-
M Ackermann,
M Ajello,
A Allafort,
K Asano,
W ~B Atwood,
L Baldini,
J Ballet,
G Barbiellini,
D Bastieri,
K Bechtol, [......],
V Vitale,
A Kienlin,
A ~P Waite,
E Wallace,
P Weltevrede,
B ~L Winer,
K ~S Wood,
M Wood,
Z Yang,
S Zimmer
apj. 03/2013; 765:54.
-
M Ackermann,
M Ajello,
A Allafort,
L Baldini,
J Ballet,
G Barbiellini,
M G Baring,
D Bastieri,
K Bechtol,
R Bellazzini, [......],
G Vianello,
V Vitale,
A P Waite,
M Werner,
B L Winer,
K S Wood,
M Wood,
R Yamazaki,
Z Yang,
S Zimmer
[show abstract]
[hide abstract]
ABSTRACT: Cosmic rays are particles (mostly protons) accelerated to relativistic speeds. Despite wide agreement that supernova remnants (SNRs) are the sources of galactic cosmic rays, unequivocal evidence for the acceleration of protons in these objects is still lacking. When accelerated protons encounter interstellar material, they produce neutral pions, which in turn decay into gamma rays. This offers a compelling way to detect the acceleration sites of protons. The identification of pion-decay gamma rays has been difficult because high-energy electrons also produce gamma rays via bremsstrahlung and inverse Compton scattering. We detected the characteristic pion-decay feature in the gamma-ray spectra of two SNRs, IC 443 and W44, with the Fermi Large Area Telescope. This detection provides direct evidence that cosmic-ray protons are accelerated in SNRs.
Science 02/2013; 339(6121):807-11. · 31.20 Impact Factor
-
H J Pletsch,
L Guillemot,
H Fehrmann,
B Allen,
M Kramer,
C Aulbert,
M Ackermann,
M Ajello,
A de Angelis,
W B Atwood, [......],
J Vandenbroucke,
V Vasileiou,
G Vianello,
V Vitale,
A P Waite,
B L Winer,
K S Wood,
M Wood,
Z Yang,
S Zimmer
[show abstract]
[hide abstract]
ABSTRACT: Millisecond pulsars, old neutron stars spun-up by accreting matter from a companion star, can reach high rotation rates of hundreds of revolutions per second. Until now, all such "recycled" rotation-powered pulsars have been detected by their spin-modulated radio emission. In a computing-intensive blind search of gamma-ray data from the Fermi Large Area Telescope (with partial constraints from optical data), we detected a 2.5-millisecond pulsar, PSR J1311-3430. This unambiguously explains a formerly unidentified gamma-ray source that had been a decade-long enigma, confirming previous conjectures. The pulsar is in a circular orbit with an orbital period of only 93 minutes, the shortest of any spin-powered pulsar binary ever found.
Science 10/2012; · 31.20 Impact Factor
-
A ~A Abdo,
M Ackermann,
M Ajello,
W ~B Atwoo,
L Baldini,
J Ballet,
G Barbiellini,
D Bastieri,
K Bechtol,
R Bellazzini, [......],
V Vasileiou,
G Vianello,
V Vitale,
A ~P Waite,
P Wang,
B ~L Winer,
D ~L Wood,
K ~S Wood,
Z Yang,
S Zimmer
apj. 10/2012; 758:140.
-
M Ackermann,
M Ajello,
A Allafort,
L Baldini,
J Ballet,
G Barbiellini,
D Bastieri,
K Bechtol,
R Bellazzini,
B Berenji, [......],
V Vasileiou,
G Vianello,
V Vitale,
A ~P Waite,
P Wang,
B ~L Winer,
K ~S Wood,
H Yamamoto,
Z Yang,
S Zimmer
apj. 08/2012; 755:22.
-
M. Ackermann,
M. Ajello,
J. Ballet,
G. Barbiellini,
D. Bastieri,
R. Bellazzini,
R. D. Blandford,
E. D. Bloom,
E. Bonamente,
A. W. Borgland, [......],
M. Yoshida,
T. Belloni,
G. Tagliaferri,
E. W. Bonning,
J. Isler,
C. M. Urry,
E. Hoversten,
A. Falcone,
C. Pagani,
M. Stroh
[show abstract]
[hide abstract]
ABSTRACT: The blazar AO 0235+164 (z = 0.94) has been one of the most active objects
observed by Fermi Large Area Telescope (LAT) since its launch in Summer 2008.
In addition to the continuous coverage by Fermi, contemporaneous observations
were carried out from the radio to {\gamma} -ray bands between 2008 September
and 2009 February. In this paper, we summarize the rich multi-wavelength data
collected during the campaign (including F-GAMMA, GASP- WEBT, Kanata, OVRO,
RXTE, SMARTS, Swift, and other instruments), examine the cross-correlation
between the light curves measured in the different energy bands, and interpret
the resulting spectral energy distributions in the context of well-known blazar
emission models. We find that the {\gamma} -ray activity is well correlated
with a series of near-IR/optical flares, accompanied by an increase in the
optical polarization degree. On the other hand, the X-ray light curve shows a
distinct 20 day high state of unusually soft spectrum, which does not match the
extrapolation of the optical/UV synchrotron spectrum. We tentatively interpret
this feature as the bulk Compton emission by cold electrons contained in the
jet, which requires an accretion disk corona with an effective covering factor
of 19% at a distance of 100 Rg . We model the broadband spectra with a leptonic
model with external radiation dominated by the infrared emission from the dusty
torus.
07/2012;
-
M. Ackermann,
M. Ajello,
A. Albert,
L. Baldini,
G. Barbiellini,
K. Bechtol,
R. Bellazzini,
B. Berenji,
R. D. Blandford,
E. D. Bloom, [......],
E. Troja,
J. Vandenbroucke,
V. Vasileiou,
G. Vianello,
V. Vitale,
A. P. Waite,
B. L. Winer,
K. S. Wood,
Z. Yang,
S. Zimmer
[show abstract]
[hide abstract]
ABSTRACT: Dark matter particle annihilation or decay can produce monochromatic gamma-ray lines and contribute to the diffuse gamma-ray background. Flux upper limits are presented for gamma-ray spectral lines from 7 to 200 GeV and for the diffuse gamma-ray background from 4.8 GeV to 264 GeV obtained from two years of Fermi Large Area Telescope data integrated over most of the sky. We give cross-section upper limits and decay lifetime lower limits for dark matter models that produce gamma-ray lines or contribute to the diffuse spectrum, including models proposed as explanations of the PAMELA and Fermi cosmic-ray data.
Phys. Rev. D. 07/2012; 86(2).
-
M. Ackermann,
M. Ajello,
A. Allafort,
E. Antolini,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri,
K. Bechtol,
R. Bellazzini, [......],
J. Vandenbroucke,
V. Vasileiou,
G. Vianello,
V. Vitale,
A. P. Waite,
P. Wang,
B. L. Winer,
K. S. Wood,
Z. Yang,
S. Zimmer
[show abstract]
[hide abstract]
ABSTRACT: We report on the gamma-ray observations of giant molecular clouds Orion A and
B with the Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space
Telescope. The gamma-ray emission in the energy band between \sim100 MeV and
\sim100 GeV is predicted to trace the gas mass distribution in the clouds
through nuclear interactions between the Galactic cosmic rays (CRs) and
interstellar gas. The gamma-ray production cross-section for the nuclear
interaction is known to \sim10% precision which makes the LAT a powerful tool
to measure the gas mass column density distribution of molecular clouds for a
known CR intensity. We present here such distributions for Orion A and B, and
correlate them with those of the velocity integrated CO intensity (WCO) at a
1{\deg} \times1{\deg} pixel level. The correlation is found to be linear over a
WCO range of ~10 fold when divided in 3 regions, suggesting penetration of
nuclear CRs to most of the cloud volumes. The Wco-to-mass conversion factor,
Xco, is found to be \sim2.3\times10^20 cm-2(K km s-1)-1 for the high-longitude
part of Orion A (l > 212{\deg}), \sim1.7 times higher than \sim1.3 \times 10^20
found for the rest of Orion A and B. We interpret the apparent high Xco in the
high-longitude region of Orion A in the light of recent works proposing a
non-linear relation between H2 and CO densities in the diffuse molecular gas.
Wco decreases faster than the H2 column density in the region making the gas
"darker" to Wco.
07/2012;
-
M. Ackermann,
M. Ajello,
A. Allafort,
E. Antolini,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri,
R. Bellazzini,
B. Berenji, [......],
V. Vitale,
A. P. Waite,
E. Wallace,
P. Wang,
B. L. Winer,
M. T. Wolff,
D. L. Wood,
K. S. Wood,
Z. Yang,
and S. Zimmer
[show abstract]
[hide abstract]
ABSTRACT: The Fermi Large Area Telescope (LAT) First Source Catalog (1FGL) provided spatial, spectral, and temporal properties for a large number of γ-ray sources using a uniform analysis method. After correlating with the most-complete catalogs of source types known to emit γ rays, 630 of these sources are "unassociated" (i.e., have no obvious counterparts at other wavelengths). Here, we employ two statistical analyses of the primary γ-ray characteristics for these unassociated sources in an effort to correlate their γ-ray properties with the active galactic nucleus (AGN) and pulsar populations in 1FGL. Based on the correlation results, we classify 221 AGN-like and 134 pulsar-like sources in the 1FGL unassociated sources. The results of these source "classifications" appear to match the expected source distributions, especially at high Galactic latitudes. While useful for planning future multiwavelength follow-up observations, these analyses use limited inputs, and their predictions should not be considered equivalent to "probable source classes" for these sources. We discuss multiwavelength results and catalog cross-correlations to date, and provide new source associations for 229 Fermi-LAT sources that had no association listed in the 1FGL catalog. By validating the source classifications against these new associations, we find that the new association matches the predicted source class in ~80% of the sources.
The Astrophysical Journal 06/2012; 753(1):83. · 6.02 Impact Factor
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The Fermi-LAT collaboration: M. Ackermann,
M. Ajello,
W. B. Atwood,
L. Baldini,
G. Barbiellini,
D. Bastieri,
K. Bechtol,
R. Bellazzini,
R. D. Blandford,
E. D. Bloom, [......],
V. Vasileiou,
G. Vianello,
V. Vitale,
A. P. Waite,
E. Wallace,
K. S. Wood,
M. Wood,
Z. Yang,
G. Zaharijas,
S. Zimmer
[show abstract]
[hide abstract]
ABSTRACT: We have performed an analysis of the diffuse gamma-ray emission with the
Fermi Large Area Telescope in the Milky Way Halo region searching for a signal
from dark matter annihilation or decay. In the absence of a robust dark matter
signal, constraints are presented. We consider both gamma rays produced
directly in the dark matter annihilation/decay and produced by inverse Compton
scattering of the e+e- produced in the annihilation/decay. Conservative limits
are derived requiring that the dark matter signal does not exceed the observed
diffuse gamma-ray emission. A second set of more stringent limits is derived
based on modeling the foreground astrophysical diffuse emission using the
GALPROP code. Uncertainties in the height of the diffusive cosmic-ray halo, the
distribution of the cosmic-ray sources in the Galaxy, the index of the
injection cosmic-ray electron spectrum and the column density of the
interstellar gas are taken into account using a profile likelihood formalism,
while the parameters governing the cosmic-ray propagation have been derived
from fits to local cosmic-ray data. The resulting limits impact the range of
particle masses over which dark matter thermal production in the early Universe
is possible, and challenge the interpretation of the PAMELA/Fermi-LAT cosmic
ray anomalies as annihilation of dark matter.
05/2012;
-
Fermi-LAT Collaboration: M. Ackermann,
M. Ajello,
A. Albert,
L. Baldini,
G. Barbiellini,
K. Bechtol,
R. Bellazzini,
B. Berenji,
R. D. Blandford,
E. D. Bloom, [......],
E. Troja,
J. Vandenbroucke,
V. Vasileiou,
G. Vianello,
V. Vitale,
A. P. Waite,
B. L. Winer,
K. S. Wood,
Z. Yang,
S. Zimmer
[show abstract]
[hide abstract]
ABSTRACT: Dark matter particle annihilation or decay can produce monochromatic
gamma-ray lines and contribute to the diffuse gamma-ray background. Flux upper
limits are presented for gamma-ray spectral lines from 7 to 200 GeV and for the
diffuse gamma-ray background from 4.8 GeV to 264 GeV obtained from two years of
Fermi Large Area Telescope data integrated over most of the sky. We give cross
section upper limits and decay lifetime lower limits for dark matter models
that produce gamma-ray lines or contribute to the diffuse spectrum, including
models proposed as explanations of the PAMELA and Fermi cosmic-ray data.
05/2012;
-
M Ackermann,
M Ajello,
A Albert,
L Baldini,
J Ballet,
G Barbiellini,
D Bastieri,
K Bechtol,
R Bellazzini,
E ~D Bloom, [......],
V Vasileiou,
G Vianello,
V Vitale,
A ~P Waite,
B ~L Winer,
K ~S Wood,
M Wood,
Z Yang,
S Zimmer,
E Komatsu
prd. 05/2012; 85(10):109901.
-
M. Ackermann,
M. Ajello,
W. B. Atwood,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri,
K. Bechtol,
R. Bellazzini,
B. Berenji, [......],
G. Vianello,
V. Vitale,
A. P. Waite,
P. Wang,
B. L. Winer,
K. S. Wood,
M. Wood,
Z. Yang,
M. Ziegler,
and S. Zimmer
[show abstract]
[hide abstract]
ABSTRACT: The γ-ray sky >100 MeV is dominated by the diffuse emissions from interactions of cosmic rays with the interstellar gas and radiation fields of the Milky Way. Observations of these diffuse emissions provide a tool to study cosmic-ray origin and propagation, and the interstellar medium. We present measurements from the first 21 months of the Fermi Large Area Telescope (Fermi-LAT) mission and compare with models of the diffuse γ-ray emission generated using the GALPROP code. The models are fitted to cosmic-ray data and incorporate astrophysical input for the distribution of cosmic-ray sources, interstellar gas, and radiation fields. To assess uncertainties associated with the astrophysical input, a grid of models is created by varying within observational limits the distribution of cosmic-ray sources, the size of the cosmic-ray confinement volume (halo), and the distribution of interstellar gas. An all-sky maximum-likelihood fit is used to determine the X CO factor, the ratio between integrated CO-line intensity and H2 column density, the fluxes and spectra of the γ-ray point sources from the first Fermi-LAT catalog, and the intensity and spectrum of the isotropic background including residual cosmic rays that were misclassified as γ-rays, all of which have some dependency on the assumed diffuse emission model. The models are compared on the basis of their maximum-likelihood ratios as well as spectra, longitude, and latitude profiles. We also provide residual maps for the data following subtraction of the diffuse emission models. The models are consistent with the data at high and intermediate latitudes but underpredict the data in the inner Galaxy for energies above a few GeV. Possible explanations for this discrepancy are discussed, including the contribution by undetected point-source populations and spectral variations of cosmic rays throughout the Galaxy. In the outer Galaxy, we find that the data prefer models with a flatter distribution of cosmic-ray sources, a larger cosmic-ray halo, or greater gas density than is usually assumed. Our results in the outer Galaxy are consistent with other Fermi-LAT studies of this region that used different analysis methods than employed in this paper.
The Astrophysical Journal 04/2012; 750(1):3. · 6.02 Impact Factor
-
M Ackermann,
M Ajello,
A Allafort,
W ~B Atwood,
L Baldini,
G Barbiellini,
D Bastieri,
K Bechtol,
R Bellazzini,
P ~N Bhat, [......],
J Vandenbroucke,
V Vasileiou,
G Vianello,
V Vitale,
A Kienlin,
A ~P Waite,
C Wilson-Hodge,
D ~L Wood,
K ~S Wood,
Z Yang
apj. 04/2012; 748:151.
-
P. L. Nolan,
A. A. Abdo,
M. Ackermann,
M. Ajello,
A. Allafort,
E. Antolini,
W. B. Atwood,
M. Axelsson,
L. Baldini,
J. Ballet, [......],
A. P. Waite,
E. Wallace,
P. Wang,
M. Werner,
B. L. Winer,
D. L. Wood,
K. S. Wood,
M. Wood,
Z. Yang,
and S. Zimmer
[show abstract]
[hide abstract]
ABSTRACT: We present the second catalog of high-energy γ-ray sources detected by the Large Area Telescope (LAT), the primary science instrument on the Fermi Gamma-ray Space Telescope (Fermi), derived from data taken during the first 24 months of the science phase of the mission, which began on 2008 August 4. Source detection is based on the average flux over the 24 month period. The second Fermi-LAT catalog (2FGL) includes source location regions, defined in terms of elliptical fits to the 95% confidence regions and spectral fits in terms of power-law, exponentially cutoff power-law, or log-normal forms. Also included are flux measurements in five energy bands and light curves on monthly intervals for each source. Twelve sources in the catalog are modeled as spatially extended. We provide a detailed comparison of the results from this catalog with those from the first Fermi-LAT catalog (1FGL). Although the diffuse Galactic and isotropic models used in the 2FGL analysis are improved compared to the 1FGL catalog, we attach caution flags to 162 of the sources to indicate possible confusion with residual imperfections in the diffuse model. The 2FGL catalog contains 1873 sources detected and characterized in the 100 MeV to 100 GeV range of which we consider 127 as being firmly identified and 1171 as being reliably associated with counterparts of known or likely γ-ray-producing source classes.
The Astrophysical Journal Supplement Series 03/2012; 199(2):31. · 13.46 Impact Factor
-
Fermi-LAT collaboration,
M. Ajello,
L. Baldini,
G. Barbiellini,
D. Bastieri,
K. Bechtol,
R. Bellazzini,
B. Berenji,
E.D. Bloom,
E. Bonamente, [......],
J. Vandenbroucke,
V. Vasileiou,
G. Vianello,
V. Vitale,
A.P. Waite,
B.L. Winer,
K.S. Wood,
M. Wood,
Z. Yang,
S. Zimmer
[show abstract]
[hide abstract]
ABSTRACT: We present limits for the compactification scale in the theory of Large Extra Dimensions (LED) proposed by Arkani-Hamed, Dimopoulos, and Dvali. We use 11 months of data from the Fermi Large Area Telescope (Fermi-LAT) to set gamma ray flux limits for 6 gamma-ray faint neutron stars (NS). To set limits on LED we use the model of Hannestad and Raffelt (HR) that calculates the Kaluza-Klein (KK) graviton production in supernova cores and the large fraction subsequently gravitationally bound around the resulting NS. The predicted decay of the bound KK gravitons to γγ should contribute to the flux from NSs. Considering 2 to 7 extra dimensions of the same size in the context of the HR model, we use Monte Carlo techniques to calculate the expected differential flux of gamma-rays arising from these KK gravitons, including the effects of the age of the NS, graviton orbit, and absorption of gamma-rays in the magnetosphere of the NS. We compare our Monte Carlo-based differential flux to the experimental differential flux using maximum likelihood techniques to obtain our limits on LED. Our limits are more restrictive than past EGRET-based optimistic limits that do not include these important corrections. Additionally, our limits are more stringent than LHC based limits for 3 or fewer LED, and comparable for 4 LED. We conclude that if the effective Planck scale is around a TeV, then for 2 or 3 LED the compactification topology must be more complicated than a torus.
Journal of Cosmology and Astroparticle Physics 02/2012; 2012(02):012. · 5.72 Impact Factor
-
M. Ackermann,
M. Ajello,
A. Albert,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri,
K. Bechtol,
R. Bellazzini,
E. D. Bloom, [......],
G. Vianello,
V. Vitale,
A. P. Waite,
B. L. Winer,
K. S. Wood,
M. Wood,
Z. Yang,
S. Zimmer,
for the Fermi LAT Collaboration,
E. Komatsu
[show abstract]
[hide abstract]
ABSTRACT: The contribution of unresolved sources to the diffuse gamma-ray background
could induce anisotropies in this emission on small angular scales. We analyze
the angular power spectrum of the diffuse emission measured by the Fermi LAT at
Galactic latitudes |b| > 30 deg in four energy bins spanning 1 to 50 GeV. At
multipoles \ell \ge 155, corresponding to angular scales \lesssim 2 deg,
angular power above the photon noise level is detected at >99.99% CL in the 1-2
GeV, 2-5 GeV, and 5-10 GeV energy bins, and at >99% CL at 10-50 GeV. Within
each energy bin the measured angular power takes approximately the same value
at all multipoles \ell \ge 155, suggesting that it originates from the
contribution of one or more unclustered source populations. The amplitude of
the angular power normalized to the mean intensity in each energy bin is
consistent with a constant value at all energies, C_P/ ^2 = 9.05 +/- 0.84 x
10^{-6} sr, while the energy dependence of C_P is consistent with the
anisotropy arising from one or more source populations with power-law photon
spectra with spectral index \Gamma_s = 2.40 +/- 0.07. We discuss the
implications of the measured angular power for gamma-ray source populations
that may provide a contribution to the diffuse gamma-ray background.
02/2012;
-
Fermi LAT Collaboration,
M Ackermann,
M Ajello,
J Ballet,
G Barbiellini,
D Bastieri,
A Belfiore,
R Bellazzini,
B Berenji,
R D Blandford, [......],
M Wood,
Z Yang,
S Zimmer,
M J Coe,
F Di Mille,
P G Edwards,
M D Filipović,
J L Payne,
J Stevens,
M A P Torres
[show abstract]
[hide abstract]
ABSTRACT: Gamma-ray binaries are stellar systems containing a neutron star or black hole, with gamma-ray emission produced by an interaction between the components. These systems are rare, even though binary evolution models predict dozens in our Galaxy. A search for gamma-ray binaries with the Fermi Large Area Telescope (LAT) shows that 1FGL J1018.6-5856 exhibits intensity and spectral modulation with a 16.6-day period. We identified a variable x-ray counterpart, which shows a sharp maximum coinciding with maximum gamma-ray emission, as well as an O6V((f)) star optical counterpart and a radio counterpart that is also apparently modulated on the orbital period. 1FGL J1018.6-5856 is thus a gamma-ray binary, and its detection suggests the presence of other fainter binaries in the Galaxy.
Science 01/2012; 335(6065):189-93. · 31.20 Impact Factor
-
The Fermi LAT Collaboration,
M. Ackermann,
A. Albert,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri,
K. Bechtol,
R. Bellazzini,
R. D. Blandford, [......],
V. Vasileiou,
G. Vianello,
V. Vitale,
A. P. Waite,
P. Wang,
B. L. Winer,
K. S. Wood,
Z. Yang,
S. Zalewski,
S. Zimmer
[show abstract]
[hide abstract]
ABSTRACT: Numerical simulations based on the Lambda-CDM model of cosmology predict a
large number of as yet unobserved Galactic dark matter satellites. We report
the results of a Large Area Telescope (LAT) search for these satellites via the
gamma-ray emission expected from the annihilation of weakly interacting massive
particle (WIMP) dark matter. Some dark matter satellites are expected to have
hard gamma-ray spectra, finite angular extents, and a lack of counterparts at
other wavelengths. We sought to identify LAT sources with these
characteristics, focusing on gamma-ray spectra consistent with WIMP
annihilation through the $b \bar b$ channel. We found no viable dark matter
satellite candidates using one year of data, and we present a framework for
interpreting this result in the context of numerical simulations to constrain
the velocity-averaged annihilation cross section for a conventional 100 GeV
WIMP annihilating through the $b \bar b$ channel.
01/2012;
-
M Ackermann,
M Ajello,
A Allafort,
W B Atwood,
L Baldini,
G Barbiellini,
D Bastieri,
K Bechtol,
R Bellazzini,
B Berenji, [......],
J Vandenbroucke,
V Vasileiou,
G Vianello,
V Vitale,
A P Waite,
B L Winer,
K S Wood,
M Wood,
Z Yang,
S Zimmer
[show abstract]
[hide abstract]
ABSTRACT: We measured separate cosmic-ray electron and positron spectra with the Fermi Large Area Telescope. Because the instrument does not have an onboard magnet, we distinguish the two species by exploiting Earth's shadow, which is offset in opposite directions for opposite charges due to Earth's magnetic field. We estimate and subtract the cosmic-ray proton background using two different methods that produce consistent results. We report the electron-only spectrum, the positron-only spectrum, and the positron fraction between 20 and 200 GeV. We confirm that the fraction rises with energy in the 20-100 GeV range. The three new spectral points between 100 and 200 GeV are consistent with a fraction that is continuing to rise with energy.
Physical Review Letters 01/2012; 108(1):011103. · 7.37 Impact Factor
