38 research outputs found
Search for dark matter signals with Fermi-LAT observation of globular clusters NGC 6388 and M 15
The globular clusters are probably good targets for dark matter (DM) searches
in -rays due to the possible adiabatic contraction of DM by baryons. In
this work we analyse the three-year data collected by {\it Fermi} Large Area
Telescope of globular clusters NGC 6388 and M 15 to search for possible DM
signals. For NGC 6388 the detection of -ray emission was reported by
{\it Fermi} collaboration, which is consistent with the emission of a
population of millisecond pulsars. The spectral shape of NGC 6388 is also shown
to be consistent with a DM contribution if assuming the annihilation final
state is . No significant -ray emission from M 15 is
observed. We give the upper limits of DM contribution to -ray emission
in both NGC 6388 and M 15, for annihilation final states , ,
, and monochromatic line. The constraints are
stronger than that derived from observation of dwarf galaxies by {\it Fermi}.Comment: 17 pages, 6 figures, accepted by JCA
Multi-wavelength observations of the BL Lac object Fermi J1544-0649: One year after its awakening
© 2020 Elsevier B.V. We report observations of a transient source Fermi J1544-0649 from radio to γ-rays. Fermi J1544-0649 was discovered by the Fermi-LAT in May 2017. Follow-up Swift-XRT observations revealed three flaring episodes through March 2018, and the peak X-ray flux is about 103 higher than the ROSAT all-sky survey (RASS) flux upper limit. Optical spectral measurements taken by the Magellan 6.5-m telescope and the Lick-Shane telescope both show a largely featureless spectrum, strengthening the BL Lac interpretation first proposed by Bruni et al. (2018). The optical and mid-infrared (MIR) emission goes to a higher state in 2018, when the flux in high energies goes down to a lower level. Our RATAN-600 m measurements at 4.8 GHz and 8.2 GHz do not indicate any significant radio flux variation over the monitoring seasons in 2017 and 2018, nor deviate from the archival NVSS flux level. During GeV flaring times, the spectrum is very hard (Γγ∼1.7) in the GeV band and at times also very hard ((ΓX≲2) in the X-rays, similar to a high-synchrotron-peak (or even an extreme) BL Lac object, making Fermi J1544-0649 a good target for ground-based Cherenkov telescopes
Multi-wavelength study of the galactic PeVatron candidate LHAASO J2108+5157
LHAASO J2108+5157 is one of the few known unidentified Ultra-High-Energy (UHE) gamma-ray sources with no Very-High-Energy (VHE) counterpart, recently discovered by the LHAASO collaboration. We observed LHAASO J2108+5157 in the X-ray band with XMM-Newton in 2021 for a total of 3.8 hours and at TeV energies with the Large-Sized Telescope prototype (LST-1), yielding 49 hours of good quality data. In addition, we analyzed 12 years of Fermi-LAT data, to better constrain emission of its High-Energy (HE) counterpart 4FGL J2108.0+5155. We found an excess (3.7 sigma) in the LST-1 data at energies E > 3 TeV. Further analysis in the whole LST-1 energy range assuming a point-like source, resulted in a hint (2.2 sigma) of hard emission which can be described with a single power law with photon index Gamma = 1.6 +- 0.2 between 0.3 - 100 TeV. We did not find any significant extended emission which could be related to a Supernova Remnant (SNR) or Pulsar Wind Nebula (PWN) in the XMM-Newton data, which puts strong constraints on possible synchrotron emission of relativistic electrons. The LST-1 and LHAASO observations can be explained as inverse Compton dominated leptonic emission of relativistic electrons with cutoff energy of 100+70-30 TeV. The low magnetic field in the source imposed by the X-ray upper limits on synchrotron emission is compatible with a hypothesis of a TeV halo. Furthermore, the spectral properties of the HE counterpart are consistent with a hypothesis of Geminga-like pulsar, which would be able to power the VHE-UHE emission. LST-1 and Fermi-LAT upper limits impose strong constraints on hadronic scenario of pi-0 decay dominated emission from accelerated protons interacting with nearby molecular clouds, requiring hard spectral index, which is incompatible with the standard diffusive acceleration scenario
Stringent Tests of Lorentz Invariance Violation from LHAASO Observations of GRB 221009A
International audienceVery recently, the Large High Altitude Air Shower Observatory (LHAASO) reported the observation of the very early TeV afterglow of the brightest-of-all-time GRB 221009A, recording the highest photon statistics in the TeV band ever from a gamma-ray burst. We use this unique observation to place stringent constraints on an energy dependence of the speed of light in vacuum, a manifestation of Lorentz invariance violation (LIV) predicted by some quantum gravity (QG) theories. Our results show that the 95% confidence level lower limits on the QG energy scales are times of the Planck energy for the linear, and for the quadratic LIV effects, respectively. Our limits on the quadratic LIV case improve previous best bounds by factors of 5--7
Does or did the supernova remnant Cassiopeia A operate as a PeVatron?
International audienceFor decades, supernova remnants (SNRs) have been considered the prime sources of Galactic Cosmic rays (CRs). But whether SNRs can accelerate CR protons to PeV energies and thus dominate CR flux up to the knee is currently under intensive theoretical and phenomenological debate. The direct test of the ability of SNRs to operate as CR PeVatrons can be provided by ultrahigh-energy (UHE; ~TeV) -rays. In this context, the historical SNR Cassiopeia A (Cas A) is considered one of the most promising target for UHE observations. This paper presents the observation of Cas A and its vicinity by the LHAASO KM2A detector. The exceptional sensitivity of LHAASO KM2A in the UHE band, combined with the young age of Cas A, enabled us to derive stringent model-independent limits on the energy budget of UHE protons and nuclei accelerated by Cas A at any epoch after the explosion. The results challenge the prevailing paradigm that Cas A-type SNRs are major suppliers of PeV CRs in the Milky Way
LHAASO-KM2A detector simulation using Geant4
International audienceKM2A is one of the main sub-arrays of LHAASO, working on gamma ray astronomy and cosmic ray physics at energies above 10 TeV. Detector simulation is the important foundation for estimating detector performance and data analysis. It is a big challenge to simulate the KM2A detector in the framework of Geant4 due to the need to track numerous photons from a large number of detector units (>6000) with large altitude difference (30 m) and huge coverage (1.3 km^2). In this paper, the design of the KM2A simulation code G4KM2A based on Geant4 is introduced. The process of G4KM2A is optimized mainly in memory consumption to avoid memory overffow. Some simpliffcations are used to signiffcantly speed up the execution of G4KM2A. The running time is reduced by at least 30 times compared to full detector simulation. The particle distributions and the core/angle resolution comparison between simulation and experimental data of the full KM2A array are also presented, which show good agreement
Multi-wavelength study of the galactic PeVatron candidate LHAASO J2108+5157
LHAASO J2108+5157 is one of the few known unidentified Ultra-High-Energy (UHE) gamma-ray sources with no Very-High-Energy (VHE) counterpart, recently discovered by the LHAASO collaboration. We observed LHAASO J2108+5157 in the X-ray band with XMM-Newton in 2021 for a total of 3.8 hours and at TeV energies with the Large-Sized Telescope prototype (LST-1), yielding 49 hours of good quality data. In addition, we analyzed 12 years of Fermi-LAT data, to better constrain emission of its High-Energy (HE) counterpart 4FGL J2108.0+5155. We found an excess (3.7 sigma) in the LST-1 data at energies E > 3 TeV. Further analysis in the whole LST-1 energy range assuming a point-like source, resulted in a hint (2.2 sigma) of hard emission which can be described with a single power law with photon index Gamma = 1.6 +- 0.2 between 0.3 - 100 TeV. We did not find any significant extended emission which could be related to a Supernova Remnant (SNR) or Pulsar Wind Nebula (PWN) in the XMM-Newton data, which puts strong constraints on possible synchrotron emission of relativistic electrons. The LST-1 and LHAASO observations can be explained as inverse Compton dominated leptonic emission of relativistic electrons with cutoff energy of 100+70-30 TeV. The low magnetic field in the source imposed by the X-ray upper limits on synchrotron emission is compatible with a hypothesis of a TeV halo. Furthermore, the spectral properties of the HE counterpart are consistent with a hypothesis of Geminga-like pulsar, which would be able to power the VHE-UHE emission. LST-1 and Fermi-LAT upper limits impose strong constraints on hadronic scenario of pi-0 decay dominated emission from accelerated protons interacting with nearby molecular clouds, requiring hard spectral index, which is incompatible with the standard diffusive acceleration scenario
Observations of the Crab Nebula and Pulsar with the Large-Sized Telescope Prototype of the Cherenkov Telescope Array
International audienceCTA (Cherenkov Telescope Array) is the next generation ground-based observatory for gamma-ray astronomy at very-high energies. The Large-Sized Telescope prototype (\LST) is located at the Northern site of CTA, on the Canary Island of La Palma. LSTs are designed to provide optimal performance in the lowest part of the energy range covered by CTA, down to GeV. \LST started performing astronomical observations in November 2019, during its commissioning phase, and it has been taking data since then. We present the first \LST observations of the Crab Nebula, the standard candle of very-high energy gamma-ray astronomy, and use them, together with simulations, to assess the basic performance parameters of the telescope. The data sample consists of around 36 hours of observations at low zenith angles collected between November 2020 and March 2022. \LST has reached the expected performance during its commissioning period - only a minor adjustment of the preexisting simulations was needed to match the telescope behavior. The energy threshold at trigger level is estimated to be around 20 GeV, rising to GeV after data analysis. Performance parameters depend strongly on energy, and on the strength of the gamma-ray selection cuts in the analysis: angular resolution ranges from 0.12 to 0.40 degrees, and energy resolution from 15 to 50%. Flux sensitivity is around 1.1% of the Crab Nebula flux above 250 GeV for a 50-h observation (12% for 30 minutes). The spectral energy distribution (in the 0.03 - 30 TeV range) and the light curve obtained for the Crab Nebula agree with previous measurements, considering statistical and systematic uncertainties. A clear periodic signal is also detected from the pulsar at the center of the Nebula