Fermi LAT measurements of diffuse gamma-ray emission: results at the first-year milestone

For more than one year the Fermi Large Area Telescope has been surveying the gamma-ray sky from 20 MeV to more than 300 GeV with unprecedented statistics and angular resolution. One of the key science targets of the Fermi mission is diffuse gamma-ray emission. Galactic interstellar gamma-ray emission is produced by interactions of high-energy cosmic rays with the interstellar gas and radiation field. We review the most important results on the subject obtained so far: the non-confirmation of the excess of diffuse GeV emission seen by EGRET, the measurement of the gamma-ray emissivity spectrum of local interstellar gas, the study of the gradient of cosmic-ray densities and of the X(CO)=N(H2)/W(CO) ratio in the outer Galaxy. We also catch a glimpse at diffuse gamma-ray emission in the Large Magellanic Cloud. These results allow the improvement of large-scale models of Galactic diffuse gamma-ray emission and new measurements of the extragalactic gamma-ray background.Comment: Contribution to the Workshop SciNeGHe 2009/Gamma-ray Physics in the LHC era (Assisi - Italy, Oct. 7-9 2009); 10 pages, 6 figure

Searching for Dark Matter Annihilation in the Smith High-Velocity Cloud

Recent observations suggest that some high-velocity clouds may be confined by massive dark matter halos. In particular, the proximity and proposed dark matter content of the Smith Cloud make it a tempting target for the indirect detection of dark matter annihilation. We argue that the Smith Cloud may be a better target than some Milky Way dwarf spheroidal satellite galaxies and use gamma-ray observations from the Fermi Large Area Telescope to search for a dark matter annihilation signal. No significant gamma-ray excess is found coincident with the Smith Cloud, and we set strong limits on the dark matter annihilation cross section assuming a spatially-extended dark matter profile consistent with dynamical modeling of the Smith Cloud. Notably, these limits exclude the canonical thermal relic cross section ($\sim 3\times10^{-26}{\rm cm}^{3}{\rm s}^{-1}$) for dark matter masses $\lesssim 30$ GeV annihilating via the $b \bar b$ or $\tau^{+}\tau^{-}$ channels for certain assumptions of the dark matter density profile; however, uncertainties in the dark matter content of the Smith Cloud may significantly weaken these constraints.Comment: 7 pages, 5 figures. Published in Ap

Population synthesis of pulsar wind nebulae and pulsar halos in the Milky Way -- Predicted contributions to the very-high-energy sky

The discovery of extended gamma-ray emission toward a number of middle-aged pulsars suggests the possibility of long-lived particle confinement beyond the classical pulsar wind nebula (PWN) stage. How this emerging source class can be extrapolated to a Galactic population remains unclear. We aim to evaluate how pulsar halos fit in existing TeV observations, under the assumption that all middle-aged pulsars develop halos similar to those observed toward the J0633+1746 or B0656+14 pulsars. We modeled the populations of supernova remnants, PWNe, and pulsar halos in the Milky Way. The PWN-halo evolutionary sequence is described in a simple yet coherent framework, and both kinds of objects are assumed to share the same particle injection properties. We then assessed the contribution of the different source classes to the very-high-energy emission from the Galaxy. The synthetic population can be made consistent with the flux distribution of all known objects, including unidentified objects, for a reasonable set of parameters. The fraction of the populations predicted to be detectable in surveys of the Galactic plane with HESS. and HAWC is then found to be in good agreement with their actual outcome, with a number of detectable halos ranging from 30 to 80% of the number of detectable PWNe. Prospects for CTA involve the detection of 250-300 sources in the Galactic Plane Survey, including 170 PWNe and up to 100 halos. The extent of diffusion suppression in halos has a limited impact on such prospects but its magnitude has a strong influence. The level of diffuse emission from unresolved populations in each survey is found to be dominated by halos and comparable to large-scale interstellar radiation powered by cosmic rays above 0.1-1TeV. Pulsar halos are shown to be viable counterparts to a fraction of the currently unidentified sources if they develop around most middle-aged pulsars (abridged).Comment: 16 pages, 22 figures, accepted for publication in A&

Creating a high-resolution picture of Cygnus with the Cherenkov Telescope Array

The Cygnus region hosts one of the most remarkable star-forming regions in the Milky Way. Indeed, the total mass in molecular gas of the Cygnus X complex exceeds 10 times the total mass of all other nearby star-forming regions. Surveys at all wavelengths, from radio to gamma-rays, reveal that Cygnus contains such a wealth and variety of sources---supernova remnants (SNRs), pulsars, pulsar wind nebulae (PWNe), H II regions, Wolf-Rayet binaries, OB associations, microquasars, dense molecular clouds and superbubbles---as to practically be a galaxy in microcosm. The gamma-ray observations along reveal a wealth of intriguing sources at energies between 1 GeV and tens of TeV. However, a complete understanding of the physical phenomena producing this gamma-ray emission first requires us to disentangle overlapping sources and reconcile discordant pictures at different energies. This task is made more challenging by the limited angular resolution of instruments such as the Fermi Large Area Telescope, ARGO-YBJ, and HAWC and the limited sensitivity and field of view of current imaging atmospheric Cherenkov telescopes (IACTs). The Cherenkov Telescope Array (CTA), with its improved angular resolution, large field of view, and order of magnitude gain in sensitivity over current IACTs, has the potential to finally create a coherent and well-resolved picture of the Cygnus region between a few tens of GeV and a hundred TeV. We describe a proposed strategy to study the Cygnus region using CTA data, which combines a survey of the whole region at $65^\circ < l < 85^\circ$ and $-3.5^\circ < b < 3.5^\circ$ with deeper observations of two sub-regions that host rich groups of known gamma-ray sources

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

Sensitivity Projections for Dark Matter Searches with the Fermi Large Area Telescope

The nature of dark matter is a longstanding enigma of physics; it may consist of particles beyond the Standard Model that are still elusive to experiments. Among indirect search techniques, which look for stable products from the annihilation or decay of dark matter particles, or from axions coupling to high-energy photons, observations of the $\gamma$-ray sky have come to prominence over the last few years, because of the excellent sensitivity of the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope mission. The LAT energy range from 20 MeV to above 300 GeV is particularly well suited for searching for products of the interactions of dark matter particles. In this report we describe methods used to search for evidence of dark matter with the LAT, and review the status of searches performed with up to six years of LAT data. We also discuss the factors that determine the sensitivities of these searches, including the magnitudes of the signals and the relevant backgrounds, considering both statistical and systematic uncertainties. We project the expected sensitivities of each search method for 10 and 15 years of LAT data taking. In particular, we find that the sensitivity of searches targeting dwarf galaxies, which provide the best limits currently, will improve faster than the square root of observing time. Current LAT limits for dwarf galaxies using six years of data reach the thermal relic level for masses up to 120 GeV for the $b\bar{b}$ annihilation channel for reasonable dark matter density profiles. With projected discoveries of additional dwarfs, these limits could extend to about 250 GeV. With as much as 15 years of LAT data these searches would be sensitive to dark matter annihilations at the thermal relic cross section for masses to greater than 400 GeV (200 GeV) in the $b\bar{b}$ ($\tau^+ \tau^-$) annihilation channels.Comment: Updated with a few additional and corrected references; otherwise, text is identical to previous version. Submitted on behalf of the Fermi-LAT collaboration. Accepted for publication in Physics Reports, 59 pages, 34 figures; corresponding author: Eric Charles ([email protected]

Discovery of GeV Emission from the Circinus galaxy with the Fermi-LAT

We report the discovery of gamma-ray emission from the Circinus galaxy using the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope. Circinus is a nearby (~4 Mpc) starburst with a heavily obscured Seyfert-type active nucleus, bipolar radio lobes perpendicular to the spiral disk, and kpc-scale jet-like structures. Our analysis of 0.1-100 GeV events collected during 4 years of LAT observations reveals a significant (~ 7.3 sigma) excess above the background. We find no indications of variability or spatial extension beyond the LAT point-spread function. A power-law model used to describe the 0.1-100 GeV gamma-ray spectrum yields a flux of (18.8+/-5.8)x10^{-9} ph cm^{-2} s^{-1} and photon index 2.19+/-0.12, corresponding to an isotropic gamma-ray luminosity of 3 x 10^{40} erg s^{-1}. This observed gamma-ray luminosity exceeds the luminosity expected from cosmic-ray interactions in the interstellar medium and inverse Compton radiation from the radio lobes. Thus the origin of the GeV excess requires further investigation.Comment: 7 pages, 7 figures, accepted for publication in the Astrophysical Journa