All sky archival search for FRB high energy counterparts with Swift and Fermi

Fast radio bursts (FRBs) are millisecond-duration radio signals from unknown cosmic origin. Many models associate FRBs with high-energy astrophysical objects such as magnetars. In this attempt to find counterparts to FRBs, we explore gamma-ray bursts (GRBs) from the Swift and Fermi missions. We first search for spatial correlations between FRB and GRB populations as a whole and then search for a one-by-one correlation between each of the FRBs and GRBs investigated. Temporal coincidences are not considered. To evaluate the significance of any correlation found, we generate background realizations that take into account instrumentally induced anisotropies in the distribution of the sources. Neither study yields any significant counterpart detection. We estimate that less than 4\% of the FRBs are associated with GRBs in the studied samplesComment: Proceedings 38th International Cosmic Ray Conference (ICRC2023

tilepy: rapid tiling strategies in mid/small FoV observatories

The challenges inherent to time-domain multi-messenger astronomy require strategic actions so that adapted, optimized follow-up observations are performed efficiently. In particular, poorly localized events require dedicated tiling and/or targeted, follow-up campaigns so that the region in which the source really is can be efficiently covered, increasing the chances to detect the multi-wavelength counterpart. We have developed the python package "tilepy" to rapidly derive the observation scheduling of large uncertainty localization events by small/mid-FoV instruments. We will describe several mature follow-up scheduling strategies. These range from an option to use of low-resolution grids, to the full integration of sky regions and targeted observations using galaxy catalogs. The algorithms consider the visibility constraints of customisable observatories and allow to schedule observations in both astronomical darkness and in moonlight conditions. Developed initially to provide a rapid response to gravitational wave (GW) alerts by Imaging Atmospheric Cherenkov Telescopes (IACTs), they have been proven successful, as shown by the GW follow-up during O2 and O3 with the H.E.S.S. telescopes, and particularly in the follow-up of GW170817, the first binary neutron star (BNS) merger ever detected. Here we will present a generalisation of these rapid strategies to other alerts showing large uncertainties in the localization, like Gamma-Ray Burst (GRB) alerts from Fermi-GBM. We will also demonstrate the flexibility of {\it tilepy} in scheduling observations for a large variety of observatories. We will conclude by describing the latest developments of these algorithms that are able to derive optimised follow-up schedules across multiple observatories and networks of telescopes.Comment: Proceedings 38th International Cosmic Ray Conference (ICRC2023

H.E.S.S. realtime follow-ups of IceCube high-energy neutrino alerts

The evidence for multi-messenger photon and neutrino emission from the blazar TXS 0506+056 has demonstrated the importance of realtime follow-up of neutrino events by various ground- and space-based facilities. The effort of H.E.S.S. and other experiments in coordinating observations to obtain quasi-simultaneous multiwavelength flux and spectrum measurements has been critical in measuring the chance coincidence with the high-energy neutrino event IC-170922A and constraining theoretical models. For about a decade, the H.E.S.S. transient program has included a search for gamma-ray emission associated with high-energy neutrino alerts, looking for gamma-ray activity from known sources and newly detected emitters consistent with the neutrino location. In this contribution, we present an overview of follow-up activities for realtime neutrino alerts with H.E.S.S. in 2021 and 2022. Our analysis includes both public IceCube neutrino alerts and alerts exchanged as part of a joint H.E.S.S.-IceCube program. We focus on interesting coincidences observed with gamma-ray sources, particularly highlighting the significant detection of PKS 0625-35, an AGN previously detected by H.E.S.S., and three IceCube neutrinos.Comment: Presented at the 38th International Cosmic Ray Conference (ICRC2023). See arXiv:2307.13047 for all IceCube contribution

nectarchain\texttt{nectarchain}, the scientific software for the Cherenkov Telescope Array -- NectarCAM

The NectarCAM is a camera that will be mounted on the Medium-Sized Telescopes of the Cherenkov Telescope Array (CTA) observatory. Along with the hardware integration of the camera, the scientific software, $\texttt{nectarchain}$, is being developed. The software is responsible for transforming the raw data from the camera into analysis-ready calibrated data. In this contribution, we present the structure of the software, which consists of two modules: the calibration pipeline and the data quality check pipeline. The calibration pipeline reduces the data, performs flat fielding, and determines the gain for the analysis. The data quality monitoring pipeline is used to select the data that meets the necessary standards for analysis. Additionally, we discuss the format of the downstream data and the integration of the $\texttt{nectarchain}$ modules in the general software framework of CTA. We also present the necessary tests for validating each part of the code. We conclude by mentioning the prospects for the future of the software.Comment: Presented at the 38th International Cosmic Ray Conference (ICRC 2023), 2023 (arXiv:submit/5126940

Astrophysique transitoire et multimessager avec les télescopes à imagerie Tcherenkov atmosphérique

I hunt very-high-energy (VHE) emission from three classes of transient sources: Gravitational waves (GW), their possible gamma-ray bursts (GRB) counterparts and fast radio bursts (FRB). I tackle three types of challenges: Preobservation, focusing on efficient scheduling, observation planning and fast telescope response; in-observation, focusing on efficient data acquisition on the transients as an expert on call; post-observation, focusing on efficient data analysis in the search for a significant signal. For the GW events, the poor localization requires a special treatment for the follow-up. I work on the optimization of GW follow-up strategies that rely on the maximization of the probability of detecting counterparts to transient GW events either by using solely GW localization maps or by correlating the local distribution of the galaxies in the Universe with GW maps. The tools are combined in a GW scheduler, tested and assessed. Using the developed tools and acquired skills, I develop automatic response schemes to GW alerts for the H.E.S.S. and LST telescopes and a response to UTMOST FRB alerts for H.E.S.S. I also contribute to the update of the GRB program in H.E.S.S. on the technical follow-up front. With the implemented tools, H.E.S.S observed 6 GW events, of which 4 BBH mergers. Other transient sources, for which I am involved in the analysis, are observed also by H.E.S.S.: GRB 190829A, GW170817 (long-term), FRB events and the soft gamma-ray repeater (SGR) SGR 1935+2145. GRB 190829A is the third GRB to be observed at VHE energies on the ground and the second one by H.E.S.S. The VHE data (in addition to the Xray and UV data for FRB 20171019A) is analysed for the other transients. No significant VHE emission was found during the BBH merger follow-up and the GW170817 long-term follow-up observation campaigns. The same non-detection was found for the FRBs and SGR 1935+2154. For those, a search for variability on the minutes to sub-seconds scale did not lead to any significant detection either. Upper limits are derived from the observations of the transients presented above. These results are published and the upper limits permit to constrain the magnetic field in the GW170817 remnant, to constrain the persistent and transient emission from SGR bursts and to derive prospects for the fourth GW observation run, O4.Je recherche des émissions de très hautes énergies (VHE) à partir de trois types de sources transitoires : les ondes gravitationnelles, les sursauts gamma qui peuvent en émerger et les sursauts radio rapides. Pour les ondes gravitationnelles, la localisation des évènements sur plusieurs dizaines de degrés dans le ciel nécessite un traitement spécial pour le suivi. J'optimise des stratégies de suivi ondes gravitationnelles qui reposent sur la maximisation de la probabilité de détection d'émission électromagnétique homologue, soit en utilisant uniquement des cartes de localisation, soit en utilisant aussi la distribution locale des galaxies dans l'Univers. Les outils développés sont testés et évalués. À l'aide de ces outils et des compétences acquises, je développe un système de réponse automatique aux alertes ondes gravitationnelles pour les télescopes H.E.S.S. et LST et aux alertes UTMOST pour H.E.S.S. Je contribue à la mise à jour du programme de H.E.S.S. de suivi de sursaut gamma. Avec les outils mis en place, H.E.S.S a observé 6 événements ondes gravitationnelles, dont 4 fusions de trous noires. D'autres sources transitoires sont également observées par H.E.S.S., dont le sursaut gamma GRB 190829A, la fusion d'étoile à neutron GW170817 (à long terme aussi), des sursauts radio rapides et la source SGR 1935+2145. GRB 190829A est le troisième sursaut gamma détecté à très hautes énergies au sol. Les données VHE, sont analysées (ainsi que les données X et UV pour FRB 20171019A) pour les autres sources. Aucune émission significative de VHE n'a été trouvée durant les suivis des fusions de trous noirs et durant la campagne d'observation à long terme de GW170817, ainsi que pour les sursauts radio rapide et la source SGR 1935+2154. Je déduis des limites supérieures et publie ces résultats. Les limites déduites permettent de contraindre le champ magnétique dans le vestige de GW170817, de placer des limites supérieures sur l'émission VHE persistante et transitoire provenant des sursauteurs gamma mou, et d'établir des perspectives pour O4

Searches for TeV gamma-ray counterparts to Gravitational Wave events with H.E.S.S

International audienceThe search for electromagnetic counterparts for gravitational waves events is one of the main topics of multi-messenger Astrophysics. Among these searches is the one for high energy gamma-ray emission with the H.E.S.S. Imaging Atmospheric Cherenkov Telescopes in Namibia. During their second Observation Run O2, the Advanced Virgo detector in Italy and the two advanced LIGO detectors in Washington and Louisiana while conducting joint observations, detected for the first time, on August $14^{th}$, 2017 a transient GW signal due to the coalescence of two stellar masses black holes, an event labeled GW170814. The alert announcing the event was issued two hours later and H.E.S.S. observations could be scheduled for the nights of $16^{th}$, $17^{th}$ and $18^{th}$ August 2017. Three days after the binary BH merger, on August $17^{th}$, the coalescence of two neutron star was detected for the first time, followed by a GRB detection by Fermi's GBM starting a new era in multi-messenger Astronomy. Observations started 5.3 h after the merge and contained the counterpart SSS17a that was identified several hours later. It stands as the first data obtained by a ground-based pointing instrument on this object. In this contribution, we will present the results of the search of high-energy gamma ray emission as electromagnetic counterpart of these two GW events. No significant gamma ray emission was detected for either event. Nevertheless upper limit maps were derived constraining, for the first time, the non-thermal, high-energy emission on the remnant of a three detector binary black hole coalescence (GW170814), and a binary neutron star coalescence (GW170817)

The case of the missing VHE GRBs: A retrospective study of Swift gamma-ray bursts with Imaging Atmospheric Cherenkov Telescopes

International audienceGamma-ray bursts (GRBs) are particle acceleration sites that can emit photons in the very high-energy (VHE) domain through non-thermal processes. From 2004 until 2018, the current generation of Imaging Atmospheric Cherenkov Telescopes (IACTs) did not detect any GRB in the VHE domain. However, from 2018 to 2020, five detections have been reported. In this work, we try to solve the case of the missing VHE GBRs prior to 2018. We aim to identify GRBs that might have eluded VHE detection in the past years by the H.E.S.S., MAGIC, and VERITAS IACTs. To do so, we study GRBs with known redshift detected by \emph{Swift} from 2004 until June 2022. We first identify all GRBs that could have been observed by these IACTs since 2004, considering observation conditions and visibility constraints. We assume a relation between the X-rays and the VHE gamma rays based on the VHE GRBs detected to date and combine this with the redshift measurements, instrument response information, and observation conditions to predict the observed VHE gamma-ray flux from the \emph{Swift}-XRT measurements. We report findings on 12 bright low-redshift GRBs that we identify as most likely to have been detected in the VHE domain by current IACTs. The rate of IACT-detectable GRBs with ideal observation conditions is $<$1 VHE GRB per year with the current configuration. With its lower energy threshold and higher sensitivity, this rate increases to $\sim$4 VHE GRBs per year with CTA

H.E.S.S. realtime follow-ups of IceCube high-energy neutrino alerts

International audienceThe evidence for multi-messenger photon and neutrino emission from the blazar TXS 0506+056 has demonstrated the importance of realtime follow-up of neutrino events by various ground- and space-based facilities. The effort of H.E.S.S. and other experiments in coordinating observations to obtain quasi-simultaneous multiwavelength flux and spectrum measurements has been critical in measuring the chance coincidence with the high-energy neutrino event IC-170922A and constraining theoretical models. For about a decade, the H.E.S.S. transient program has included a search for gamma-ray emission associated with high-energy neutrino alerts, looking for gamma-ray activity from known sources and newly detected emitters consistent with the neutrino location. In this contribution, we present an overview of follow-up activities for realtime neutrino alerts with H.E.S.S. in 2021 and 2022. Our analysis includes both public IceCube neutrino alerts and alerts exchanged as part of a joint H.E.S.S.--IceCube program. We focus on interesting coincidences observed with gamma-ray sources, particularly highlighting the significant detection of PKS 0625-35, an AGN previously detected by H.E.S.S., and three IceCube neutrinos

Follow-up of gravitational waves alerts with IACTs using Astro-COLIBRI

International audienceFollow-up of gravitational wave alerts has proven to be challenging in the past due to the large uncertainty on the localisation, much larger than the field of view of most instruments. A smart pointing strategy helps to increase the chance of observing the true position of the underlying compact binary merger event and so to detect an electromagnetic counterpart. To tackle this, a software called tilepy has been developed and was successfully used by the H.E.S.S. collaboration to search for very-high energy gamma-ray emission from GWs during the O2 and O3 runs. The optimised tiling strategies implemented in tilepy allowed H.E.S.S. to be the first ground facility to point toward the true position of GW 170817. Here we present the main strategy used by the software to compute an optimal observation schedule. The Astro-COLIBRI platform helps to plan follow-up of a large range of transient phenomena including GW alerts. The integration of tilepy in this tool allow for an easy planning and visualisation of of follow-up of gravitational wave alert helping the astronomer to maximise the chance of detecting a counterpart. The platform also provides an overview of the multi-wavelength context by grouping and visualising information coming from different observatories alongside GW alerts