7,935 research outputs found
Fermi detected blazars seen by INTEGRAL
Multiwavelength observations are essential to constrain physical parameters
of the blazars observed by Fermi/LAT. Among the 187 AGN significantly detected
in public INTEGRAL data above 20 keV by the imager IBIS/ISGRI, 20 blazars were
detected. 15 of these sources allowed significant spectral extraction. They
show hard X-ray spectra with an average photon index of 2.1+-0.1 and a hard
X-ray luminosity of L(20-100 keV) = 1.3e46 erg/s. 15 of the INTEGRAL blazars
are also visible in the first 16 months of the Fermi/LAT data, thus allowing to
constrain the inverse Compton branch in these cases. Among others, we analyse
the LAT data of four blazars which were not included in the Fermi LAT Bright
AGN Sample based on the first 3 months of the mission: QSO B0836+710, H
1426+428, RX J1924.8-2914, and PKS 2149-306. Especially for blazars during
bright outbursts, as already observed simultaneously by INTEGRAL and Fermi
(e.g. 3C 454.3 and Mrk 421), INTEGRAL provides unique spectral coverage up to
several hundred keV. We present the spectral analysis of INTEGRAL and Fermi
data and demonstrate the potential of INTEGRAL observations of Fermi detected
blazars in outburst by analysing the combined data set of the persistent radio
galaxy Cen A.Comment: 5 pages, 5 figures, 2009 Fermi Symposium, eConf Proceedings C09112
Zooming in on supermassive black holes: how resolving their gas cloud host renders their accretion episodic
Born in rapidly evolving mini-halos during the first billion years of the
Universe, super- massive black holes (SMBH) feed from gas flows spanning many
orders of magnitude, from the cosmic web in which they are embedded to their
event horizon. As such, accretion onto SMBHs constitutes a formidable challenge
to tackle numerically, and currently requires the use of sub-grid models to
handle the flow on small, unresolved scales. In this paper, we study the impact
of resolution on the accretion pattern of SMBHs initially inserted at the heart
of dense galactic gas clouds, using a custom super-Lagrangian refinement scheme
to resolve the black hole (BH) gravitational zone of influence. We find that
once the self-gravitating gas cloud host is sufficiently well re- solved,
accretion onto the BH is driven by the cloud internal structure, independently
of the BH seed mass, provided dynamical friction is present during the early
stages of cloud collapse. For a pristine gas mix of hydrogen and helium, a slim
disc develops around the BH on sub-parsec scales, turning the otherwise chaotic
BH accretion duty cycle into an episodic one, with potentially important
consequences for BH feedback. In the presence of such a nuclear disc, BH mass
growth predominantly occurs when infalling dense clumps trigger disc
instabilities, fuelling intense albeit short-lived gas accretion episodes.Comment: Resubmitted to mnras after reviewer comments, 24 page
Nonlinear thermoelectric effects in high-field superconductor-ferromagnet tunnel junctions
Thermoelectric effects result from the coupling of charge and heat transport,
and can be used for thermometry, cooling and harvesting of thermal energy. The
microscopic origin of thermoelectric effects is a broken electron-hole
symmetry, which is usually quite small in metal structures, and vanishes at low
temperatures. We report on a combined experimental and theoretical
investigation of thermoelectric effects in superconductor/ferromagnet hybrid
structures. We investigate the depencence of thermoelectric currents on the
thermal excitation, as well as on the presence of a dc bias voltage across the
junction. Large thermoelectric effects are observed in
superconductor/ferromagnet and superconductor/normal-metal hybrid structures.
The spin-independent signals observed under finite voltage bias are shown to be
reciprocal to the physics of superconductor/normal-metal microrefrigerators.
The spin-dependent thermoelectric signals in the linear regime are due to the
coupling of spin and heat transport, and can be used to design more efficient
refrigeratorsComment: 11 pages, submitted to Beilstein Journal of Nanotechnolog
Gamma-Ray Bursts Observed with the Spectrometer SPI Onboard INTEGRAL
The spectrometer SPI is one of the main detectors of ESA's INTEGRAL mission.
The instrument offers two interesting and valuable capabilities for the
detection of the prompt emission of Gamma-ray bursts. Within a field of view of
16 degrees, SPI is able to localize Gamma-ray bursts with an accuracy of 10
arcmin. The large anticoincidence shield, ACS, consisting of 512 kg of BGO
crystals, detects Gamma-ray bursts quasi omnidirectionally above ~70 keV. Burst
alerts from SPI/ACS are distributed to the interested community via the
INTEGRAL Burst Alert System. The ACS data have been implemented into the 3rd
Interplanetary Network and have proven valuable for the localization of bursts
using the triangulation method. During the first 8 months of the mission
approximately one Gamma-ray burst per month was localized within the field of
fiew of SPI and 145 Gamma-ray burst candidates were detected by the ACS from
which 40 % have been confirmed by other instruments.Comment: 4 pages, 2 figures, to appear in the Proceedings of the Conference
"30 Years of GRB Discovery", Santa Fe, New Mexico, USA, September 8-12, 200
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