one would naturally expect a much more smoother turn-over. See here for details.

H.E.S.S. also reported on improvements on the event reconstruction and analysis techniques. The most important new technique is probably the introduction of time dependent Monte Carlo simulations which reproduce the actual data taking conditions much more precisely and thus lower the systematic uncertainties. Coincidentally the introduced scheme is very similar to the one I introduced for the Pierre Auger Collaboration during my PhD thesis and which is still used to measure the energy spectrum of cosmic rays. The first impressive application of the new MC technique is the measurement of the Crab nebula at TeV energies, roughly matching the one seen in X-rays. See here for the talk presenting the analysis.

The MAGIC collaboration showed a hint for the long-awaited detection of a gamma-ray burst (GRB) with their Imaging Air Cherenkov Telescope system. The burst itself (GRB 160821B) is one of the brightest short GRBs seen by the GBM instrument onboard the Fermi satellite and MAGIC could react to the incoming alert message within only 24 seconds. Unfortunately the weather conditions at that moment were pretty unfavorable with clouds, high moonlight illuminations and high zenith angles leading to a rather high energy threshold around 500GeV. The observed hotspot has a statistical significance of (only) about 3sigma. The talk discussing the observations is here.