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Register a new userH.E.S.S. ToO program on nearby core-collapse Supernovae: search for very-high energy $gamma$-ray emission towards the SN candidate AT2019krl in M74
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While the youngest known supernova remnants, such as Cassiopeia A, have been proven to be able to accelerate cosmic rays only up to $sim$10$^{14},mathrm{eV}$ at their present evolutionary stages, recent studies have shown that particle energies larger than a few PeV ($10^{15},mathrm{eV}$) could be reached during the early stages of a core-collapse Supernova, when the high-velocity forward shock expands into the dense circumstellar medium shaped by the stellar progenitor wind. Such environments, in particular the type IIn SNe whose progenitors may exhibit mass-loss rates as high as $10^{-2}M_odot,mathrm{yr}^{-1}$ cite{smith14}, could thus lead to $gamma$-ray emission from $pi^0$ decay in hadronic interactions, potentially detectable with current Cherenkov telescopes at very-high energies. Such a detection would provide direct evidence for efficient acceleration of CR protons/nuclei in supernovae, and hence new insights on the long-standing issue of the origin of Galactic Cosmic Rays. In that context, the High Energy Stereoscopic System (hess) has been carrying out a Target of Opportunity program since 2016 to search for such an early very-high-energy $gamma$-ray emission towards nearby core-collapse supernovae and supernova candidates (up to $sim 10~mathrm{Mpc}$), within a few weeks after discovery. After giving an overview of this hess Target of Opportunity program, we present the results obtained from the July 2019 observations towards the transient at, originally classified as a type IIn supernova, which occurred in the galaxy M74 at $sim 9.8,mathrm{Mpc}$. Although its nature still remains unclear, the derived hess constraints on this transient are placed in the general context of the expected VHE $gamma$-ray emission from core-collapse supernovae.
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Young core-collapse supernovae with dense-wind progenitors may be able to accelerate cosmic-ray hadrons beyond the knee of the cosmic-ray spectrum, and this may result in measurable gamma-ray emission. We searched for gamma-ray emission from ten supernovae observed with the High Energy Stereoscopic System (H.E.S.S.) within a year of the supernova event. Nine supernovae were observed serendipitously in the H.E.S.S. data collected between December 2003 and December 2014, with exposure times ranging from 1.4 hours to 53 hours. In addition we observed SN 2016adj as a target of opportunity in February 2016 for 13 hours. No significant gamma-ray emission has been detected for any of the objects, and upper limits on the $>1$ TeV gamma-ray flux of the order of $sim$10$^{-13}$ cm$^{-2}$s$^{-1}$ are established, corresponding to upper limits on the luminosities in the range $sim$2 $times$ 10$^{39}$ erg s$^{-1}$ to $sim$1 $times$ 10$^{42}$ erg s$^{-1}$. These values are used to place model-dependent constraints on the mass-loss rates of the progenitor stars, implying upper limits between $sim$2 $times 10^{-5}$ and $sim$2 $times 10^{-3}$M$_{odot}$yr$^{-1}$ under reasonable assumptions on the particle acceleration parameters.
Globular clusters (GCs) are established emitters of high-energy (HE, 100 MeV<E<100 GeV) gamma-ray radiation which could originate from the cumulative emission of the numerous millisecond pulsars (msPSRs) in the clusters cores or from inverse Compton (IC) scattering of relativistic leptons accelerated in the GC environment. GCs could also constitute a new class of sources in the very-high-energy (VHE, E>100 GeV) gamma-ray regime, judging from the recent detection of emission from the direction of Terzan 5 with the H.E.S.S. telescope array. To search for VHE gamma-ray sources associated with other GCs, and to put constraints on leptonic emission models, we systematically analyzed the observations towards 15 GCs taken with H.E.S.S. We searched for individual sources of VHE gamma-rays from each GC in our sample and also performed a stacking analysis combining the data from all GCs to investigate the hypothesis of a population of faint emitters. Assuming IC emission as the source of emission from Terzan 5, we calculated the expected gamma-ray flux for each of the 15 GCs, based on their number of millisecond pulsars, their optical brightness and the energy density of background photon fields. We did not detect significant emission from any of the 15 GCs. The obtained flux upper limits allow to rule out the simple IC/msPSR scaling model for NGC 6388 and NGC 7078. The upper limits derived from the stacking analyses are factors between 2 and 50 below the flux predicted by the simple leptonic model, depending on the assumed source extent and the dominant target photon fields. Therefore, Terzan 5 still remains exceptional among all GCs, as the VHE gamma-ray emission either arises from extra-ordinarily efficient leptonic processes, or from a recent catastrophic event, or is even unrelated to the GC itself.
Globular clusters (GCs) are established emitters of high-energy (HE, 100 MeV<E<100 GeV) gamma-ray radiation which could originate from the cumulative emission of the numerous millisecond pulsars (msPSRs) in the clusters cores or from inverse Compton (IC) scattering of relativistic leptons accelerated in the GC environment. These stellar clusters could also constitute a new class of sources in the very-high-energy (VHE, E>100 GeV) gamma-ray regime, judging from the recent detection of a signal from the direction of Terzan 5 with the H.E.S.S. telescope array. We searched for point-like and extended VHE gamma-ray emission from 15 GCs serendipitously covered by H.E.S.S observations and also performed a stacking analysis combining the data from all GCs to investigate the hypothesis of a population of faint emitters. Assuming IC emission as the origin of the VHE gamma-ray signal from the direction of Terzan 5, we calculated the expected gamma-ray flux from each of the 15 GCs, based on their number of millisecond pulsars, their optical brightness and the energy density of background photon fields. We did not detect significant VHE gamma-ray emission from any of the 15 GCs in either of the two analyses. Given the uncertainties related to the parameter determinations, the obtained flux upper limits allow to rule out the simple IC/msPSR scaling model for NGC 6388 and NGC 7078. The upper limits derived from the stacking analyses are factors between 2 and 50 below the flux predicted by the simple leptonic scaling model, depending on the assumed source extent and the dominant target photon fields. Therefore, Terzan 5 still remains exceptional among all GCs, as the VHE gamma-ray emission either arises from extra-ordinarily efficient leptonic processes, or from a recent catastrophic event, or is even unrelated to the GC itself.
The synchrotron self-Compton (SSC) emission from Gamma-ray Burst (GRB) forward shock can extend to the very-high-energy (VHE; $E_gamma > $100 GeV) range. Such high energy photons are rare and are attenuated by the cosmic infrared background before reaching us. In this work, we discuss the prospect to detect these VHE photons using the current ground-based Cherenkov detectors. Our calculated results are consistent with the upper limits obtained with several Cherenkov detectors for GRB 030329, GRB 050509B, and GRB 060505 during the afterglow phase. For 5 bursts in our nearby GRB sample (except for GRB 030329), current ground-based Cherenkov detectors would not be expected to detect the modeled VHE signal. Only for those very bright and nearby bursts like GRB 030329, detection of VHE photons is possible under favorable observing conditions and a delayed observation time of $la$10 hours.
Some core-collapse supernovae are likely to be efficient cosmic-ray accelerators up to the PeV range, and therefore, to potentially play an important role in the overall Galactic cosmic-ray population. The TeV gamma-ray domain can be used to study particle acceleration in the multi-TeV and PeV range. This motivates the study of the detectability of such supernovae by current and future gamma-ray facilities. The gamma-ray emission of core-collapse supernovae strongly depends on the level of the two-photon annihilation process: high-energy gamma-ray photons emitted at the expanding shock wave following the supernova explosion can interact with soft photons from the supernova photosphere through the pair production channel, thereby strongly suppressing the flux of gamma rays leaving the system. In the case of SN 1993J, whose photospheric and shock-related parameters are well measured, we calculate the temporal evolution of the expected gamma-ray attenuation by accounting for the temporal and geometrical effects. We find the attenuation to be of about $10$ orders of magnitude in the first few days after the SN explosion. The probability of detection of a supernova similar to SN 1993J with the Cherenkov Telescope Array is highest if observations are performed either earlier than 1 day, or later than 10 days after the explosion, when the gamma-ray attenuation decreases to about $2$ orders of magnitude.
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