Very high energy (VHE, energy E ≲ 100 GeV) γ-rays from cosmological sources are attenuated due to the interaction with photons of the extragalactic background light (EBL) in the ultraviolet to ...infrared wavelength band. The EBL, thus, leaves an imprint on the observed energy spectra of these objects. In the last four years, the number of extragalactic VHE sources discovered with imaging atmospheric Cherenkov telescopes (IACTs), such as MAGIC, H.E.S.S., and VERITAS, has doubled. Furthermore, the measurements of the Fermi satellite brought new insights into the intrinsic spectra of the sources at GeV energies. In this paper, upper limits on the EBL intensity are derived by considering the most extensive VHE source sample ever used in this context. This is accomplished by constructing a large number of generic EBL shapes and combining spectral informations from Fermi and IACTs together with minimal assumptions about the source physics at high and very high γ-ray energies. The evolution of the EBL with redshift is accounted for and the possibility of the formation of an electromagnetic cascade and the implications on the upper limits are explored. The EBL density at z = 0 is constrained over a broad wavelength range between 0.4 and 100 μm. At optical wavelengths, the EBL density is constrained below 24 nW m-2 sr-1 and below 5 nW m-2 sr-1 between 8 μm and 31 μm.
Recent observations of the Galactic center with ground based gamma-ray instruments (CANGAROO-II, VERITAS, H.E.S.S.) have revealed a TeV (
10
12
eV
) gamma-ray signal consistent with the position of
...Sgr
A
*
. The observed energy spectrum and angular distribution of the excess could be explained by a massive Dark Matter candidate with a minimum mass of 12 TeV (90% c.l.) and an upper limit on the WIMP density for
r
<
10
pc
of
(
1261
M
⊙
pc
−3
)
(
〈
σ
v
〉
/
(
3
×
10
−26
cm
3
s
−1
)
)
−
1
/
2
. The angular distribution of the excess events is consistent with a cuspy profile with
ρ
(
r
)
∝
r
−
α
with
α
>
1.0
at a confidence level of 90%. Even though the mass and the cross section of the Dark Matter constituents are unexpectedly high in the framework of most models of non-baryonic Dark Matter, it cannot be ruled out.
The continuum high-energy gamma-ray emission between 1 GeV and 105 GeV from the Crab Nebula has been measured for the first time in overlapping energy bands by the Fermi large-area telescope ...(Fermi/LAT) below ≈100 GeV and by ground-based imaging air Cherenkov telescopes (IACTs) above ≈60 GeV. To follow up on the phenomenological approach suggested by Hillas et al. (1998), the broad band spectral and spatial measurement (from radio to low-energy gamma-rays <1 GeV) is used to extract the shape of the electron spectrum. While this model per construction provides an excellent description of the data at energies <1 GeV, the predicted inverse Compton component matches the combined Fermi/LAT and IACT measurements remarkably well after including all relevant seed photon fields and fitting the average magnetic field to $B = (124\,{}\pm6\,\mathrm{(stat.)}\,{}^{+15}_{-6}\,\mathrm{(sys.)})\,\mu\mathrm{G}$ B = ( 124 ± 6 ( stat . ) -6 + 15 ( sys . ) ) μ G . The close match of the resulting broad band inverse Compton component with the combined Fermi/LAT and IACTs data is used to derive instrument specific energy-calibration factors. These factors can be used to combine data from Fermi/LAT and IACTs without suffering from systematic uncertainties on the common energy scale. As a first application of the cross calibration, we derive an upper limit to the diffuse gamma-ray emission between 250 GeV and 1 TeV based upon the combined measurements of Fermi/LAT and the H.E.S.S. ground-based Cherenkov telescopes. Finally, the predictions of the magneto-hydrodynamic flow model of Kennel & Coroniti (1984) are compared to the measured SED.
The well-known Crab Nebula is at the center of the SN1054 supernova remnant. It consists of a rotationally powered pulsar interacting with a surrounding nebula through a relativistic particle wind. ...The emissions originating from the pulsar and nebula have been considered to be essentially stable. Here, we report the detection of strong gamma-ray (100 mega-electron volts to 10 giga-electron volts) flares observed by the AGILE satellite in September 2010 and October 2007. In both cases, the total gamma-ray flux increased by a factor of three compared with the non-flaring flux. The flare luminosity and short time scale favor an origin near the pulsar, and we discuss Chandra Observatory x-ray and Hubble Space Telescope optical follow-up observations of the nebula. Our observations challenge standard models of nebular emission and require power-law acceleration by shock-driven plasma wave turbulence within an approximately 1-day time scale.
The standard paradigm of hierarchical structure formation in a ΛCDM universe predicts the presence of dark matter subhaloes, hosted by Milky Way-sized galaxies. Anticipated subhalo masses range from ...1010 down to a cut-off mass between 10-3 and 10-11 M⊙. If dark matter is composed of heavy self-annihilating or decaying particles, these subhaloes could be visible in the γ-ray band as faint and temporally constant sources without astrophysical counterparts. Based upon realistic subhalo models and current observational constraints on annihilating dark matter scenarios, we predict that one massive Galactic subhalo between 106 and 108 M⊙ may already be present in the 11-month catalogue of Fermi-LAT. Indeed, at least twelve objects in the first Fermi catalogue qualify as candidates. The most promising object, 1FGL J0030.7+0724, is investigated in detail using a dedicated Swift X-ray follow-up observation and a refined positional analysis of the 24-month Fermi-LAT data. With the new observations, seven point-like X-ray sources have been discovered, of which SWIFT J003119.8+072454, which coincides with a faint radio source (12 mJy at 1.4 GHz), serves as a counterpart candidate of 1FGL J0030.7+0724. The broad-band spectral energy distribution is consistent with a high-energy-peaked blazar. However, flux and extent of 1FGL J0030.7+0724 may also be compatible with a dark matter subhalo. Detection of temporal variability or improved astrometry of 1FGL J0030.7+0724 are necessary to rule out or confirm an astrophysical origin. We discuss strategies to identify γ-ray sources that are associated with self-annihilating dark matter subhaloes.
We present a detailed analysis of week-long simultaneous observations of the blazar Mrk 421 at 2-60 keV X-rays (RXTE) and TeV -rays (Whipple and HEGRA) in 2001. Accompanying optical monitoring was ...performed with the Mt. Hopkins 48 inch telescope. The unprecedented quality of this data set enables us to establish the existence of the correlation between the TeV and X-ray luminosities, and also to start unveiling some of its characteristics, in particular its energy dependence and time variability. The source shows strong variations in both X-ray and -ray bands, which are highly correlated. No evidence of an X-ray/-ray interband lag tau is found on the full week data set, with tau image 3 ks. A detailed analysis of the March 19 flare, however, reveals that data are not consistent with the peak of the outburst in the 2-4 keV X-ray and TeV band being simultaneous. We estimate a image ks TeV lag. The amplitudes of the X-ray and -ray variations are also highly correlated, and the TeV luminosity increases more than linearly with respect to the X-ray one. The high degree of correlation lends further support to the standard model in which a unique electron population produces the X-rays by synchrotron radiation and the -ray component by inverse Compton scattering. However, the finding that for the individual best observed flares the -ray flux scales approximately quadratically with respect to the X-ray flux poses a serious challenge to emission models for TeV blazars, as it requires rather special conditions and/or fine tuning of the temporal evolution of the physical parameters of the emission region. We briefly discuss the astrophysical consequences of these new findings in the context of the competing models for the jet emission in blazars.
Aims. The optical variability of the blazar PKS 2155-304 is investigated to characterise the red noise behaviour at largely different time scales from 20 days to \hbox{${\cal O}(>$}𝒪(> 10 yrs). ...Methods. The long-term optical light curve of PKS 2155-304 is assembled from archival data as well as from so-far unpublished observations mostly carried out with the ROTSE-III and the ASAS robotic telescopes. A forward folding technique is used to determine the best-fit parameters for a model of a power law with a break in the power spectral density function (PSD). The best-fit parameters are estimated using a maximum-likelihood method with simulated light curves in conjunction with the Lomb-Scargle periodogram and the first-order structure function (SF). In addition, a new approach based upon the so-called multiple fragments variance function (MFVF) is introduced and compared to the other methods. Simulated light curves have been used to confirm the reliability of these methods as well as to estimate the uncertainties of the best-fit parameters. Results. The light curve is consistent with the assumed broken power-law PSD. All three methods agree within the estimated uncertainties with the MFVF providing the most accurate results. The red-noise behaviour of the PSD in frequency f follows a power law with f − β, \hbox{$\beta=1.8^{+0.1}_{-0.2}$}β=1.8-0.2+0.1 and a break towards f0 at frequencies lower than \hbox{$f_\mathrm{min}=(2.7^{+2.2}_{-1.6}~\mathrm{yrs})^{-1}$}fmin=(2.7-1.6+2.2 yrs)-1.
The high efficiency, low background, and single-photon detection with transition-edge sensors (TES) is making this type of detector attractive in widely different types of applications. In this ...paper, we present first characterizations of a TES to be used in the Any Light Particle Search (ALPS) experiment searching for new fundamental ultra-light particles. Firstly, we describe the setup and the main components of the ALPS TES detector (TES, millikelvin-cryostat and SQUID readout) and their performances. Secondly, we explain a dedicated analysis method for single-photon spectroscopy and rejection of non-photon background. Finally, we report on results from extensive background measurements. Considering an event selection, optimized for a wavelength of 1064 nm, we achieved a background suppression of
with a
% efficiency for photons passing the selection. The resulting overall efficiency was 23% with a dark count rate of
. We observed that pile-up events of thermal photons are the main background components.
We report on very long baseline interferometry (VLBI) observations of the radio emission from the inner region of the Crab nebula, made at 1.6 GHz and 5 GHz after a recent high-energy flare in this ...object. The 5 GHz data have provided only upper limits of 0.4 milli-Jansky (mJy) on the flux density of the pulsar and 0.4 mJy/beam on the brightness of the putative flaring region. The 1.6 GHz data have enabled imaging the inner regions of the nebula on scales of up to ≈ 40′′. The emission from the inner “wisps” is detected for the first time with VLBI observations. A likely radio counterpart (designated “C1”) of the putative flaring region observed with Chandra and HST is detected in the radio image, with an estimated flux density of 0.5 ± 0.3 mJy and a size of 0\hbox{$\farcs$}″̣2–0\hbox{$\farcs$}″̣6. Another compact feature (“C2”) is also detected in the VLBI image closer to the pulsar, with an estimated flux density of 0.4 ± 0.2 mJy and a size smaller than 0\hbox{$\farcs$}″̣2. Combined with the broad-band SED of the flare, the radio properties of C1 yield a lower limit of ≈ 0.5 mG for the magnetic field and a total minimum energy of 1.2 × 1041 erg vested in the flare (corresponding to using about 0.2% of the pulsar spin-down power). The 1.6 GHz observations provide upper limits for the brightness (0.2 mJy/beam) and total flux density (0.4 mJy) of the optical Knot 1 located at 0\hbox{$\farcs$}″̣6 from the pulsar. The absolute position of the Crab pulsar is determined, and an estimate of the pulsar proper motion (μα = −13.0 ± 0.2 mas/yr, μδ = + 2.9 ± 0.1 mas/yr) is obtained.
PDF
