486 research outputs found
Data model issues in the Cherenkov Telescope Array project
The planned Cherenkov Telescope Array (CTA), a future ground-based
Very-High-Energy (VHE) gamma-ray observatory, will be the largest project of
its kind. It aims to provide an order of magnitude increase in sensitivity
compared to currently operating VHE experiments and open access to guest
observers. These features, together with the thirty years lifetime planned for
the installation, impose severe constraints on the data model currently being
developed for the project.
In this contribution we analyze the challenges faced by the CTA data model
development and present the requirements imposed to face them. While the full
data model is still not completed we show the organization of the work, status
of the design, and an overview of the prototyping efforts carried out so far.
We also show examples of specific aspects of the data model currently under
development.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
The doubling of the superorbital period of Cyg X-1
We study properties of the superorbital modulation of the X-ray emission of
Cyg X-1. We find that it has had a stable period of about 300 d in soft and
hard X-rays and in radio since 2005 until at least 2010, which is about double
the previously seen period. This new period, seen in the hard spectral state
only, is detected not only in the light curves but also in soft X-ray hardness
ratios and in the amplitude of the orbital modulation. On the other hand, the
spectral slope in hard X-rays, >20 keV, averaged over superorbital bins is
constant, and the soft and hard X-rays and the radio emission change in phase.
This shows that the superorbital variability consists of changing the
normalization of an intrinsic spectrum of a constant shape and of changes of
the absorbing column density with the phase. The maximum column density is
achieved at the superorbital minimum. The amplitude changes are likely to be
caused by a changing viewing angle of an anisotropic emitter, most likely a
precessing accretion disc. The constant shape of the intrinsic spectrum shows
that this modulation is not caused by a changing accretion rate. The modulated
absorbing column density shows the presence of a bulge around the disc centre,
as proposed previously. We also find the change of the superorbital period from
about 150 d to about 300 d to be associated with almost unchanged average X-ray
fluxes, making the period change difficult to explain in the framework of
disc-irradiation models. Finally, we find no correlation of the X-ray and radio
properties with the reported detections in the GeV and TeV gamma-ray range.Comment: MNRAS, in press, 8 page
Cherenkov Telescope Array Data Management
Very High Energy gamma-ray astronomy with the Cherenkov Telescope Array (CTA)
is evolving towards the model of a public observatory. Handling, processing and
archiving the large amount of data generated by the CTA instruments and
delivering scientific products are some of the challenges in designing the CTA
Data Management. The participation of scientists from within CTA Consortium and
from the greater worldwide scientific community necessitates a sophisticated
scientific analysis system capable of providing unified and efficient user
access to data, software and computing resources. Data Management is designed
to respond to three main issues: (i) the treatment and flow of data from remote
telescopes; (ii) "big-data" archiving and processing; (iii) and open data
access. In this communication the overall technical design of the CTA Data
Management, current major developments and prototypes are presented.Comment: 8 pages, 2 figures, In Proceedings of the 34th International Cosmic
Ray Conference (ICRC2015), The Hague, The Netherlands. All CTA contributions
at arXiv:1508.0589
A giant radio flare from Cygnus X-3 with associated Gamma-ray emission
With frequent flaring activity of its relativistic jets, Cygnus X-3 is one of
the most active microquasars and is the only Galactic black hole candidate with
confirmed high energy Gamma-ray emission, thanks to detections by Fermi/LAT and
AGILE. In 2011, Cygnus X-3 was observed to transit to a soft X-ray state, which
is known to be associated with high-energy Gamma-ray emission. We present the
results of a multi-wavelength campaign covering a quenched state, when radio
emission from Cygnus X-3 is at its weakest and the X-ray spectrum is very soft.
A giant (~ 20 Jy) optically thin radio flare marks the end of the quenched
state, accompanied by rising non-thermal hard X-rays. Fermi/LAT observations (E
>100 MeV) reveal renewed Gamma-ray activity associated with this giant radio
flare, suggesting a common origin for all non-thermal components. In addition,
current observations unambiguously show that the Gamma-ray emission is not
exclusively related to the rare giant radio flares. A 3-week period of
Gamma-ray emission is also detected when Cygnus X-3 was weakly flaring in
radio, right before transition to the radio quenched state. No Gamma rays are
observed during the ~ one-month long quenched state, when the radio flux is
weakest. Our results suggest transitions into and out of the ultrasoft X-ray
(radio quenched) state trigger Gamma-ray emission, implying a connection to the
accretion process, and also that the Gamma-ray activity is related to the level
of radio flux (and possibly shock formation), strengthening the connection to
the relativistic jets.Comment: Accepted for publication in MNRAS. 10 pages 5 figures, 1 tabl
Neutral pion emission from accelerated protons in the supernova remnant W44
We present the AGILE gamma-ray observations in the energy range 50 MeV - 10
GeV of the supernova remnant (SNR) W44, one of the most interesting systems for
studying cosmic-ray production. W44 is an intermediate-age SNR (20, 000 years)
and its ejecta expand in a dense medium as shown by a prominent radio shell,
nearby molecular clouds, and bright [SII] emitting regions. We extend our
gamma-ray analysis to energies substantially lower than previous measurements
which could not conclusively establish the nature of the radiation. We find
that gamma-ray emission matches remarkably well both the position and shape of
the inner SNR shocked plasma. Furthermore, the gamma-ray spectrum shows a
prominent peak near 1 GeV with a clear decrement at energies below a few
hundreds of MeV as expected from neutral pion decay. Here we demonstrate that:
(1) hadron-dominated models are consistent with all W44 multiwavelength
constraints derived from radio, optical, X-ray, and gamma-ray observations; (2)
ad hoc lepton-dominated models fail to explain simultaneously the
well-constrained gamma-ray and radio spectra, and require a circumstellar
density much larger than the value derived from observations; (3) the hadron
energy spectrum is well described by a power-law (with index s = 3.0 \pm 0.1)
and a low-energy cut-off at Ec = 6 \pm 1 GeV. Direct evidence for pion emission
is then established in an SNR for the first time.Comment: accepted for publication on ApJ
AGILE detection of delayed gamma-ray emission from GRB 080514B
GRB 080514B is the first gamma ray burst (GRB), since the time of EGRET, for
which individual photons of energy above several tens of MeV have been detected
with a pair-conversion tracker telescope. This burst was discovered with the
Italian AGILE gamma-ray satellite. The GRB was localized with a cooperation by
AGILE and the interplanetary network (IPN). The gamma-ray imager (GRID)
estimate of the position, obtained before the SuperAGILE-IPN localization, is
found to be consistent with the burst position. The hard X-ray emission
observed by SuperAGILE lasted about 7 s, while there is evidence that the
emission above 30 MeV extends for a longer duration (at least ~13 s). Similar
behavior was seen in the past from a few other GRBs observed with EGRET.
However, the latter measurements were affected, during the brightest phases, by
instrumental dead time effects, resulting in only lower limits to the burst
intensity. Thanks to the small dead time of the AGILE/GRID we could assess that
in the case of GRB 080514B the gamma-ray to X-ray flux ratio changes
significantly between the prompt and extended emission phase.Comment: A&A letters, in pres
AGILE detection of GeV gamma-ray emission from the SNR W28
Supernova remnants (SNRs) are believed to be the main sources of Galactic
cosmic rays. Molecular clouds associated with SNRs can produce gamma-ray
emission through the interaction of accelerated particles with the concentrated
gas. The middle aged SNR W28, for its associated system of dense molecular
clouds, provides an excellent opportunity to test this hypothesis. We present
the AGILE/GRID observations of SNR W28, and compare them with observations at
other wavelengths (TeV and 12CO J=1-->0 molecular line emission). The gamma-ray
flux detected by AGILE from the dominant source associated with W28 is (14 +-
5) 10^-8 ph cm^-2 s^-1 for E > 400 MeV. This source is positionally well
correlated with the TeV emission observed by the HESS telescope. The local
variations of the GeV to TeV flux ratio suggest a difference between the CR
spectra of the north-west and south molecular cloud complexes. A model based on
a hadronic-induced interaction and diffusion with two molecular clouds at
different distances from the W28 shell can explain both the morphological and
spectral features observed by AGILE in the MeV-GeV energy range and by the HESS
telescope in the TeV energy range. The combined set of AGILE and H.E.S.S. data
strongly support a hadronic model for the gamma-ray production in W28.Comment: Accepted for publication in Astronomy & Astrophysics Letter
The brightest gamma-ray flaring blazar in the sky: AGILE and multi-wavelength observations of 3C 454.3 during November 2010
Since 2005, the blazar 3C 454.3 has shown remarkable flaring activity at all
frequencies, and during the last four years it has exhibited more than one
gamma-ray flare per year, becoming the most active gamma-ray blazar in the sky.
We present for the first time the multi-wavelength AGILE, SWIFT, INTEGRAL, and
GASP-WEBT data collected in order to explain the extraordinary gamma-ray flare
of 3C 454.3 which occurred in November 2010. On 2010 November 20 (MJD 55520),
3C 454.3 reached a peak flux (E>100 MeV) of F_gamma(p) = (6.8+-1.0)E-5 ph/cm2/s
on a time scale of about 12 hours, more than a factor of 6 higher than the flux
of the brightest steady gamma-ray source, the Vela pulsar, and more than a
factor of 3 brighter than its previous super-flare on 2009 December 2-3. The
multi-wavelength data make a thorough study of the present event possible: the
comparison with the previous outbursts indicates a close similarity to the one
that occurred in 2009. By comparing the broadband emission before, during, and
after the gamma-ray flare, we find that the radio, optical and X-ray emission
varies within a factor 2-3, whereas the gamma-ray flux by a factor of 10. This
remarkable behavior is modeled by an external Compton component driven by a
substantial local enhancement of soft seed photons.Comment: Accepted for publication in ApJ Letters. 18 Pages, 4 Figures, 1 Tabl
Expected performance of the ASTRI-SST-2M telescope prototype
ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) is an
Italian flagship project pursued by INAF (Istituto Nazionale di Astrofisica)
strictly linked to the development of the Cherenkov Telescope Array, CTA.
Primary goal of the ASTRI program is the design and production of an end-to-end
prototype of a Small Size Telescope for the CTA sub-array devoted to the
highest gamma-ray energy region. The prototype, named ASTRI SST-2M, will be
tested on field in Italy during 2014. This telescope will be the first
Cherenkov telescope adopting the double reflection layout in a
Schwarzschild-Couder configuration with a tessellated primary mirror and a
monolithic secondary mirror. The collected light will be focused on a compact
and light-weight camera based on silicon photo-multipliers covering a 9.6 deg
full field of view. Detailed Monte Carlo simulations have been performed to
estimate the performance of the planned telescope. The results regarding its
energy threshold, sensitivity and angular resolution are shown and discussed.Comment: In Proceedings of the 33rd International Cosmic Ray Conference
(ICRC2013), Rio de Janeiro (Brazil). All CTA contributions at arXiv:1307.223
Two Active States of the Narrow-Line Gamma-Ray-Loud AGN GB 1310 + 487
Context. Previously unremarkable, the extragalactic radio source GB1310 487 showed gamma-ray flare on 2009 November 18, reaching a daily flux of approximately 10(exp -6) photons cm(exp -2) s(exp -1) at energies E greater than 100MeV and became one of the brightest GeV sources for about two weeks. Its optical spectrum shows strong forbidden-line emission while lacking broad permitted lines, which is not typical for a blazar. Instead, the spectrum resembles those of narrow emission-line galaxies. Aims. We investigate changes in the object's radio-to-GeV spectral energy distribution (SED) during and after the prominent gamma-ray flare with the aim of determining the nature of the object and of constraining the origin of the variable high-energy emission. Methods. The data collected by the Fermi and AGILE satellites at gamma-ray energies; Swift at X-ray and ultraviolet (UV); the Kanata, NOT, and Keck telescopes at optical; OAGH and WISE at infrared (IR); and IRAM30m, OVRO 40m, Effelsberg 100m, RATAN-600, and VLBA at radio are analyzed together to trace the SED evolution on timescales of months. Results. The gamma-ray radio-loud narrow-line active galactic nucleus (AGN) is located at redshift z = 0.638. It shines through an unrelated foreground galaxy at z = 0.500. The AGN light is probably amplified by gravitational lensing. The AGN SED shows a two-humped structure typical of blazars and gamma-ray-loud narrow-line Seyfert 1 galaxies, with the high-energy (inverse-Compton) emission dominating by more than an order of magnitude over the low-energy (synchrotron) emission during gamma-ray flares. The difference between the two SED humps is smaller during the low-activity state. Fermi observations reveal a strong correlation between the gamma-ray flux and spectral index, with the hardest spectrum observed during the brightest gamma-ray state. The gamma-ray flares occurred before and during a slow rising trend in the radio, but no direct association between gamma-ray and radio flares could be established. Conclusions. If the gamma-ray flux is a mixture of synchrotron self-Compton (SSC) and external Compton (EC) emission, the observed GeV spectral variability may result from varying relative contributions of these two emission components. This explanation fits the observed changes in the overall IR to gamma-ray SED
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