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V A Acciari,
E Aliu,
T Aune,
M Beilicke,
W Benbow,
M Böttcher,
S M Bradbury,
J H Buckley,
V Bugaev,
Y Butt, [......],
F De Sabata,
A Dominguez,
D Dorner,
M Doro,
D Elsaesser,
M Errando,
D Ferenc,
E Fer Andez,
R Firpo,
M Mariotti
[show abstract]
[hide abstract]
ABSTRACT: We report on the results of two coordinated multiwavelength campaigns that focused on the blazar Markarian 421 during its 2006 and 2008 outbursts. These campaigns obtained UV and X-ray data us-ing the XMM-Newton satellite, while the gamma-ray data were obtained utilizing three imaging atmo-spheric Cerenkov telescopes, the Whipple 10 m telescope and VERITAS, both based in Arizona, as well as the MAGIC telescope, based on La Palma in the Canary Islands. The coordinated effort between the gamma-ray groups allowed for truly simultaneous data in UV/X-ray/gamma-ray wavelengths during a sig-nificant portion of the XMM-Newton observations. This simultaneous coverage allowed for a reliable search for correlations between UV, X-ray, and gamma-ray variability over the course of the observations. In-vestigations of spectral hysteresis and modeling of the spectral energy distributions are also presented.
The Astrophysical Journal J. Moldón J. Ninkovic E. Prandini N. Puchades I. Reichardt J. Rico T. Y. Saito V. Scalzotto S. N. Shore N. Sidro A. Sierpowska-Bartosik J. Sitarek J. Zapatero. 03/2037; 703455657454052443847(35):169-178.
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N. Otte,
M. Errando,
S. Griffiths, P. Kaaret,
H. Krawczynski,
A. McCann,
G. Sinnis,
F. Stecker,
I. Taboada,
V. Vasileiou,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: We discuss the prospects of doing tests of Lorentz invariance with gamma-rays
observed with present and future ground based gamma-ray observatories.
05/2013;
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E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
A. Bouvier,
J. H. Buckley,
V. Bugaev,
A. Cesarini, [......],
A. Varlotta,
V. V. Vassiliev,
S. Vincent,
M. Vivier,
S. P. Wakely,
T. C. Weekes,
A. Weinstein,
R. Welsing,
D. A. Williams,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: We report the discovery of TeV gamma-ray emission coincident with the
shell-type radio supernova remnant (SNR) CTA 1 using the VERITAS gamma-ray
observatory. The source, VER J0006+729, was detected as a 6.5 standard
deviation excess over background and shows an extended morphology, approximated
by a two-dimensional Gaussian of semi-major (semi-minor) axis 0.30 degree (0.24
degree) and a centroid 5' from the Fermi gamma-ray pulsar PSR J0007+7303 and
its X-ray pulsar wind nebula (PWN). The photon spectrum is well described by a
power-law dN/dE = N_0 (E/3 TeV)^(-\Gamma), with a differential spectral index
of \Gamma = 2.2 +- 0.2_stat +- 0.3_sys, and normalization N_0 = (9.1 +-
1.3_stat +- 1.7_sys) x 10^(-14) cm^(-2) s^(-1) TeV^(-1). The integral flux,
F_\gamma = 4.0 x 10^(-12) erg cm^(-2) s^(-1) above 1 TeV, corresponds to 0.2%
of the pulsar spin-down power at 1.4 kpc. The energetics, co-location with the
SNR, and the relatively small extent of the TeV emission strongly argue for the
PWN origin of the TeV photons. We consider the origin of the TeV emission in
CTA 1.
The Astrophysical Journal 12/2012; 764(1):38. · 6.02 Impact Factor
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M. Feroci,
J. W. den Herder,
E. Bozzo,
D. Barret,
S. Brandt,
M. Hernanz,
M. van der Klis,
M. Pohl,
A. Santangelo,
L. Stella, [......],
D. Lai,
F. K. Lamb,
F. Lebrun,
D. Lin,
D. Linder,
G. Lodato,
F. Longo,
N. Lund,
T. J. Maccarone,
D. Macera
-
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
A. Bouvier,
J. H. Buckley,
V. Bugaev,
A. Cesarini,
L. Ciupik,
M. P. Connolly, [......],
W. Max-Moerbeck,
A. Readhead,
M. L. Lister,
Y. Y. Kovalev,
A. B. Pushkarev,
M. A. Gurwell,
A. Lahteenmaki,
E. Nieppola,
M. Tornikoski,
E. Jarvela
[show abstract]
[hide abstract]
ABSTRACT: We report on the detection of a very rapid TeV gamma-ray flare from BL
Lacertae on 2011 June 28 with the Very Energetic Radiation Imaging Telescope
Array System (VERITAS). The flaring activity was observed during a 34.6-minute
exposure, when the integral flux above 200 GeV reached $(3.4\pm0.6) \times
10^{-6} \;\text{photons}\;\text{m}^{-2}\text{s}^{-1}$, roughly 125% of the Crab
Nebula flux measured by VERITAS. The light curve indicates that the
observations missed the rising phase of the flare but covered a significant
portion of the decaying phase. The exponential decay time was determined to be
$13\pm4$ minutes, making it one of the most rapid gamma-ray flares seen from a
TeV blazar. The gamma-ray spectrum of BL Lacertae during the flare was soft,
with a photon index of $3.6\pm 0.4$, which is in agreement with the measurement
made previously by MAGIC in a lower flaring state. Contemporaneous radio
observations of the source with the Very Long Baseline Array (VLBA) revealed
the emergence of a new, superluminal component from the core around the time of
the TeV gamma-ray flare, accompanied by changes in the optical polarization
angle. Changes in flux also appear to have occurred at optical, UV, and GeV
gamma-ray wavelengths at the time of the flare, although they are difficult to
quantify precisely due to sparse coverage. A strong flare was seen at radio
wavelengths roughly four months later, which might be related to the gamma-ray
flaring activities. We discuss the implications of these multiwavelength
results.
The Astrophysical Journal 11/2012; 762(2):92. · 6.02 Impact Factor
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E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
M. Bottcher,
A. Bouvier,
J. H. Buckley,
V. Bugaev, [......],
T. C. Weekes,
A. Weinstein,
R. Welsing,
D. A. Williams,
B. Zitzer,
P. Fortin,
D. Horan,
M. Fumagalli,
K. Kaplan,
J. X. Prochaska
[show abstract]
[hide abstract]
ABSTRACT: We report on VERITAS very-high-energy (VHE; E>100 GeV) observations of six
blazars selected from the Fermi Large Area Telescope First Source Catalog
(1FGL). The gamma-ray emission from 1FGL sources was extrapolated up to the VHE
band, taking gamma-ray absorption by the extragalactic background light into
account. This allowed the selection of six bright, hard-spectrum blazars that
were good candidate TeV emitters. Spectroscopic redshift measurements were
attempted with the Keck Telescope for the targets without Sloan Digital Sky
Survey (SDSS) spectroscopic data. No VHE emission is detected during the
observations of the six sources described here. Corresponding TeV upper limits
are presented, along with contemporaneous Fermi observations and non-concurrent
Swift UVOT and XRT data. The blazar broadband spectral energy distributions
(SEDs) are assembled and modeled with a single-zone synchrotron self-Compton
model. The SED built for each of the six blazars show a synchrotron peak
bordering between the intermediate- and high-spectrum-peak classifications,
with four of the six resulting in particle-dominated emission regions.
The Astrophysical Journal 10/2012; 759(2):102. · 6.02 Impact Factor
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E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
A. Bouvier,
J. H. Buckley,
V. Bugaev,
K. Byrum, [......],
S. Vincent,
M. Vivier,
R. G. Wagner,
S. P. Wakely,
T. C. Weekes,
A. Weinstein,
R. Welsing,
D. A. Williams,
B. Zitzer,
V. Kondratiev
[show abstract]
[hide abstract]
ABSTRACT: We present the results of a joint observational campaign between the Green
Bank radio telescope and the VERITAS gamma-ray telescope, which searched for a
correlation between the emission of very-high-energy (VHE) gamma rays
($E_{\gamma} >$ 150 GeV) and Giant Radio Pulses (GRPs) from the Crab pulsar at
8.9 GHz. A total of 15366 GRPs were recorded during 11.6 hours of simultaneous
observations, which were made across four nights in December 2008 and in
November and December 2009. We searched for an enhancement of the pulsed
gamma-ray emission within time windows placed around the arrival time of the
GRP events. In total, 8 different time windows with durations ranging from
0.033 ms to 72 s were positioned at three different locations relative to the
GRP to search for enhanced gamma-ray emission which lagged, led, or was
concurrent with, the GRP event. Further, we performed separate searches on main
pulse GRPs and interpulse GRPs and on the most energetic GRPs in our data
sample. No significant enhancement of pulsed VHE emission was found in any of
the preformed searches. We set upper limits of 5-10 times the average VHE flux
of the Crab pulsar on the flux simultaneous with interpulse GRPs on
single-rotation-period time scales. On $\sim$8-second time scales around
interpulse GRPs, we set an upper limit of 2-3 times the average VHE flux.
Within the framework of recent models for pulsed VHE emission from the Crab
pulsar, the expected VHE-GRP emission correlations are below the derived
limits.
The Astrophysical Journal 10/2012; 760(2):136. · 6.02 Impact Factor
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M. Feroci,
J. W. den Herder,
E. Bozzo,
D. Barret,
S. Brandt,
M. Hernanz,
M. van der Klis,
M. Pohl,
A. Santangelo,
L. Stella, [......],
R. Wijnands,
M. Wille,
C. A. Wilson-Hodge,
B. Winter,
K. Wood,
G. Zampa,
N. Zampa,
L. Zampieri,
A. Zdziarski,
B. Zhang
[show abstract]
[hide abstract]
ABSTRACT: The LOFT mission concept is one of four candidates selected by ESA for the M3
launch opportunity as Medium Size missions of the Cosmic Vision programme. The
launch window is currently planned for between 2022 and 2024. LOFT is designed
to exploit the diagnostics of rapid X-ray flux and spectral variability that
directly probe the motion of matter down to distances very close to black holes
and neutron stars, as well as the physical state of ultra-dense matter. These
primary science goals will be addressed by a payload composed of a Large Area
Detector (LAD) and a Wide Field Monitor (WFM). The LAD is a collimated (<1
degree field of view) experiment operating in the energy range 2-50 keV, with a
10 m^2 peak effective area and an energy resolution of 260 eV at 6 keV. The WFM
will operate in the same energy range as the LAD, enabling simultaneous
monitoring of a few-steradian wide field of view, with an angular resolution of
<5 arcmin. The LAD and WFM experiments will allow us to investigate variability
from submillisecond QPO's to year-long transient outbursts. In this paper we
report the current status of the project.
09/2012;
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The Veritas Collaboration with C. Pfrommer,
A. Pinzke: T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
A. Bouvier,
J. H. Buckley,
V. Bugaev,
K. Byrum,
A. Cannon, [......],
S. Thibadeau,
K. Tsurusaki,
A. Varlotta,
M. Vivier,
S. P. Wakely,
J. E. Ward,
A. Weinstein,
R. Welsing,
D. A. Williams,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: Observations of radio halos and relics in galaxy clusters indicate efficient
electron acceleration. Protons should likewise be accelerated, suggesting that
clusters may also be sources of very high-energy (VHE; E>100 GeV) gamma-ray
emission. We report here on VHE gamma-ray observations of the Coma galaxy
cluster with the VERITAS array of imaging Cherenkov telescopes, with
complementing Fermi-LAT observations at GeV energies. No significant gamma-ray
emission from the Coma cluster was detected. Integral flux upper limits at the
99% confidence level were measured to be on the order of (2-5)*10^-8\ ph. m^-2
s^-1 (VERITAS, >220 GeV} and ~2*10^-6 ph. m^-2 s^-1 (Fermi, 1-3 GeV),
respectively. We use the gamma-ray upper limits to constrain CRs and magnetic
fields in Coma. Using an analytical approach, the CR-to-thermal pressure ratio
is constrained to be < 16% from VERITAS data and < 1.7% from Fermi data
(averaged within the virial radius). These upper limits are starting to
constrain the CR physics in self-consistent cosmological cluster simulations
and cap the maximum CR acceleration efficiency at structure formation shocks to
be <50%. Assuming that the radio-emitting electrons of the Coma halo result
from hadronic CR interactions, the observations imply a lower limit on the
central magnetic field in Coma of (2 - 5.5) muG, depending on the radial
magnetic-field profile and on the gamma-ray spectral index. Since these values
are below those inferred by Faraday rotation measurements in Coma (for most of
the parameter space), this {renders} the hadronic model a very plausible
explanation of the Coma radio halo. Finally, since galaxy clusters are
dark-matter (DM) dominated, the VERITAS upper limits have been used to place
constraints on the thermally-averaged product of the total self-annihilation
cross section and the relative velocity of the DM particles, <\sigma v>. (abr.)
The Astrophysical Journal 08/2012; 757(2):123. · 6.02 Impact Factor
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E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
M. Bottcher,
A. Bouvier,
V. Bugaev,
A. Cannon, [......],
K. Tsurusaki,
A. Varlotta,
V. V. Vassiliev,
M. Vivier,
S. P. Wakely,
J. E. Ward,
A. Weinstein,
R. Welsing,
D. A. Williams,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: We present observations of the BL Lac object 1ES 0414+009 in the >200 GeV
gamma-ray band by the VERITAS array of Cherenkov telescopes. 1ES 0414+009 was
observed by VERITAS between January 2008 and February 2011, resulting in 56.2
hours of good quality pointed observations. These observations resulted in a
detection of 822 events from the source corresponding to a statistical
significance of 6.4 standard deviations (6.4 sigma) above the background. The
source flux, showing no evidence for variability, is measured as 5.2 +/-
1.1_stat +/- 2.6_sys * 10^-12 photons cm^-2 s^-1 above 200 GeV, equivalent to
approximately 2% of the Crab Nebula flux above this energy. The differential
photon spectrum from 230 GeV to 850 GeV is well fit by a power law with an
photon index of Gamma 3.4 +/- 0.5_stat +/- 0.3_sys and a flux normalization of
1.6 +/- 0.3_stat +/- 0.8_sys * 10^-11 photons cm^-2 s^-1 at 300 GeV. We also
present multiwavelength results taken in the optical (MDM), X-ray (Swift-XRT),
and GeV (Fermi-LAT) bands and use these results to construct a broadband
spectral energy distribution (SED). Modeling of this SED indicates that
homogenous one-zone leptonic scenarios are not adequate to describe emission
from the system, with a lepto-hadronic model providing a better fit to the
data.
The Astrophysical Journal 06/2012; 755(2):118. · 6.02 Impact Factor
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E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
A. Bouvier,
S. M. Bradbury,
J. H. Buckley,
V. Bugaev, [......],
S. Vincent,
M. Vivier,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A. Weinstein,
T. Weisgarber,
R. Welsing,
D. A. Williams,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: We report on very high energy (E > 100 GeV) gamma-ray observations of V407
Cygni, a symbiotic binary that underwent a nova outburst producing 0.1-10 GeV
gamma rays during 2010 March 10-26. Observations were made with the Very
Energetic Radiation Imaging Telescope Array System during 2010 March 19-26 at
relatively large zenith angles, due to the position of V407 Cyg. An improved
reconstruction technique for large zenith angle observations is presented and
used to analyze the data. We do not detect V407 Cygni and place a differential
upper limit on the flux at 1.6 TeV of 2.3 \times 10^(-12) erg cm^(-2) s^(-1)
(at the 95% confidence level). When considered jointly with data from
Fermi-LAT, this result places limits on the acceleration of very high energy
particles in the nova.
05/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: We present the results of a spectroscopic study of the Fe K{\alpha} emission
of the persistent neutron-star atoll low-mass X-ray binary and type I X-ray
burster GX 3+1 with the EPIC-PN on board XMM-Newton. The source shows a flux
modulation over several years and we observed it during its fainter phase,
which corresponds to an X-ray luminosity of Lx~10^37 ergs/s. When fitted with a
two-component model, the X-ray spectrum shows broad residuals at \sim6-7 keV
that can be ascribed to an iron K{\alpha} fluorescence line. In addition, lower
energy features are observed at \sim3.3 keV, \sim3.9 keV and might originate
from Ar XVIII and Ca XIX. The broad iron line feature is well fitted with a
relativistically smeared profile. This result is robust against possible
systematics caused by instrumental pile-up effects. Assuming that the line is
produced by reflection from the inner accretion disk, we infer an inner disk
radius of \sim25 Rg and a disk inclination of 35{\deg} < i < 44{\deg}.
05/2012;
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[show abstract]
[hide abstract]
ABSTRACT: In this paper we report on Expanded Very Large Array radio and Chandra and
Swift X-ray observations of the outburst decay of the transient black hole
candidate MAXI J1659-152 in 2011. We discuss the distance to the source taking
the high inclination into account and we conclude that the source distance is
probably 6+-2 kpc. The lowest observed flux corresponds to a luminosity of
2x10^31 (d/6 kpc)^2 erg/s This, together with the orbital period of 2.4 hr
reported in the literature, suggests that the quiescent X-ray luminosity is
higher than predicted on the basis of the orbital period -- quiescent X-ray
luminosity relationship. The relation between the accretion and ejection
mechanisms can be studied using the observed correlation between the radio and
X-ray luminosities as these evolve over an outburst. We determine the behaviour
of MAXI J1659-152 in the radio -- X-ray diagram at low X-ray luminosities using
the observations reported in this paper and at high X-ray luminosities using
values reported in the literature. At high X-ray luminosities the source lies
closer to the sources that follow a correlation index steeper than 0.6-0.7.
However, when compared to other sources that follow a steeper correlation
index, the X-ray luminosity in MAXI J1659-152 is also lower. The latter can
potentially be explained by the high inclination of MAXI J1659-152 if the X-ray
emission comes from close to the source and the radio emission is originating
in a more extended region. However, it is probable that the source was not in
the canonical low-hard state during these radio observations and this may
affect the behaviour of the source as well. At intermediate X-ray luminosities
the source makes the transition from the radio underluminous sources in the
direction of the relation traced by the 'standard' correlation similar to what
has been reported for H1743-322. (abridged)
04/2012;
-
E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
M. Böttcher,
A. Bouvier,
S. M. Bradbury,
J. H. Buckley, [......],
M. Vivier,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A. Weinstein,
T. Weisgarber,
D. A. Williams,
B. Zitzer,
P. Fortin,
and D. Horan
[show abstract]
[hide abstract]
ABSTRACT: We report on the discovery of high-energy (HE; E > 0.1 GeV) and very high energy (VHE; E > 100 GeV) γ-ray emission from the high-frequency-peaked BL Lac object RBS 0413. VERITAS, a ground-based γ-ray observatory, detected VHE γ rays from RBS 0413 with a statistical significance of 5.5 standard deviations (σ) and a γ-ray flux of (1.5 ± 0.6stat ± 0.7syst) × 10–8 photons m–2 s–1 (~1% of the Crab Nebula flux) above 250 GeV. The observed spectrum can be described by a power law with a photon index of 3.18 ± 0.68stat ± 0.30syst. Contemporaneous observations with the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope detected HE γ rays from RBS 0413 with a statistical significance of more than 9σ, a power-law photon index of 1.57 ± 0.12stat +0.11 – 0.12sys, and a γ-ray flux between 300 MeV and 300 GeV of (1.64 ± 0.43stat +0.31 – 0.22sys) × 10–5 photons m–2 s–1. We present the results from Fermi-LAT and VERITAS, including a spectral energy distribution modeling of the γ-ray, quasi-simultaneous X-ray (Swift-XRT), ultraviolet (Swift-UVOT), and R-band optical (MDM) data. We find that, if conditions close to equipartition are required, both the combined synchrotron self-Compton/external-Compton and the lepto-hadronic models are preferred over a pure synchrotron self-Compton model.
The Astrophysical Journal 04/2012; 750(2):94. · 6.02 Impact Factor
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E. M. Ratti,
P. G. Jonker,
J. C. A. Miller-Jones,
M. A. P. Torres,
J. Homan,
S. Markoff,
J. A. Tomsick, P. Kaaret,
R. Wijnands,
E. Gallo,
F. Ozel,
D. T. H. Steeghs,
R. P. Fender
[show abstract]
[hide abstract]
ABSTRACT: We present optical, X-ray and radio observations of the black hole transient
(BHT) XTE J1752-223 towards and in quiescence. Optical photometry shows that
the quiescent magnitude of XTE J1752-223 is fainter than 24.4 magnitudes in the
i'-band. A comparison with measurements of the source during its 2009-2010
outburst shows that the outburst amplitude is more than 8 magnitudes in the
i'-band. Known X-ray properties of the source combined with the faintness of
the quiescence optical counterpart and the large outburst optical amplitude
point towards a short orbital period system (Porb<~6.8 h) with an M type (or
later) mass donor, at a distance of 3.5<~d<~8 kpc. Simultaneous X-ray and radio
data were collected with Chandra and the EVLA, allowing constraints to be
placed on the quiescent X-ray and radio flux of XTE J1752-223. Furthermore,
using data covering the final stage of the outburst decay, we investigated the
low luminosity end of the X-ray - radio correlation for this source and
compared it with other BHTs. We found that XTE J1752-223 adds to the number of
outliers with respect to the `standard' X-ray - radio luminosity relation.
Furthermore, XTE J1752-223 is the second source, after the BHT H1743-322, that
shows a transition from the region of the outliers towards the `standard'
correlation at low luminosity. Finally, we report on a faint, variable X-ray
source we discovered with Chandra at an angular distance of ~2.9" to XTE
J1752-223 and at a position angle consistent with that of the radio jets
previously observed from the BHT. We discuss the possibility that we detected
X-ray emission associated with a jet from XTE J1752-223.
04/2012;
-
VERITAS Collaboration,
E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
M. Boettcher,
A. Bouvier,
S. M. Bradbury, [......],
M. Vivier,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A. Weinstein,
T. Weisgarber,
D. A. Williams,
B. Zitzer,
P. Fortin,
D. Horan
[show abstract]
[hide abstract]
ABSTRACT: We report on the discovery of high-energy (HE; E > 0.1 GeV) and very
high-energy (VHE; E > 100 GeV) gamma-ray emission from the
high-frequency-peaked BL Lac object RBS 0413. VERITAS, a ground-based gamma-ray
observatory, detected VHE gamma rays from RBS 0413 with a statistical
significance of 5.5 standard deviations (sigma) and a gamma-ray flux of (1.5
\pm 0.6stat \pm 0.7syst) \times 10^(-8) photons m^(-2) s^(-1) (\sim 1% of the
Crab Nebula flux) above 250 GeV. The observed spectrum can be described by a
power law with a photon index of 3.18 \pm 0.68stat \pm 0.30syst.
Contemporaneous observations with the Large Area Telescope (LAT) on the Fermi
Gamma-ray Space Telescope detected HE gamma rays from RBS 0413 with a
statistical significance of more than 9 sigma, a power-law photon index of 1.57
\pm 0.12stat +0.11sys -0.12sys and a gamma-ray flux between 300 MeV and 300 GeV
of (1.64 \pm 0.43stat +0.31sys -0.22sys) \times 10^(-5) photons m^(-2) s^(-1).
We present the results from Fermi-LAT and VERITAS, including a spectral energy
distribution modeling of the gamma-ray, quasi-simultaneous X-ray (Swift-XRT),
ultraviolet (Swift-UVOT) and R-band optical (MDM) data. We find that, if
conditions close to equipartition are required, both the combined synchrotron
self-Compton/external-Compton and the lepto-hadronic models are preferred over
a pure synchrotron self-Compton model.
04/2012;
-
E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
A. Bouvier,
S. M. Bradbury,
J. H. Buckley,
V. Bugaev, [......],
M. Vivier,
R. G. Wagner,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A. Weinstein,
T. Weisgarber,
D. A. Williams,
B. Zitzer,
for the VERITAS Collaboration
[show abstract]
[hide abstract]
ABSTRACT: The VERITAS array of Cherenkov telescopes has carried out a deep
observational program on the nearby dwarf spheroidal galaxy Segue 1. We report
on the results of nearly 48 hours of good quality selected data, taken between
January 2010 and May 2011. No significant $\gamma$-ray emission is detected at
the nominal position of Segue 1, and upper limits on the integrated flux are
derived. According to recent studies, Segue 1 is the most dark matter-dominated
dwarf spheroidal galaxy currently known. We derive stringent bounds on various
annihilating and decaying dark matter particle models. The upper limits on the
velocity-weighted annihilation cross-section are $\mathrm{<\sigma v >^{95% CL}
\lesssim 10^{-23} cm^{3} s^{-1}}$, improving our limits from previous
observations of dwarf spheroidal galaxies by at least a factor of two for dark
matter particle masses $\mathrm{m_{\chi}\gtrsim 300 GeV}$. The lower limits on
the decay lifetime are at the level of $\mathrm{\tau^{95% CL} \gtrsim 10^{24}
s}$. Finally, we address the interpretation of the cosmic ray lepton anomalies
measured by ATIC and PAMELA in terms of dark matter annihilation, and show that
the VERITAS observations of Segue 1 disfavor such a scenario.
02/2012;
-
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
Y. Becherini,
J. Becker, [......],
N. P. Lee,
C. Ly,
J. Madrid,
F. Massaro,
C. G. Mundell,
H. Nagai,
E. S. Perlman,
I. A. Steele,
R. C. Walker,
and D. L. Wood
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ABSTRACT: The giant radio galaxy M 87 with its proximity (16 Mpc), famous jet, and very massive black hole ((3 – 6) × 109 M ☉) provides a unique opportunity to investigate the origin of very high energy (VHE; E > 100 GeV) γ-ray emission generated in relativistic outflows and the surroundings of supermassive black holes. M 87 has been established as a VHE γ-ray emitter since 2006. The VHE γ-ray emission displays strong variability on timescales as short as a day. In this paper, results from a joint VHE monitoring campaign on M 87 by the MAGIC and VERITAS instruments in 2010 are reported. During the campaign, a flare at VHE was detected triggering further observations at VHE (H.E.S.S.), X-rays (Chandra), and radio (43 GHz Very Long Baseline Array, VLBA). The excellent sampling of the VHE γ-ray light curve enables one to derive a precise temporal characterization of the flare: the single, isolated flare is well described by a two-sided exponential function with significantly different flux rise and decay times of τrise d = (1.69 ± 0.30) days and τdecay d = (0.611 ± 0.080) days, respectively. While the overall variability pattern of the 2010 flare appears somewhat different from that of previous VHE flares in 2005 and 2008, they share very similar timescales (~day), peak fluxes (Φ>0.35 TeV (1-3) × 10–11 photons cm–2 s–1), and VHE spectra. VLBA radio observations of 43 GHz of the inner jet regions indicate no enhanced flux in 2010 in contrast to observations in 2008, where an increase of the radio flux of the innermost core regions coincided with a VHE flare. On the other hand, Chandra X-ray observations taken ~3 days after the peak of the VHE γ-ray emission reveal an enhanced flux from the core (flux increased by factor ~2; variability timescale <2 days). The long-term (2001-2010) multi-wavelength (MWL) light curve of M 87, spanning from radio to VHE and including data from Hubble Space Telescope, Liverpool Telescope, Very Large Array, and European VLBI Network, is used to further investigate the origin of the VHE γ-ray emission. No unique, common MWL signature of the three VHE flares has been identified. In the outer kiloparsec jet region, in particular in HST-1, no enhanced MWL activity was detected in 2008 and 2010, disfavoring it as the origin of the VHE flares during these years. Shortly after two of the three flares (2008 and 2010), the X-ray core was observed to be at a higher flux level than its characteristic range (determined from more than 60 monitoring observations: 2002-2009). In 2005, the strong flux dominance of HST-1 could have suppressed the detection of such a feature. Published models for VHE γ-ray emission from M 87 are reviewed in the light of the new data.
The Astrophysical Journal 02/2012; 746(2):151. · 6.02 Impact Factor
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[show abstract]
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ABSTRACT: We present new radio, optical, and X-ray observations of three Ultraluminous
X-ray sources (ULXs) that are associated with large-scale nebulae. We report
the discovery of a radio nebula associated with the ULX IC342 X-1 using the
Very Large Array (VLA). Complementary VLA observations of the nebula around
Holmberg II X-1, and high-frequency Australia Telescope Compact Array (ATCA)
and Very Large Telescope (VLT) spectroscopic observations of NGC5408 X-1 are
also presented. We study the morphology, ionization processes, and the
energetics of the optical/radio nebulae of IC342 X-1, Holmberg II X-1 and
NGC5408 X-1. The energetics of the optical nebula of IC342 X-1 is discussed in
the framework of standard bubble theory. The total energy content of the
optical nebula is 6 x 10^52 erg. The minimum energy needed to supply the
associated radio nebula is 9.2 x 10^50 erg. In addition, we detected an
unresolved radio source at the location of IC342 X-1 at VLA scales. However,
our Very Long Baseline Interferometry (VLBI) observations using the European
VLBI Network likely rule out the presence of any compact radio source at
milli-arcsecond (mas) scales. Using a simultaneous Swift X-ray Telescope
measurement, we estimate an upper limit on the mass of the black hole in IC342
X-1 using the "fundamental plane" of accreting black holes and obtain M_BH <
(1.0\pm0.3) x 10^3 M_Sun. Arguing that the nebula of IC342 X-1 is possibly
inflated by a jet, we estimate accretion rates and efficiencies for the jet of
IC342 X-1 and compare with sources like S26, SS433, IC10 X-1.
01/2012;
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E. Aliu,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
A. Bouvier,
S. M. Bradbury,
J. H. Buckley,
V. Bugaev,
K. Byrum, [......],
V. V. Vassiliev,
S. Vincent,
M. Vivier,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A Weinstein,
T. Weisgarber,
D A Williams,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: VERITAS has been monitoring the very-high-energy (VHE; >100GeV) gamma-ray
activity of the radio galaxy M87 since 2007. During 2008, flaring activity on a
timescale of a few days was observed with a peak flux of (0.70 +- 0.16) X
10^{-11} cm^{-2} s^{-1} at energies above 350GeV. In 2010 April, VERITAS
detected a flare from M87 with peak flux of (2.71 +- 0.68) X 10^{-11} cm^{-2}
s^{-1} for E>350GeV. The source was observed for six consecutive nights during
the flare, resulting in a total of 21 hr of good quality data. The most rapid
flux variation occurred on the trailing edge of the flare with an exponential
flux decay time of 0.90^{+0.22}_{-0.15} days. The shortest detected exponential
rise time is three times as long, at 2.87^{+1.65}_{-0.99} days. The quality of
the data sample is such that spectral analysis can be performed for three
periods: rising flux, peak flux, and falling flux. The spectra obtained are
consistent with power-law forms. The spectral index at the peak of the flare is
equal to 2.19 +- 0.07. There is some indication that the spectrum is softer in
the falling phase of the flare than the peak phase, with a confidence level
corresponding to 3.6 standard deviations. We discuss the implications of these
results for the acceleration and cooling rates of VHE electrons in M87 and the
constraints they provide on the physical size of the emitting region.
12/2011;
