411 research outputs found
The gamma-ray emitting microquasar LSI+61303
LS I +61 303 is one of the most studied X-ray binary systems because of its
two peculiarities: On the one hand being the probable counterpart of the
variable gamma ray source 2CG 135+01 (Gregory and Taylor 1978; Tavani et al.
1998) and on the other hand being a periodic radio source (Taylor and Gregory
1982). The recent discovery of a radio emitting jet extending ca. 200 AU at
both sides of a central core (Massi et al. 2004) in all evidence has shown the
occurrence of accretion/ejection processes in this system. However, the radio
outbursts do not occur at periastron passage, where the accretion is at its
maximum, but several days later. In addition, when the gamma-ray emission of
2CG 135+01 is examined along the orbital phase of LS I +61 303 one sees that
this emission seems to peak at periastron passage (Massi 2004). Here in detail
we analyse the trend of the gamma-ray data versus orbital phase and discuss the
delay between peaks at gamma-rays and in the radio band within the framework of
a two-peak accretion/ejection model proposed by Taylor et al. (1992) and
further developed by Marti' and Paredes (1995).Comment: To be published in the proceedings of the Symposium on High-Energy
Gamma-Ray Astronomy, Heidelberg, July 26-30, 2004 (AIP Proceedings Series
Kinematics of Black Hole X-ray Binary GRS 1915+105
The space velocity of a stellar black hole encodes the history of its
formation and evolution. Here we measure the 3-dimensional motion of the
microquasar GRS 1915+105, using a decade of astrometry with the NRAO Very Long
Baseline Array, together with the published radial velocity. The velocity in
the Galactic Plane deviates from circular rotation by 53-80 +_ 8 km/s, where
the range covers any specific distance from 6-12 kpc. Perpendicular to the
plane, the velocity is only 10 +_ 4 km/s. The peculiar velocity is minimized at
a distance 9-10 kpc, and is then nearly in the radial direction towards the
Galactic Center. We discuss mechanisms for the origin of the peculiar velocity,
and conclude that it is most likely a consequence of Galactic velocity
diffusion on this old binary, rather than the result of a supernova kick during
the formation of the 14 Mo black hole. Finally, a brief comparison is made with
4 other BH binaries whose kinematics are well determined.Comment: 16 pages, 4 figures. ApJ accepte
On the origin of the X-ray emission from a narrow-line radioquasar at z>1
We present new XMM-Newton X-ray observations of the z=1.246 narrow-line
radioquasar RX J1011.2+5545 serendipitously discovered by ROSAT. The flat X-ray
spectrum previously measured by ROSAT and ASCA is shown to be the result of a
steep Gamma~1.8 power law spectrum seen through a moderate intrinsic absorbing
column NH~4E21 cm^-2. The position of the X-ray source is entirely coincident
with the nucleus of the radio source that we have resolved in new sensitive VLA
observations at 3.6 and 6 cm, implying that scattering in the radio lobes is
not responsible for the bulk of X-ray emission. In the EPIC pn image, a faint
patch of X-ray emission is apparent 14'' to the NE of the main X-ray source.
The former is positionally coincident with an apparently extended optical
object with R~21.9, but there is no associated radio emission, thus ruling out
the possibility that this represents a hotspot in a jet emanating from the
primary X-ray source. No reflection features are detected in the X-ray spectrum
of the narrow-line radioquasar, although an Fe line with equivalent width of up
to 600 eV cannot be ruled out.Comment: 7 pages, 6 figures, MNRAS in the pres
Binaries with the eyes of CTA
The binary systems that have been detected in gamma rays have proven very
useful to study high-energy processes, in particular particle acceleration,
emission and radiation reprocessing, and the dynamics of the underlying
magnetized flows. Binary systems, either detected or potential gamma-ray
emitters, can be grouped in different subclasses depending on the nature of the
binary components or the origin of the particle acceleration: the interaction
of the winds of either a pulsar and a massive star or two massive stars;
accretion onto a compact object and jet formation; and interaction of a
relativistic outflow with the external medium. We evaluate the potentialities
of an instrument like the Cherenkov telescope array (CTA) to study the
non-thermal physics of gamma-ray binaries, which requires the observation of
high-energy phenomena at different time and spatial scales. We analyze the
capability of CTA, under different configurations, to probe the spectral,
temporal and spatial behavior of gamma-ray binaries in the context of the known
or expected physics of these sources. CTA will be able to probe with high
spectral, temporal and spatial resolution the physical processes behind the
gamma-ray emission in binaries, significantly increasing as well the number of
known sources. This will allow the derivation of information on the particle
acceleration and emission sites qualitatively better than what is currently
available.Comment: 23 pages, 13 figures, accepted for publication in Astroparticle
Physics, special issue on Physics with the Cherenkov Telescope Arra
Temperature dependence of the charge carrier mobility in gated quasi-one-dimensional systems
The many-body Monte Carlo method is used to evaluate the frequency dependent
conductivity and the average mobility of a system of hopping charges,
electronic or ionic on a one-dimensional chain or channel of finite length. Two
cases are considered: the chain is connected to electrodes and in the other
case the chain is confined giving zero dc conduction. The concentration of
charge is varied using a gate electrode. At low temperatures and with the
presence of an injection barrier, the mobility is an oscillatory function of
density. This is due to the phenomenon of charge density pinning. Mobility
changes occur due to the co-operative pinning and unpinning of the
distribution. At high temperatures, we find that the electron-electron
interaction reduces the mobility monotonically with density, but perhaps not as
much as one might intuitively expect because the path summation favour the
in-phase contributions to the mobility, i.e. the sequential paths in which the
carriers have to wait for the one in front to exit and so on. The carrier
interactions produce a frequency dependent mobility which is of the same order
as the change in the dc mobility with density, i.e. it is a comparably weak
effect. However, when combined with an injection barrier or intrinsic disorder,
the interactions reduce the free volume and amplify disorder by making it
non-local and this can explain the too early onset of frequency dependence in
the conductivity of some high mobility quasi-one-dimensional organic materials.Comment: 9 pages, 8 figures, to be published in Physical Review
MAGIC upper limits on the very high energy emission from GRBs
The fast repositioning system of the MAGIC Telescope has allowed during its
first data cycle, between 2005 and the beginning of year 2006, observing nine
different GRBs as possible sources of very high energy gammas. These
observations were triggered by alerts from Swift, HETE-II, and Integral; they
started as fast as possible after the alerts and lasted for several minutes,
with an energy threshold varying between 80 and 200 GeV, depending upon the
zenith angle of the burst. No evidence for gamma signals was found, and upper
limits for the flux were derived for all events, using the standard analysis
chain of MAGIC. For the bursts with measured redshift, the upper limits are
compatible with a power law extrapolation, when the intrinsic fluxes are
evaluated taking into account the attenuation due to the scattering in the
Metagalactic Radiation Field (MRF).Comment: 25 pages, 9 figures, final version accepted by ApJ. Changet title to
"MAGIC upped limits on the VERY high energy emission from GRBs", re-organized
chapter with description of observation, removed non necessaries figures,
added plot of effective area depending on zenith angle, added an appendix
explaining the upper limit calculation, added some reference
Discovery of VHE Gamma Radiation from IC443 with the MAGIC Telescope
We report the detection of a new source of very high energy (VHE, E_gamma >=
100GeV) gamma-ray emission located close to the Galactic Plane, MAGIC
J0616+225, which is spatially coincident with SNR IC443. The observations were
carried out with the MAGIC telescope in the periods December 2005 - January
2006 and December 2006 - January 2007. Here we present results from this
source, leading to a VHE gamma-ray signal with a statistical significance of
5.7 sigma in the 2006/7 data and a measured differential gamma-ray flux
consistent with a power law, described as dN_gamma/(dA dt dE) = (1.0 +/-
0.2)*10^(-11)(E/0.4 TeV)^(-3.1 +/- 0.3) cm^(-2)s^(-1)TeV^(-1). We briefly
discuss the observational technique used and the procedure implemented for the
data analysis. The results are put in the perspective of the multiwavelength
emission and the molecular environment found in the region of IC443.Comment: Accepted by ApJ Letter
First bounds on the very high energy gamma-ray emission from Arp 220
Using the Major Atmospheric Gamma Imaging Cherenkov Telescope (MAGIC), we
have observed the nearest ultra-luminous infrared galaxy Arp 220 for about 15
hours. No significant signal was detected within the dedicated amount of
observation time. The first upper limits to the very high energy -ray
flux of Arp 220 are herein reported and compared with theoretical expectations.Comment: Accepted for publication in Ap
- …