The blazar AO 0235+164 (z = 0.94) has been one of the most active objects observed by Fermi Large Area Telescope (LAT) since its launch in Summer 2008. In addition to the continuous coverage by ...Fermi, contemporaneous observations were carried out from the radio to gamma -ray bands between 2008 September and 2009 February. In this paper, we summarize the rich multi-wavelength data collected during the campaign (including F-GAMMA, GASPWEBT, Kanata, OVRO, RXTE, SMARTS, Swift, and other instruments), examine the cross-correlation between the light curves measured in the different energy bands, and interpret the resulting spectral energy distributions in the context of well-known blazar emission models. We find that the gamma -ray activity is well correlated with a series of near-IR/optical flares, accompanied by an increase in the optical polarization degree. On the other hand, the X-ray light curve shows a distinct 20 day high state of unusually soft spectrum, which does not match the extrapolation of the optical/UV synchrotron spectrum. We tentatively interpret this feature as the bulk Compton emission by cold electrons contained in the jet, which requires an accretion disk corona with an effective covering factor of 19% at a distance of 100 R sub(g). We model the broadband spectra with a leptonic model with external radiation dominated by the infrared emission from the dusty torus.
Context . Hadronic interactions between cosmic rays (CRs) and interstellar gas have been probed in γ rays across the Galaxy. A fairly uniform CR distribution is observed up to a few hundred parsecs ...from the Sun, except in the Eridu cloud, which shows an unexplained 30–50% deficit in GeV to TeV CR flux. Aims . To explore the origin of this deficit, we studied the Reticulum cloud, which shares notable traits with Eridu: a comparable distance in the low-density region of the Local Valley and a filamentary structure of atomic hydrogen extending along a bundle of ordered magnetic-field lines that are steeply inclined to the Galactic plane. Methods . We measured the γ -ray emissivity per gas nucleon in the Reticulum cloud in the 0.16–63 GeV energy band using 14 years of Fermi -LAT data. We also derived interstellar properties that are important for CR propagation in both the Eridu and Reticulum clouds, at the same parsec scale. Results . The γ -ray emissivity in the Reticulum cloud is fully consistent with the average spectrum measured in the solar neighbourhood, but this emissivity, and therefore the CR flux, is 1.57 ± 0.09 times larger than in Eridu across the whole energy band. The difference cannot be attributed to uncertainties in gas mass. Nevertheless, we find that the two clouds are similar in many respects: both have magnetic-field strengths of a few micro-Gauss in the plane of the sky; both are in approximate equilibrium between magnetic and thermal pressures; they have similar turbulent velocities and sonic Mach numbers; and both show magnetic-field regularity with a dispersion in orientation lower than 10°–15° over large zones. The gas in Reticulum is colder and denser than in Eridu, but we find similar parallel diffusion coefficients around a few times 10 28 cm 2 s −1 in both clouds if CRs above 1 GV in rigidity diffuse on resonant, self-excited Alfvén waves that are damped by ion-neutral interactions. Conclusions . The loss of CRs in Eridu remains unexplained, but these two clouds provide important test cases to further study how magnetic turbulence, line tangling, and ion-neutral damping regulate CR diffusion in the dominant gas phase of the interstellar medium.
Neutron stars generate powerful winds of relativistic particles that form bright synchrotron nebulae around them. Polarimetry provides a unique insight into the geometry and magnetic configuration of ...the wind, but high-energy measurements have failed until recently. The INTEGRAL/IBIS telescope has been used in its Compton mode to search for linearly polarized emission for energies above 200 keV from the Crab Nebula. The asymmetries in the instrument response are small and we obtain evidence for a strongly polarized signal at an angle parallel to the pulsar rotation axis. This result confirms the detection recently reported by Dean and coworkers, and extends the polarization measure for all the pulsar's phases. The hard X-ray/soft -ray observations therefore probe the inner jets or equatorial flow of the wind. The achieved sensitivity opens a new window for polarimetric studies at energies above 200 keV.
Aims. The Orion-Eridanus superbubble has been blown by supernovae and supersonic winds of the massive stars in the Orion OB associations. It is the nearest site at which stellar feedback on the ...interstellar medium that surrounds young massive clusters can be studied. The formation history and current structure of the superbubble are still poorly understood, however. It has been pointed out that the picture of a single expanding object should be replaced by a combination of nested shells that are superimposed along the line of sight. We have investigated the composite structure of the Eridanus side of the superbubble in the light of a new decomposition of the atomic and molecular gas. Methods. We used H I 21 cm and CO (J = 1−0) emission lines to separate coherent gas shells in space and velocity, and we studied their relation to the warm ionised gas probed in Hα emission, the hot plasma emitting X-rays, and the magnetic fields traced by dust polarised emission. We also constrained the relative distances to the clouds using dust reddening maps and X-ray absorption. We applied the Davis–Chandrasekhar–Fermi method to the dust polarisation data to estimate the plane-of-sky components of the magnetic field in several clouds and along the outer rim of the superbubble. Results. Our gas decomposition has revealed several shells inside the superbubble that span distances from about 150–250 pc. One of these shells forms a nearly complete ring filled with hot plasma. Other shells likely correspond to the layers of swept-up gas that is compressed behind the expanding outer shock wave. We used the gas and magnetic field data downstream of the shock to derive the shock expansion velocity, which is close to ~20 km s−1. Taking the X-ray absorption by the gas into account, we find that the hot plasma inside the superbubble is over-pressured compared to plasma in the Local Bubble. The plasma comprises a mix of hotter and cooler gas along the lines of sight, with temperatures of (3–9) and (0.3 − 1.2) × 106 K, respectively. The magnetic field along the western and southern rims and in the approaching wall of the superbubble appears to be shaped and compressed by the ongoing expansion. We find plane-of-sky magnetic field strengths from 3 to 15 μG along the rim.
Cosmic Rays in Superbubbles Tolksdorf, T.; Grenier, I. A.; Joubaud, T. ...
Astrophysical journal/The Astrophysical journal,
07/2019, Letnik:
879, Številka:
2
Journal Article
Recenzirano
Odprti dostop
A solution of the transport equation for cosmic rays in turbulent magnetic fields in a spherically symmetric geometry is presented. The results are applied to particle propagation in superbubbles. In ...the fully analytical calculation, various energy-loss processes are considered. From the distribution function of the cosmic-ray particles, the distribution for pions from continuous losses is computed. Folding with the appropriate cross section yields the gamma-ray distribution. It is shown that in the case of efficient acceleration the volume-integrated π0-decay gamma-ray emission from the superbubble exhibits the characteristic hard differential number density spectrum at large gamma-ray energies, Eγ > 70 MeV. This prediction is useful for identifying efficient superbubble cosmic-ray accelerators from gamma-ray observations.
We describe a uniform all-sky survey of bright blazars, selected primarily by their flat radio spectra, that is designed to provide a large catalog of likely Y-ray active galactic nuclei (AGNs). The ...defined sample has 1625 targets with radio and X-ray properties similar to those of the EGRET blazars, spread uniformly across the degree 'b degree ' > 10 degree sky. We also report progress toward optical characterization of the sample; of objects with known R < 23, 85% have been classified and 81% have measured redshifts. One goal of this program is to focus attention on the most interesting (e.g., high-redshift, high-luminosity,...) sources for intensive multiwavelength study during the observations by the Large Area Telescope (LAT) on GLAST.
From the comparison of interstellar gas tracers in the solar neighborhood (HI and CO lines from the atomic and molecular gas, dust thermal emission, and g rays from cosmic-ray interactions with gas), ...we unveil vast clouds of cold dust and dark gas, invisible in HI and CO but detected in gamma rays. They surround all the nearby CO clouds and bridge the dense cores to broader atomic clouds, thus providing a key link in the evolution of interstellar clouds. The relation between the masses in the molecular, dark, and atomic phases in the local clouds implies a dark gas mass in the Milky Way comparable to the molecular one.
Aims. We aim to explore the capabilities of dust emission and γ rays for probing the properties of the interstellar medium in the nearby anti-centre region, using γ-ray observations with the Fermi ...Large Area Telescope (LAT), and the thermal dust optical depth inferred from Planck and IRAS observations. We also aim to study massive star-forming clouds including the well known Taurus, Auriga, Perseus, and California molecular clouds, as well as a more diffuse structure which we refer to as Cetus. In particular, we aim at quantifying potential variations in cosmic-ray density and dust properties per gas nucleon across the different gas phases and different clouds, and at measuring the CO-to-H2 conversion factor, XCO, in different environments. Methods. We have separated six nearby anti-centre clouds that are coherent in velocities and distances, from the Galactic-disc background in H i 21-cm and 12CO 2.6-mm line emission. We have jointly modelled the γ-ray intensity recorded between 0.4 and 100 GeV, and the dust optical depth τ353 at 353 GHz as a combination of H i-bright, CO-bright, and ionised gas components. The complementary information from dust emission and γ rays was used to reveal the gas not seen, or poorly traced, by H i, free-free, and 12CO emissions, namely (i) the opaque H iand diffuse H2 present in the Dark Neutral Medium at the atomic-molecular transition, and (ii) the dense H2 to be added where 12CO lines saturate. Results. The measured interstellar γ-ray spectra support a uniform penetration of the cosmic rays with energies above a few GeV through the clouds, from the atomic envelopes to the 12CO-bright cores, and with a small ± 9% cloud-to-cloud dispersion in particle flux. We detect the ionised gas from the H iiregion NGC 1499 in the dust and γ-ray emissions and measure its mean electron density and temperature. We find a gradual increase in grain opacity as the gas (atomic or molecular) becomes more dense. The increase reaches a factor of four to six in the cold molecular regions that are well shielded from stellar radiation. Consequently, the XCO factor derived from dust is systematically larger by 30% to 130% than the γ-ray estimate. We also evaluate the average γ-ray XCO factor for each cloud, and find that XCO tends to decrease from diffuse to more compact molecular clouds, as expected from theory. We find XCO factors in the anti-centre clouds close to or below 1020 cm-2 K-1 km-1 s, in agreement with other estimates in the solar neighbourhood. Together, they confirm the long-standing unexplained discrepancy, by a factor of two, between the mean XCO values measured at parsec scales in nearby clouds and those obtained at kiloparsec scale in the Galaxy. Our results also highlight large quantitative discrepancies in 12CO intensities between simulations and observations at low molecular gas densities. Key words: gamma rays: ISM / solar neighborhood / ISM: clouds / cosmic rays / dust, extinction
ABSTRACT It is widely accepted that cosmic rays (CRs) up to at least PeV energies are Galactic in origin. Accelerated particles are injected into the interstellar medium where they propagate to the ...farthest reaches of the Milky Way, including a surrounding halo. The composition of CRs coming to the solar system can be measured directly and has been used to infer the details of CR propagation that are extrapolated to the whole Galaxy. In contrast, indirect methods, such as observations of γ-ray emission from CR interactions with interstellar gas, have been employed to directly probe the CR densities in distant locations throughout the Galactic plane. In this article we use 73 months of data from the Fermi Large Area Telescope in the energy range between 300 MeV and 10 GeV to search for γ-ray emission produced by CR interactions in several high- and intermediate-velocity clouds (IVCs) located at up to ∼7 kpc above the Galactic plane. We achieve the first detection of IVCs in γ rays and set upper limits on the emission from the remaining targets, thereby tracing the distribution of CR nuclei in the halo for the first time. We find that the γ-ray emissivity per H atom decreases with increasing distance from the plane at 97.5% confidence level. This corroborates the notion that CRs at the relevant energies originate in the Galactic disk. The emissivity of the upper intermediate-velocity Arch hints at a 50% decline of CR densities within 2 kpc from the plane. We compare our results to predictions of CR propagation models.
Context. Hadronic interactions between cosmic rays (CRs) and interstellar gas have been probed in γ rays across the Galaxy. A fairly uniform CR distribution is observed up to a few hundred parsecs ...from the Sun, except in the Eridu cloud, which shows an unexplained 30–50% deficit in GeV to TeV CR flux.Aims. To explore the origin of this deficit, we studied the Reticulum cloud, which shares notable traits with Eridu: a comparable distance in the low-density region of the Local Valley and a filamentary structure of atomic hydrogen extending along a bundle of ordered magnetic-field lines that are steeply inclined to the Galactic plane.Methods. We measured the γ-ray emissivity per gas nucleon in the Reticulum cloud in the 0.16–63 GeV energy band using 14 years of Fermi-LAT data. We also derived interstellar properties that are important for CR propagation in both the Eridu and Reticulum clouds, at the same parsec scale.Results. The γ-ray emissivity in the Reticulum cloud is fully consistent with the average spectrum measured in the solar neighbourhood, but this emissivity, and therefore the CR flux, is 1.57 ± 0.09 times larger than in Eridu across the whole energy band. The difference cannot be attributed to uncertainties in gas mass. Nevertheless, we find that the two clouds are similar in many respects: both have magnetic-field strengths of a few micro-Gauss in the plane of the sky; both are in approximate equilibrium between magnetic and thermal pressures; they have similar turbulent velocities and sonic Mach numbers; and both show magnetic-field regularity with a dispersion in orientation lower than 10°–15° over large zones. The gas in Reticulum is colder and denser than in Eridu, but we find similar parallel diffusion coefficients around a few times 1028 cm2 s−1 in both clouds if CRs above 1 GV in rigidity diffuse on resonant, self-excited Alfvén waves that are damped by ion-neutral interactions.Conclusions. The loss of CRs in Eridu remains unexplained, but these two clouds provide important test cases to further study how magnetic turbulence, line tangling, and ion-neutral damping regulate CR diffusion in the dominant gas phase of the interstellar medium.
PDF
