Abstract
The Fermi-LAT (Large Area Telescope) has detected more than 3000 sources in the GeV γ-ray regime. The majority are extragalactic and these sources are dominated by blazars. However, ...∼28 per cent of the sources in Fermi 3LAC are listed as blazar candidates of uncertain type (BCU). Increasing the number of classified Fermi-LAT sources is important for improving our understanding of extragalactic γ-ray sources and can be used to search for new very high energy sources. We report on the optical spectroscopy of seven selected unclassified BCU sources during 2014 and 2015 undertaken using the SAAO 1.9-m and Southern African Large Telescope (SALT). Based on the identified spectral lines, we have classified three of the sources as FSRQs and the remaining four as BL Lac objects, determining the redshift for four sources.
The H.E.S.S. Galactic plane survey Angüner, E. O.; Arakawa, M.; Aubert, P. ...
Astronomy and astrophysics (Berlin),
04/2018, Volume:
612
Journal Article
Peer reviewed
Open access
We present the results of the most comprehensive survey of the Galactic plane in very high-energy (VHE) γ-rays, including a public release of Galactic sky maps, a catalog of VHE sources, and the ...discovery of 16 new sources of VHE γ-rays. The High Energy Spectroscopic System (H.E.S.S.) Galactic plane survey (HGPS) was a decade-long observation program carried out by the H.E.S.S. I array of Cherenkov telescopes in Namibia from 2004 to 2013. The observations amount to nearly 2700 h of quality-selected data, covering the Galactic plane at longitudes from ℓ = 250° to 65° and latitudes |b|≤ 3°. In addition to the unprecedented spatial coverage, the HGPS also features a relatively high angular resolution (0.08° ≈ 5 arcmin mean point spread function 68% containment radius), sensitivity (≲1.5% Crab flux for point-like sources), and energy range (0.2–100 TeV). We constructed a catalog of VHE γ-ray sources from the HGPS data set with a systematic procedure for both source detection and characterization of morphology and spectrum. We present this likelihood-based method in detail, including the introduction of a model component to account for unresolved, large-scale emission along the Galactic plane. In total, the resulting HGPS catalog contains 78 VHE sources, of which 14 are not reanalyzed here, for example, due to their complex morphology, namely shell-like sources and the Galactic center region. Where possible, we provide a firm identification of the VHE source or plausible associations with sources in other astronomical catalogs. We also studied the characteristics of the VHE sources with source parameter distributions. 16 new sources were previously unknown or unpublished, and we individually discuss their identifications or possible associations. We firmly identified 31 sources as pulsar wind nebulae (PWNe), supernova remnants (SNRs), composite SNRs, or gamma-ray binaries. Among the 47 sources not yet identified, most of them (36) have possible associations with cataloged objects, notably PWNe and energetic pulsars that could power VHE PWNe.
Abstract
We report on the discovery of a new fast radio burst (FRB), FRB 150215, with the Parkes radio telescope on 2015 February 15. The burst was detected in real time with a dispersion measure ...(DM) of 1105.6 ± 0.8 pc cm−3, a pulse duration of 2.8
$^{+1.2}_{-0.5}$
ms, and a measured peak flux density assuming that the burst was at beam centre of 0.7
$^{+0.2}_{-0.1}$
Jy. The FRB originated at a Galactic longitude and latitude of 24.66°, 5.28° and 25° away from the Galactic Center. The burst was found to be 43 ± 5 per cent linearly polarized with a rotation measure (RM) in the range −9 < RM < 12 rad m−2 (95 per cent confidence level), consistent with zero. The burst was followed up with 11 telescopes to search for radio, optical, X-ray, γ-ray and neutrino emission. Neither transient nor variable emission was found to be associated with the burst and no repeat pulses have been observed in 17.25 h of observing. The sightline to the burst is close to the Galactic plane and the observed physical properties of FRB 150215 demonstrate the existence of sight lines of anomalously low RM for a given electron column density. The Galactic RM foreground may approach a null value due to magnetic field reversals along the line of sight, a decreased total electron column density from the Milky Way, or some combination of these effects. A lower Galactic DM contribution might explain why this burst was detectable whereas previous searches at low latitude have had lower detection rates than those out of the plane.
The inner region of the Milky Way halo harbors a large amount of dark matter (DM). Given its proximity, it is one of the most promising targets to look for DM. We report on a search for the ...annihilations of DM particles using γ-ray observations towards the inner 300 pc of the Milky Way, with the H.E.S.S. array of ground-based Cherenkov telescopes. The analysis is based on a 2D maximum likelihood method using Galactic Center (GC) data accumulated by H.E.S.S. over the last 10 years (2004-2014), and does not show any significant γ-ray signal above background. Assuming Einasto and Navarro-Frenk-White DM density profiles at the GC, we derive upper limits on the annihilation cross section ⟨σv⟩. These constraints are the strongest obtained so far in the TeV DM mass range and improve upon previous limits by a factor 5. For the Einasto profile, the constraints reach ⟨σv⟩ values of 6×10^{-26} cm^{3} s^{-1} in the W^{+}W^{-} channel for a DM particle mass of 1.5 TeV, and 2×10^{-26} cm^{3} s^{-1} in the τ^{+}τ^{-} channel for a 1 TeV mass. For the first time, ground-based γ-ray observations have reached sufficient sensitivity to probe ⟨σv⟩ values expected from the thermal relic density for TeV DM particles.
Abstract
Supersoft X-ray sources (SSSs) are characterized by their low effective temperatures and high X-ray luminosities. The soft X-ray emission can be explained by hydrogen nuclear burning on the ...surface of a white dwarf (WD) accreting at an extremely high rate. A peculiar ∼67 s periodicity (P
67) was previously discovered in the XMM–Newton light curves of the SSS CAL 83. P
67 was detected in X-ray light curves spanning ∼9 yr, but exhibits variability of several seconds on time-scales as short as a few hours, and its properties are remarkably similar to those of dwarf nova oscillations (DNOs). DNOs are short time-scale modulations (≲1 min) often observed in dwarf novae during outburst. DNOs are explained by the well-established low-inertia magnetic accretor (LIMA) model. In this paper, we show that P
67 and its associated period variability can be satisfactorily explained by an application of the LIMA model to the more ‘extreme’ environment in an SSS (eLIMA), contrary to another recent study attempting to explain P
67 and its associated variability in terms of non-radial g-mode oscillations in the extended envelope of the rapidly accreting WD in CAL 83. In the eLIMA model, P
67 originates in an equatorial belt in the WD envelope at the boundary with the inner accretion disc, with the belt weakly coupled to the WD core by an ∼105 G magnetic field. New optical light curves obtained with the Sutherland High-speed Optical Camera are also presented, exhibiting quasi-periodic modulations on time-scales of ∼1000 s, compatible with the eLIMA framework.
Using data from contemporaneous observations with Chandra and Swift, it is shown that the X-ray emission below 10 keV is predominantly thermal, characterized by flares and emission lines and ...dominated by the soft component. The Chandra and Swift X-ray spectra (E
X≤ 10 keV) can be reproduced by multicomponent thermal emission models with a time-averaged X-ray luminosity of L
X∼ 1031 erg s−1. The pulsed 33-s soft X-ray emission below 10 keV is confirmed in both the Chandra and Swift data sets. The epoch of pulse maximum of the 33-s white dwarf spin period is consistent with the recently derived ephemeris based upon Suzaku measurements. The recently detected Suzaku hard X-ray component above 10 keV shows a non-thermal power-law nature, with a photon index of Γ∼ 1.2, possibly the result of synchrotron emission of high-energy electrons in the white dwarf magnetosphere. The hard X-ray luminosity of L
X,hard≤ 5 × 1030 erg s−1 also constitutes κ∼ 0.1 per cent of the total spin-down luminosity of the white dwarf. This places AE Aquarii in the same category as young spin-powered pulsars between 2 and 20 keV. Additionally, it is shown that electrons can be accelerated to energies in excess of 10 TeV outside the light cylinder radius, providing interesting possibilities for VHE-TeV follow-up observations. The X-ray emission below E
X≤ 10 keV, on the other hand, is explained in terms of plasma heating at the magnetospheric radius, the result of the dissipation of gravitational potential energy. It is found that a conversion efficiency of α∼ 0.01 is sufficient to heat the plasma at the magnetospheric boundary to temperatures kT≤ 10 keV, sufficient to drive the X-ray emission below 10 keV. Only a small fraction (β∼ 0.3 per cent) of the mass flow at the magnetospheric radius eventually accretes on to the surface of the white dwarf, emphasizing the very effective magnetospheric propeller process in the system.
The gamma-ray binary system PSR B1259-63/LS 2883 went through periastron in May 2014. We report on the optical spectroscopic monitoring of the system from 33 d before to 78 d after periastron, ...undertaken with the Southern African Large Telescope. The Hα and He i (λ6678) lines exhibit an orbital variation around periastron, with the line strengths reaching a maximum ∼13 d after periastron. The line strength is weaker than observed around the previous periastron in 2010. There is also a marked change in the line strength and asymmetry around the first disc crossing. These observations are consistent with the disruption of the circumstellar disc around periastron due to the interaction with the pulsar.
Spectral lines are among the most powerful signatures for dark matter (DM) annihilation searches in very-high-energy γ rays. The central region of the Milky Way halo is one of the most promising ...targets given its large amount of DM and proximity to Earth. We report on a search for a monoenergetic spectral line from self-annihilations of DM particles in the energy range from 300 GeV to 70 TeV using a two-dimensional maximum likelihood method taking advantage of both the spectral and spatial features of the signal versus background. The analysis makes use of Galactic center observations accumulated over ten years (2004-2014) with the H.E.S.S. array of ground-based Cherenkov telescopes. No significant γ-ray excess above the background is found. We derive upper limits on the annihilation cross section ⟨σv⟩ for monoenergetic DM lines at the level of 4×10^{-28} cm^{3} s^{-1} at 1 TeV, assuming an Einasto DM profile for the Milky Way halo. For a DM mass of 1 TeV, they improve over the previous ones by a factor of 6. The present constraints are the strongest obtained so far for DM particles in the mass range 300 GeV-70 TeV. Ground-based γ-ray observations have reached sufficient sensitivity to explore relevant velocity-averaged cross sections for DM annihilation into two γ-ray photons at the level expected from the thermal relic density for TeV DM particles.
PDF
