Extended Gamma-ray Emission Research Articles

Detection of extended gamma-ray emission in the vicinity of Cl Danks 1 and 2

ABSTRACT We report the detection of high-energy gamma-ray emission towards the G305 star-forming region. Using almost 15 yr of observation data from Fermi Large Area Telescope, we detected an extended gamma-ray source in this region with a significance of $\sim 13 \sigma$. The gamma-ray radiation reveals a clear pion-bump feature and can be fitted with the power-law parent proton spectrum with an index of $-2.5$. The total cosmic ray (CR) proton energy in the gamma-ray production region is estimated to be of the order of $10^{49}\ \rm erg$. We further derived the CR radial distribution from both the gamma-ray emission and gas distribution and found it roughly obeys the $1/r$ type profile, though a constant profile is not ruled out. This is consistent with other similar systems and expected from the continuous injection of CRs by the central powerful young massive star cluster Danks 1 or Danks 2 in this region. Together with former detections of similar gamma-ray structures, such as Cygnus cocoon, Westerlund 1, Westerlund 2, NGC 3603, and W40, the detection supports the hypothesis that young massive star clusters are CR accelerators.

GeV Gamma-Ray Emission from Pulsar Wind Nebula HESS J1356-645 with Fermi-Large Area Telescope

HESS J1356-645 is considered to be a pulsar wind nebula (PWN) associated with the pulsar PSR J1357-6429. We reanalyze the GeV gamma-ray emission in the direction of HESS J1356-645 with more than 13 yr of Fermi-Large Area Telescope data. The extended gamma-ray emission above 5 GeV is found to be spatially coincident with HESS J1356-645. The spectrum in the energy range of 1 GeV–1 TeV can be described by a power law with an index of Γ = 1.51 ± 0.10. The broadband spectrum of HESS J1356-645 can be reproduced by a leptonic model with a broken power-law electronic spectrum. In addition, we found evidence that the morphology of the GeV emission from HESS J1356-645 varies with energy; a behavior that is similar to that of the PWN Vela-X. More broadband observations will be helpful to study the energy-dependent characteristics of HESS J1356-645.

Population synthesis of pulsar wind nebulae and pulsar halos in the Milky Way

Context.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.Aims.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.Methods.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.Results.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 0.1–1 TeV.Conclusions.Pulsar halos are shown to be viable counterparts to a fraction of the currently unidentified sources if they develop around most middle-aged pulsars. Yet, if the phenomenon is rare, with an occurrence rate of 5 − 10% as suggested in a previous work from the local positron flux constraint, the total number of currently known TeV sources including unidentified ones cannot be accounted for in our model from young PWNe only. This calls for continued efforts to model pulsar-powered emission along the full evolutionary path, including the late stages past the young nebula phase.

Detectability of Large Correlation Length Inflationary Magnetic Field with Cherenkov Telescopes

Magnetic fields occupying the voids of the large scale structure may be a relic from the Early Universe originating from either Inflation or from cosmological phase transitions. We explore the possibility of identifying the inflationary origin of the void magnetic fields and measuring its parameters with gamma-ray astronomy methods. The large correlation length inflationary field is expected to impose a characteristic asymmetry of extended gamma-ray emission that is correlated between different sources on the sky. We show that a set of nearby blazars for which the extended emission is observable in the 0.1-1 TeV band with CTA can be used for the test of inflationary origin of the void magnetic fields.

Radial Profiles of Non-thermal Emission from Supernova Remnant RX J1713.7-3946

Abstract Supernova remnant RX J1713.7-3946 has exhibited the largest surface brightness and a detailed spectral and shell-type morphology, and is one of the brightest TeV sources. The recent H.E.S.S. observation of RX J1713.7-3946 revealed a broken power-law GeV–TeV gamma-ray and a more extended gamma-ray spatial radial profile than in the X-ray band. Based on the diffusion shock acceleration model, we solve spherically symmetric hydrodynamic equations and particle transport equations, and investigate the multi-band non-thermal emission of RX J1713.7-3946 and radial profiles of its surface brightness for two selected zones in the leptonic scenario for gamma-ray emission. We found (1) the diffusion coefficient has a weak energy dependence, and the Kolmogorov type is favored; (2) the magnetic field strength can vary linearly or nonlinearly with radius for different surrounding environments because of possible turbulence in the shock downstream region, and compressional amplification is likely to exist at the shock front; (3) the non-thermal photons from radio to X-ray bands are dominated by synchrotron emission from relativistic electrons if the GeV–TeV gamma-rays are produced by inverse Compton scattering from these electrons interacting with the background photons; then the X-ray and gamma-ray radial profiles can be reproduced except for the more extended gamma-ray emission.

Resolving acceleration to very high energies along the jet of Centaurus A.

The nearby radio galaxy Centaurus A belongs to a class of active galaxies that are luminous at radio wavelengths. Most show collimated relativistic outflows known as jets, which extend over hundreds of thousands of parsecs for the most powerful sources. Accretion of matter onto the central supermassive black hole is believed to fuel these jets and power their emission1. Synchrotron radiation from relativistic electrons causes the radio emission, and it has been suggested that the X-ray emission from Centaurus A also originates in electron synchrotron processes2-4. Another possible explanation is inverse Compton scattering with cosmic microwave background (CMB) soft photons5-7. Synchrotron radiation needs ultrarelativistic electrons (about 50teraelectronvolts) and, given their short cooling times, requires some continuous re-acceleration mechanism8. Inverse Compton scattering, on the other hand, does not require very energetic electrons, but the jets must stay highly relativistic on large scales (exceeding 1megaparsec). Some recent evidence disfavours inverse Compton-CMB models9-12, although other work seems to be compatible with them13,14. In principle, the detection of extended γ-ray emission, which directly probes the presence of ultrarelativistic electrons, could distinguish between these options. At gigaelectronvolt energies there is also an unusual spectral hardening15,16 in Centaurus A that has not yet been explained. Here we report observations of Centaurus A at teraelectronvolt energies that resolve its large-scale jet. We interpret the data as evidence for the acceleration of ultrarelativistic electrons in the jet, and favour the synchrotron explanation for the X-rays. Given that this jet is not exceptional in terms of power, length or speed, it is possible that ultrarelativistic electrons are commonplace in the large-scale jets of radio-loud active galaxies.

Constraining the properties of the magnetic turbulence in the Geminga region using HAWC γ-ray data

Observations of extended gamma-ray emission around Galactic cosmic-ray (CR) sources can be used as novel probes of interstellar magnetic fields. Using very-high-energy gamma-ray data from the HAWC Observatory, we place constraints on the properties of the magnetic turbulence within $\approx 25$ pc from Geminga. We inject and propagate individual CR electrons in 3D realizations of turbulent magnetic fields, calculate the resulting gamma-ray emission, and compare with HAWC measurements of this region. We find that HAWC data is compatible with expectations for Kolmogorov or Kraichnan turbulence, and can be well fitted for reasonable coherence lengths and strengths of the turbulence, despite implying a CR diffusion coefficient significantly smaller than those suggested by Galactic CR propagation codes. The best fit is found for a coherence length $L_{\rm c} \approx 1$ pc and a magnetic field strength $B_{\rm rms} \approx 3 \mu$G, and the preferred value for $L_{\rm c}$ increases with $B_{\rm rms}$. Moreover, the apparent lack of strong asymmetries in the observed emission allows us to constrain the coherence length to $L_{\rm c} \lesssim 5$ pc in this region.

Diffuseγ-ray emission near the young massive cluster NGC 3603

We report the Fermi Large Area Telescope's detection of extended gamma-ray emission towards the direction of the young massive star cluster NGC 3603. The emission shows a hard spectrum with a photon index of 2.3 from 1 GeV to 250 GeV. The large size and high luminosity of this structure make it unlikely a pulsar wind nebular. On the other hand the spatial correlation with the ionised gas indicate a hadronic origin. The total cosmic ray (CR) protons energy are estimated to be of the order $10^{50} ~\rm erg$ assuming the gamma-ray are produced in the interaction of CRs with ambient gas . The environment and spectral features show significant similarity with the Cygnus cocoon. It reveals that the young star clusters may be a gamma-ray source population and they can potentially accelerate a significant fraction of the Galactic cosmic rays.

GAMMA-RAY EMISSION FROM PSR J0007+7303 USING SEVEN YEARS OF FERMI LARGE AREA TELESCOPE OBSERVATIONS

ABSTRACT Based on more than seven years of Fermi Large Area Telescope Pass 8 data, we report on a detailed analysis of the bright gamma-ray pulsar (PSR) J0007+7303. We confirm that PSR J0007+7303 is significantly detected as a point source also during the off-peak phases with a test statistic value of 262 (∼16σ). In the description of the off-peak spectrum of PSR J0007+7303, a power law with an exponential cutoff at 2.7 ± 1.2 ± 1.3 GeV (the first/second uncertainties correspond to statistical/systematic errors) is preferred over a single power law at a level of 3.5σ. The possible existence of a cutoff hints at a magnetospheric origin of the emission. In addition, no extended gamma-ray emission is detected that is compatible with either the supernova remnant (CTA 1) or the very high-energy (>100 GeV) pulsar wind nebula. A flux upper limit of 6.5 × 10−12 erg cm−2 s−1 in the energy range 10–300 GeV is reported, for an extended source assuming the morphology of the VERITAS detection. During on-peak phases, a sub-exponential cutoff is significantly preferred (∼11σ) for representing the spectral energy distribution, in both the phase-averaged and phase-resolved spectra. Three glitches are detected during the observation period and we found no flux variability at the time of the glitches or in the long-term behavior. We also report the discovery of a previously unknown gamma-ray source in the vicinity of PSR J0007+7303, Fermi J0020+7328, which we associate with the z = 1.781 quasar S5 0016+73. A concurrent analysis of this source is needed to correctly characterize the behavior of CTA 1 and it is also presented in the paper.

Constraining the production of cosmic rays by pulsars

One of the possible sources of hadronic cosmic rays (CRs) are newborn pulsars. If this is indeed the case, they should feature diffusive gamma-ray halos produced by interactions of CRs with interstellar gas. In this paper we try to identify extended gamma-ray emission around young pulsars, making use of the 7-year Fermi-LAT data. For this purpose we select and analyze a set of eight pulsars that are most likely to possess detectable gamma-ray halos. We find extended emission that might be interpreted as a gamma-ray halo only in the case of PSR J0007+7303. Its luminosity accords with the total energy of injected cosmic rays $\sim 10^{50}$ erg, although other interpretations of this source are possible. Irrespectively of the nature of this source we put bounds on the luminosity of gamma-ray halos which suggest that pulsars' contribution to the overall energy budget of galactic CRs is subdominant in the GeV-TeV range.

FERMI LAT DISCOVERY OF EXTENDED GAMMA-RAY EMISSIONS IN THE VICINITY OF THE HB 3 SUPERNOVA REMNANT

ABSTRACT We report the discovery of extended gamma-ray emission measured by the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope in the region of the supernova remnant (SNR) HB 3 (G132.7+1.3) and the W3 II complex adjacent to the southeast of the remnant. W3 is spatially associated with bright 12CO (J = 1–0) emission. The gamma-ray emission is spatially correlated with this gas and the SNR. We discuss the possibility that gamma rays originate in interactions between particles accelerated in the SNR and interstellar gas or radiation fields. The decay of neutral pions produced in nucleon–nucleon interactions between accelerated hadrons and interstellar gas provides a reasonable explanation for the gamma-ray emission. The emission from W3 is consistent with irradiation of the CO clouds by the cosmic rays accelerated in HB 3.

HIGH-LATITUDE, TRANSLUSCENT MOLECULAR CLOUDS AS PROBES OF LOCAL COSMIC RAYS

We analyze the gamma-ray emission from 9 high latitude, translucent molecular clouds taken with the Fermi Large Area Telescope (LAT) between 250 MeV and 10 GeV. Observations of gamma-rays allow us to probe the density and spectrum of cosmic rays in the solar neighborhood. The clouds studied lie within $\sim\!$270 pc from the Sun and are selected from the Planck all-sky CO map. Gamma-rays in this energy range mostly result from cosmic ray interactions with the interstellar medium, which is traced with three components: HI, CO, and dark gas. Every cloud is detected and shows significant, extended gamma-ray emission from molecular gas. The gamma-ray emission is dominated by the CO-emitting gas in some clouds, but by the CO-dark gas in others. The average emissivity and gamma-ray power law index from HI above 1 GeV shows no evidence of a systematic variation. The CO-to-H$_2$ conversion factor shows no variation between clouds over this small spatial range, but shows significant variations within each cloud. The average CO-to-H$_2$ conversion factor suggests that the CO-dark gas is molecular as opposed to optically thick HI.

Annihilating asymmetric dark matter

The relic abundance of particle and antiparticle dark matter (DM) need not be vastly different in thermal asymmetric dark matter (ADM) models. By considering the effect of a primordial asymmetry on the thermal Boltzmann evolution of coupled DM and anti-DM, we derive the requisite annihilation cross section. This is used in conjunction with CMB and Fermi-LAT gamma-ray data to impose a limit on the number density of anti-DM particles surviving thermal freeze-out. When the extended gamma-ray emission from the Galactic Center is reanalyzed in a thermal ADM framework, we find that annihilation into $\tau$ leptons prefer anti-DM number densities 1-4$\%$ that of DM while the $b$-quark channel prefers 50-100$\%$.

Astrophysical and dark matter interpretations of extended gamma-ray emission from the Galactic Center

We construct empirical models of the diffuse gamma-ray background toward the Galactic Center. Including all known point sources and a template of emission associated with interactions of cosmic rays with molecular gas, we show that the extended emission observed previously in the Fermi Large Area Telescope data toward the Galactic Center is detected at high significance for all permutations of the diffuse model components. However, we find that the fluxes and spectra of the sources in our model change significantly depending on the background model. In particular, the spectrum of the central Sgr A$^\ast$ source is less steep than in previous works and the recovered spectrum of the extended emission has large systematic uncertainties, especially at lower energies. If the extended emission is interpreted to be due to dark matter annihilation, we find annihilation into pure $b$-quark and $\tau$-lepton channels to be statistically equivalent goodness of fits. In the case of the pure $b$-quark channel, we find a dark matter mass of $39.4\left(^{+3.7}_{-2.9}\rm\ stat.\right)\left(\pm 7.9\rm\ sys.\right)\rm\ GeV$, while a pure $\tau^{+} \tau^{-}$-channel case has an estimated dark matter mass of $9.43\left(^{+0.63}_{-0.52}\rm\ stat.\right)(\pm 1.2\rm\ sys.)\ GeV$. Alternatively, if the extended emission is interpreted to be astrophysical in origin such as due to unresolved millisecond pulsars, we obtain strong bounds on dark matter annihilation, although systematic uncertainties due to the dependence on the background models are significant.

A morphological analysis for searches of possible extended -ray sources associated with dark matter annihilation

We propose a morphological analysis for searches of extended gamma-ray emission associated with dark matter annihilation. Our approach is based on the likelihood analysis including the spatial templates produced by taking into account the residual count maps in the energy band where the dark matter annihilation spectrum has a prominent spectral feature. The approach is tested on the example of the possible dark matter annihilation signal from the Virgo cluster of galaxies.

Dark matter and pulsar model constraints from Galactic Center Fermi-LAT gamma-ray observations

Employing Fermi-LAT gamma-ray observations, several independent groups have found excess extended gamma-ray emission at the Galactic Center (GC). Both annihilating dark matter (DM) or a population of $\ensuremath{\sim}{10}^{3}$ unresolved millisecond pulsars (MSPs) are regarded as well-motivated possible explanations. However, there are significant uncertainties in the diffuse galactic background at the GC. We have performed a revaluation of these two models for the extended gamma-ray source at the GC by accounting for the systematic uncertainties of the Galactic diffuse emission model. We also marginalize over point-source and diffuse background parameters in the region of interest. We show that the excess emission is significantly more extended than a point source. We find that the DM (or pulsar-population) signal is larger than the systematic errors and therefore proceed to determine the sectors of parameter space that provide an acceptable fit to the data. We find that a population of 1000--2000 MSPs with parameters consistent with the average spectral shape of Fermi-LAT measured MSPs is able to fit the GC excess emission. For DM, we find that a pure ${\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ annihilation channel is not a good fit to the data. But a mixture of ${\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ and $b\overline{b}$ with a $⟨\ensuremath{\sigma}v⟩$ of order the thermal relic value and a DM mass of around 20 to 60 GeV provides an adequate fit.

Constraining extended gamma-ray emission from galaxy clusters

We report evidence for extended gamma-ray emission from the Virgo, Fornax and Coma clusters based on a maximum-likelihood analysis of the 3-year Fermi-LAT data. For all three clusters, excess emission is observed within three degrees of the center, peaking at the GeV scale. This emission cannot be accounted for by known Fermi sources or by the galactic and extragalactic backgrounds. If interpreted as annihilation emission from supersymmetric dark matter (DM) particles, the data prefer models with a particle mass in the range 20-60 GeV annihilating into the b-bbar channel, or 2-10 GeV and >1 TeV annihilating into mu-mu final states. Our results are consistent with those obtained by Hooper and Linden from a recent analysis of Fermi-LAT data in the region of the Galactic Centre. An extended DM annihilation profile dominated by emission from substructures is preferred over a simple point source model. The significance of DM detection is 4.4 sigma in Virgo and lower in the other two clusters. We also consider the possibility that the excess emission arises from cosmic ray (CR) induced gamma-rays, and infer a CR level within a factor of three of that expected from analytical models. However, the significance of a CR component is lower than the significance of a DM component, and there is no need for such a CR component in the presence of a DM component in the preferred DM mass range. We also set flux and cross-section upper limits for DM annihilation into the b-bbar and mu-mu channels in all three clusters.

Origin of TeV Galactic cosmic rays

We consider a possibility of identification of sources of cosmic rays (CR) of the energy above 1 TeV via observation of degree-scale extended gamma-ray emission which traces the locations of recent sources in the Galaxy. Such emission in the energy band above 100 GeV is produced by CR nuclei and electrons released by the sources and spreading into the interstellar medium. We use the data from the Fermi gamma-ray telescope to locate the degree-scale 100 GeV gamma-ray sources. We find that the number of such sources and their overall power match to those expected when CRs injection events happen every ~100 yr in portions of ~1e50 erg. We find that most of the sources are associated to pulsars with spin down age less than ~30 kyr and hence to the recent supernova explosions. This supports the hypothesis of supernova origin of Galactic CRs. We notice that the degree-scale extended emission does not surround shell-like supernova remnants without pulsars. Based on this observation, we argue that the presence of the pulsar is essential for the CR acceleration process. We expect that a significant fraction of the degree-scale sources should be detectable as extended sources with km3-scale neutrino detectors.

Gamma-ray burst observations by Fermi Large Area Telescope revisited: new candidates found

ABSTRACT We search the Fermi Large Area Telescope (LAT) photon data base for an extended gamma-ray emission which could be associated with any of the 581 previously detected gamma-ray bursts (GRBs) visible to the Fermi-LAT. For this purpose, we compare the number of photons with energies E > 100 MeV and E > 1 GeV which arrived in the first 1500 s after the burst from the same region, to the expected background. We require that the expected number of false detections does not exceed 0.05 for the entire search and find the high-energy emission in 19 bursts, four of which (GRB 081009, GRB 090720B, GRB 100911 and GRB 100728A) were previously unreported. The first three are detected at energies above 100 MeV, while the last one shows a statistically significant signal only above 1 GeV.

GALAXY CLUSTERS IN GAMMA-RAYS: AN ASSESSMENT FROM OBSERVATIONS

Clusters of galaxies are believed to constitute a population of astrophysical objects potentially able to emit electromagnetic radiation up to gamma-ray energies. Evidence of the existence of non-thermal radiation processes in galaxy clusters is indicated from observations of diffuse radio halos, hard X-ray and EUV excess emission. The presence of cosmic ray acceleration processes and its confinement on cosmological timescales nearly inevitably yields in predicting energetic gamma-ray emission, either directly deduceably from a cluster`s multifreqency emission characteristics or indirectly during large-scale cosmological structure formation processes. This theoretical reasoning suggests several scenarios to actually detect galaxy clusters at gamma-ray wavelengths: Either resolved as individual sources of point-like or extended gamma-ray emission, by investigating spatial-statistical correlations with unidentified gamma-ray sources or, if unresolved, through their contribution to the extragalactic diffuse gamma-ray background. In the following I review the situation concerning the proposed relation between galaxy clusters and high-energy gamma-ray observations from an observational point-of-view.