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... 

I guess most colleagues  in the high-energy astroparticle physics domain accepted the idea of the extragalactic origin of the highest energy cosmic rays (UHECR, E>10^18eV) a long time ago: we have already problems to find sources in our Galaxy which are able to accelerate particles up to the knee in the cosmic ray energy spectrum at around 10^15eV. It is therefore hard to imagine galactic accelerators reaching  to the ultra-high energies. In addition, one would expect these UHECR particles to show a clear anisotropy toward the Galactic Plane if they were produced there, something that is not observed...
Nevertheless, the recent result of the Pierre Auger Collaboration does now provide further evidence for the extragalactic origin of UHECRs. As published in Science, 22 Sep 2017, the arrival direction of particles above 8*10^18eV shows a clear dipolar structure. The amplitude is much stronger than what would be expected from the Compton-Getting effect induced by the movement of the Earth with respect to the cosmic-ray background. Even more interesting, the direction of the dipole (black cross in the figure below) seems to be consistent with the distribution of galaxies in our neighborhood (once the potential magnetic deflections in the galactic magnetic field are taken into account). The direction of galaxy distribution dipole is given as "2MRS" in the figure below, possible magnetic deflections are indicated by the two arrows.
More about UHECRs (mainly my own contributions to the field) can be found here: Cosmic rays

No, not me... although close ;-)  The NASA/JPL Voyager satellites just had their 40th birthday!
The two spacecraft have been launched in August and September 1977 and are still sending data which allow unprecedented studies of the solar system and its boundary the heliosphere. The current status of the mission and the instruments onboard is given here: Voyager mission status
Voyager 1 crossed into "Interstellar space" in 2012 and Voyager 2 is currently in the "heliosheath", the outermost layer of the heliosphere where the solar wind is slowed by the pressure of interstellar gas but still present. Voyage 1 therefore provides the first direct measurements of galactic cosmic rays, unaffected by the solar wind. In addition to the science program, the voyager crafts also provided the first and only pictures of our solar system from the 'outside'. Looking back towards the sun in 1990, voyager 1 took a series of 60 snapshots showing this family picture of 6 planets:

From: NASA/JPL

There are now three laser interferometers observing the universe with gravitational waves! The first direct detection of gravitational waves by the two Advanced Ligo detectors in 2015 was recently confirmed by a third event (see here for details). Having only two interferometers recording the events, the localisation uncertainties of the sources was very challenging (see the right figure below). This is now going to change: "Tuesday August 1st 2017, the VIRGO detector located at the European Gravitational Observatory (EGO) in Cascina (near Pisa, Italy) has officially joined the “Observation Run 2” (O2) and is now taking data alongside the American-based twin LIGO detectors. The O2 data taking period will last until August 25th." (from Virgo).
The expected improvement in the localisation accuracy is visualized in the (simulation based) estimate by Leo Singer shown below (left figure, from Ligo). We keep our fingers crossed that the joint duty-cycle of the three detectors is sufficiently high and that nature is kind enough to send us another strong gravitational wave signal in the few weeks until the end of the current data taking run... 

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.

The Ligo/Virgo collaboration just published a paper describing the detection of the third gravitational wave!! I won't try to compete with the many very well written blogs around the world describing the event and its astrophysical implications. See for example:

• LIGO/Virgo (easy to understand)
• LIGO/Virgo press-release (nice animations)
• Science News
• The Conversation
• etc.

​Obviously the best source of information is always the original paper: B.P. Abbott et al. (Ligo and Virgo Collaborations, PRL 118, 221101 (2017)
Also note that another crucial piece of information has been published at the same time: the full history of the searches for a coincident (or delayed afterglow-like) signal as reported in a large set of GCN-Notices. A significant number of observatories around the world and across the full electromagnetic spectrum, as well as both major high-energy neutrino telescopes, i.e. incl. ANTARES ;-), reacted to the alert sent out by Virgo/Ligo shortly after the detection of the event in early January 2017. Unfortunately none has detected significant emission...
For various reasons we were not able to follow this event with H.E.S.S.. Some details about our preparations for events like this are given here: Multimessenger searches

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