Constraints on a hadronic model for unidentified off-plane galactic gamma-ray sources

Recently the H.E.S.S. collaboration announced the detection of an unidentified gamma-ray source with an off-set from the galactic plane of 3.5°: HESS J1507-622. If the distance of the object is larger than about one kpc it would be physically located outside the galactic disk. The density profile of the ISM perpendicular to the galactic plane, which acts as target material for hadronic gamma-ray production, drops quite fast with increasing distance. This fact places distance dependent constraints on the energetics and properties of off-plane gamma-ray sources like HESS J1507-622 if a hadronic origin of the gamma-ray emission is assumed. For the case of this source it is found that there seems to be no simple way to link this object to the remnant of a stellar explosions.     

Radio continuum observations of the galactic supernova remnant Vela Z (G266.2-1.2)

We present multi frequency radio-continuum observations of Galactic SNR Vela Z (G266.2-1.2), associated with the prominent X-ray source RX J0852.0-4622 discovered in 1998. Our study of this Galactic supernova remnant (SNR) is based on low-resolution mosaic observations with the ATCA radio interferometer at 1384 and 2496 MHz, Parkes 4850 MHz and MOST 843 MHz survey data. We determine the radio spectral index for the most prominent features of this SNR and compare these features with X-ray observations. We note a sudden spectral turnover at higher then expected frequencies of ∼1384 MHz.     

The evolution of the central component in SN 1986J

The unusual core-collapse supernova 1986J, in the nearby spiral NGC 891, is the first modern supernova in which evidence of a compact remnant of the supernova has been seen. This evidence comes from recent VLBI images, which show the emergence of a new radio component in the center of the expanding radio shell. The new component shows an inverted radio spectrum contrasting with that of the shell. The new component is likely radio emission associated with the black-hole or neutron star compact remnant of the explosion, which would mark the first direct observational link between a modern supernova and such a compact remnant. We report here on our recent VLBI images at 22 and 5 GHz, as well as on our monitoring of the integrated radio spectrum of SN 1986J. In the 22 GHz image, the central component is marginally resolved.     

The history of SN 1986J and the emergence of a new compact radio component in its center

We review the history of the unusual Type II supernova 1986J in the nearby spiral galaxy NGC 891. A series of VLBI observations have shown the expanding shell structure and allowed the expansion curve to be determined. The integrated radio spectrum and radio lightcurve have also been monitored. The spectrum was a power-law before 1998, after which an inversion appeared above 5 GHz. Our recent high-frequency VLBI observations showed that this inverted-spectrum emission was associated not with the shell emission, but rather with a compact component almost precisely in the center of the expanding shell. The new component is likely radio emission associated with the black-hole or neutron star compact remnant of the explosion, which would mark the first direct observational link between a modern supernova and such a compact remnant.     

A multi-wavelength study of the western lobe of W50 powered by the galactic microquasar SS 433

W50 remains the only supernova remnant (SNR) confirmed to harbor a microquasar: the powerful enigmatic source SS 433. Our past study of this fascinating SNR revealed two X-ray lobes distorting the radio shell as well as non-thermal X-rays at the site of interaction between the SS 433 eastern jet and the eastern lobe of W50. In this paper we present the results of a 75 ks Chandra ACIS-I observation of the peak of W50-west targeted to: (1) determine the nature of the X-ray emission and (2) correlate the X-ray emission with that in the radio and infrared domains. We have confirmed that at the site of interaction between the western jet of SS 433 and dense interstellar gas the X-ray emission is non-thermal in nature. The helical pattern observed in radio is also seen with Chandra. No correlation was found between the infrared and X-ray emission.     

Improving the angular resolution of EGRET and new limits on supersymmetric dark matter near the galactic center

Using the EGRET data and an improved point source analysis, including an energy-dependent point spread function and an unbinned maximum likelihood technique, we have been able to place considerably lower limits on the γ-ray flux from the galactic center region. We also test this method on known sources, the Crab and Vela pulsars. In both cases, we find that our method improves the angular precision of EGRET data over the 3EG catalog.This new limit on γ-rays from the galactic center can be used to test models of annihilating supersymmetric dark matter and galactic halo profiles. We find that the present EGRET data can limit many supersymmetric models if the density of the galactic dark matter halo is cuspy or spiked toward the galactic center. We also discuss the ability of GLAST to test these models.     

The heart-shaped supernova remnant 3C 391 viewed in multi-bands

Using Chandra X-ray, Spitzer mid-IR, and 1.5 GHz radio data, we examine the spatial structure of SNR 3C 391. The X-ray surface brightness is generally anti-correlative with the IR and radio brightness. The multiband data clearly exhibit a heart-shaped morphology and show the multi-shell structure of the remnant. A previously unseen thin brace-like shell on the south detected at 24 μm is projected outside the radio border and confines the southern faint X-ray emission. The leading 24 μm knot on the SE boundary appears to be partly surrounded by soft X-ray emitting gas. The mid-IR emission is dominated by the contribution of the shocked dust grains, which may have been partly destroyed by sputtering.     

Radio-faint BL Lac objects and their impact on the radio/gamma-ray connection

Radio and gamma-ray emissions in Active Galactic Nuclei (AGNs) are both related to the presence of relativistic particles in jets. With the advent of the Fermi Large Area Telescope (LAT), and thanks to its large sensitivity up to several GeV, many observational results are changing our understanding of these phenomena. BL Lac objects, which made up only a fraction of the known extragalactic gamma-ray source population before Fermi, have now become the most abundant class. However, since they are relatively weak radio sources, most of them are poorly known as far as their parsec scale structure and multi-wavelength properties are concerned. For this reason, we have selected a complete sample of 42 low redshift BL Lacs (independently of their gamma-ray properties) to study with a multi-wavelength (radio, optical, X-ray, gamma-ray) approach. Here, we present results and images of sources in the sample (most of which have never been observed before), using new VLBA observations at 8 and 15 GHz. Beyond this sample of BL Lacs, the population of gamma-ray AGNs has also dramatically enlarged in the Fermi era, permitting us to discuss the presence of a correlation between radio and gamma-ray properties with improved statistical significance. We explore the radio-gamma relation with several hundreds sources and using both simultaneous and archival radio data, thus tackling the impact of time variability.     

Low and medium energy gamma-ray astronomy — present status and future aspects

During the last few years quite some progress has been achieved in the field of low and medium energy gamma-ray astronomy below about 30 MeV. Gamma rays from the galactic center and anti-center region have been detected, which require a high interstellar electron flux in the 100 MeV range, if they are predominantly diffuse in nature. Though the Crab pulsar and its nebula are still the only galactic gamma-ray sources which definitely have been detected, some recently determined upper limits to the gamma-ray fluxes of other radio pulsars are close to the theoretically expected values. Active galaxies seem to have a maximum of luminosity in the range between several 100 keV and a few MeV and, therefore, are of special interest. First observational results have been reported on the Seyfert galaxies NGC 4151 and MCG 8-11-11, and the radio galaxy CenA. The nature of the diffuse cosmic gamma-ray component at low gamma-ray energies is not yet solved. Unresolved active galaxies are good candidates for its origin.Considering the present status of gamma ray astronomy the study of galactic sources like radio pulsars and the unidentified high energy gamma-ray sources, the Milky Way as a whole, active galaxies and the diffuse cosmic sky seem to be the prime targets for broad band observations below 30 MeV in the GRO area. An unexplored field like that of low energy gamma-ray astronomy, however, is always open for surprises.     

空间等离子体云的回旋同步辐射

CME在行星际空间传播时, 会导致强磁场、高密度等离子体云的出现. 回旋同步辐射是此类等离子体的一种主要的射电辐射机制, 且携带行星际磁场的重要信息. 本文重点探讨了光深τν ≤1时回旋同步辐射的发射、吸收及极化特性, 包括热电子、非热电子, 投掷角各向同性、投掷角各向异性等离子体云的回旋同步辐射过程的研究分析. 在此基础上, 推导了考虑折射指数情况下, 辐射强度、辐射亮温的表达式.      

Gamma rays from giant-molecular clouds

An examination of the evidence favouring the detection of gamma rays from the irradiation of giant molecular clouds by cosmic rays is given. Data relevant to specific nearby clouds are studied in some detail and the evidence favouring some of the so-called gamma ray sources being due to the irradiation of these and more distant clouds is critically assessed. The need for enhancement of cosmic ray intensity in some clouds is discussed. The relevance of this work to interpretation of current and planned X-ray measurements as well as future gamma-ray space missions is discussed.     

Results from the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Observatory

The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory covers the high energy gamma ray energy range, approximately 30 MeV to 30 GeV, with a sensitivity considerably greater than earlier high energy gamma-ray satellites. Thus far, 4 pulsars have been detected and their properties measured, including in 3 cases the energy spectrum as a function of phase. The details of the galactic plane are being mapped and a spectra of the center region has been obtained in good agreement with that expected from cosmic ray interactions. The Magellanic clouds have been examined with the Large Magellanic Cloud having been detected at a level consistent with it having a cosmic ray density compatible with quasi-stable equilibrium. Sixteen Active Galactic Nuclei (AGN's) have been seen thus far with a high degree of certainty including 12 quasars and 4 BL Lac objects, but no Seyferts. Time variation has been detected in some of these AGN's.     

Constraints on injection parameters of cosmic rays in the Galactic center from resolved HESS observations

In this study, we investigate how restrictive the γ-ray emission from the Galactic center region, as seen by HESS and other Cherenkov air shower arrays, is against various models for cosmic ray injection. We derive diffusion coefficients which fit the observed spatial scales of diffuse γ-ray emission from the extended emission associated with the molecular clouds SgrA, B and C. Using these diffusion coefficients, we then obtain a limit for time scale of assumed recent proton acceleration near the SMBH, as the spatial size of SgrA^∗ in VHE γ-rays has to be consistent with the observed unresolved HESS point source size at this position. The signal from this hadronic component may be mixed with the expected VHE inverse Compton emission from the nearby unresolved pulsar wind nebula.     

Gamma ray astrophysics and signatures of axion-like particles

We propose that axion-like particles (ALPs) with a two-photon vertex, consistent with all astrophysical and laboratory bounds, may lead to effects in the spectra of high-energy gamma-ray sources detectable by satellite or ground-based telescopes. We discuss two kinds of signatures: (i) a peculiar spectral depletion due to gamma rays being converted into ALPs in the magnetic fields of efficient astrophysical accelerators according to the “Hillas criterion”, such as jets of active galactic nuclei or hot spots of radio galaxies; (ii) an appearance of otherwise invisible sources in the GeV or TeV sky due to back-conversion of an ALP flux (associated with gamma-ray emitters suffering some attenuation) in the magnetic field of the Milky Way. These two mechanisms might also provide an exotic way to avoid the exponential cutoff of very high energy gamma-rays expected due to the pair production onto the extragalactic background light.     

The low energy gamma-ray experiments on GRO in perspective

The NASA Gamma-Ray Observatory, GRO, will carry two instruments for low energy gamma-ray astronomy. The ‘Oriented Scintillation Spectrometer Experiment - OSSE’ represents the latest step in the evolution of collimated detectors. A large detection area, simultaneous source and background observation and rigorous control over systematic errors yield significant improvements in sensitivity over earlier instruments. The ‘Imaging Compton Telescope - COMPTEL’ brings the proven concept of the Compton telescope to the state of the art level. Position sensitive scintillation detectors make it possible to generate sky images with a resolution of about 2° over a f.o.v. of about 1 sr. The complementary nature of these two experiments promises a first in-depth exploration of the sky in a wavelength range which covers the transition from the X-ray sky to the apparently unrelated high energy gamma-ray sky. Possible directions of further evolution of these experiments will be discussed.     

Multiple Coulomb scattering method to reconstruct low-energy gamma–ray direction in the GAMMA-400 space-based gamma–ray telescope

The GAMMA-400 currently developing space-based gamma-ray telescope is designed to measure the gamma-ray fluxes in the energy range from ～20?MeV to several TeV in the highly elliptic orbit (without shadowing the telescope by the Earth) continuously for a long time. The physical characteristics of the GAMMA-400 gamma-ray telescope, especially the angular and energy resolutions (at 100-GeV gamma rays they are ～0.01° and ～1%, respectively), allow us to consider this space-based experiment as the next step in the development of extraterrestrial high-energy gamma-ray astronomy. In this paper, a method to improve the reconstruction accuracy of incident angle for low-energy gamma rays in the GAMMA-400 space-based gamma-ray telescope is presented. The special analysis of topology of pair-conversion events in thin layers of converter was performed. Applying the energy dependence of multiple Coulomb scattering for pair components, it is possible to estimate the energies for each particle, and to use these energies as weight in the angle reconstruction procedure. To identify the unique track in each projection the imaginary curvature method is applied. It allows us to obtain significantly better angular resolution in comparison with other methods applied in current space-based experiments. When using this method for 50-MeV gamma rays the GAMMA-400 gamma-ray telescope angular resolution is about 4°.     

A physical interpretation of the jet-like X-ray emission from supernova remnant W49B

In the framework of the study of supernova remnants and their complex interaction with the interstellar medium and the circumstellar material, we focus on the galactic supernova remnant W49B. Its morphology exhibits an X-ray bright elongated nebula, terminated on its eastern end by a sharp perpendicular structure aligned with the radio shell. The X-ray spectrum of W49B is characterized by strong K emission lines from Si, S, Ar, Ca, and Fe. There is a variation of the temperature in the remnant with the highest temperature found in the eastern side and the lowest one in the western side. The analysis of the recent observations of W49B indicates that the remnant may be the result of an asymmetric bipolar explosion where the ejecta are collimated along a jet-like structure and the eastern jet is hotter and more Fe-rich than the western one. Another possible scenario associates the X-ray emission with a spherical explosion where parts of the ejecta are interacting with a dense belt of ambient material. To overcome this ambiguity we present new results of the analysis of an XMM-Newton observation and we perform estimates of the mass and energy of the remnant. We conclude that the scenario of an anisotropic jet-like explosion explains quite naturally our observation results, but the association of W49B with a hypernova and a γ-ray burst, although still possible, is not directly supported by any evidence.     

Multi-wavelength study of the short term TeV flaring activity from the blazar Mrk 501 observed in June 2014

In this work, we study the short term flaring activity from the high synchrotron peaked blazar Mrk 501 detected by the FACT and H.E.S.S. telescopes in the energy range 2–20 TeV during June 23–24, 2014 (MJD 56831.86–56831.94). We revisit this major TeV flare of the source in the context of near simultaneous multi-wavelength observations of $\gamma$–rays in MeV-GeV regime with Fermi-LAT, soft X-rays in 0.3–10 keV range with Swift-XRT, hard X-rays in 10–20 keV and 15–50 keV bands with MAXI and Swift-BAT respectively, UV-Optical with Swift-UVOT and 15 GHz radio with OVRO telescope. We have performed a detailed temporal and spectral analysis of the data from Fermi-LAT, Swift-XRT and Swift-UVOT during the period June 15–30, 2014 (MJD 56823–56838). Near simultaneous archival data available from Swift-BAT, MAXI and OVRO telescope along with the V-band optical polarization measurements from SPOL observatory are also used in the study of giant TeV flare of Mrk 501 detected by the FACT and H.E.S.S. telescopes. No significant change in the multi-wavelength emission from radio to high energy $\gamma$–rays during the TeV flaring activity of Mrk 501 is observed except variation in soft X-rays. The varying soft X-ray emission is found to be correlated with the $\gamma$–ray emission at TeV energies during the flaring activity of the source. The soft X-ray photon spectral index is observed to be anti-correlated with the integral flux showing harder-when-brighter behavior. An average value of 4.5$\%$ for V-band optical polarization is obtained during the above period whereas the corresponding electric vector position angle changes significantly. We have used the minimum variability timescale from the H.E.S.S. observations to estimate the Doppler factor of the emission region which is found to be consistent with the previous studies of the source.     

Modeling the spectral evolution in the decaying tail of gamma-ray bursts observed by Swift

The detailed study of the spectral evolution during the steep decay phase of early X-ray light curves of gamma-ray bursts (GRBs) is a very important task that can give us information on different emission components contributing to the prompt-to-afterglow transition and help to understand the link between these two stages. Time resolved spectral analysis of bright GRBs detected by Swift has shown that a good modeling of the early X-ray light curves can be obtained with Band or cut-off power-law broad band spectra with evolving parameters (e.g., decaying peak energy). We developed a procedure to simultaneously fit the temporal evolution of all the spectral parameters of a GRB during the prompt-to-afterglow transition based on the analysis of the Swift Burst Alert Telescope (BAT) and the Swift X-ray Telescope (XRT) count rate and hardness ratio light curves. The procedure has been tested on GRB 060614 and GRB 090618, two very peculiar bright GRB detected by Swift that show a long exponentially decaying tail with strong softening and peak energy crossing the XRT energy band.     

A VLBI search for pulsar wind nebulae in supernovae

We examine recent supernovae which have been observed with very-long-baseline interferometry in order to detect or limit the emission from a possible compact remnant of the explosion. Such a remnant could be a neutron star, generating a pulsar wind nebula, or a black hole with an accretion disk and jets. Four supernovae, and also more than a dozen supernovae or their young remnants in M82, have structure sufficiently resolved to allow useful conclusions as to the strength of the emission from such young neutron stars or black holes. We recently discovered a compact component in the center of SN 1986J’s shell with a spectral luminosity at 15 GHz 200 times that of the Crab Nebula. This is most likely the compact remnant of the explosion, the first and only one found in any modern supernova. For other modern supernovae, the upper limits on the radio spectral luminosities of such young compact remnants range from 180 times that of the Crab Nebula for SN 1979C in M100 in the Virgo cluster to 0.001 times that of the Crab Nebula for SN 1987A in the Large Magellanic Cloud.