75 research outputs found
The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
The Boston University-Five College Radio Astronomy Observatory Galactic Ring
Survey is a new survey of Galactic 13CO (1-0) emission. The survey used the
SEQUOIA multi pixel array on the Five College Radio Astronomy Observatory 14 m
telescope to cover a longitude range of l = 18 deg-55.7 deg and a latitude
range of |b| < 1 deg, a total of 75.4 square degrees. Using both
position-switching and On-The-Fly mapping modes, we achieved an angular
sampling of 22 arcsec, better than half of the telescope's 46 arcsec angular
resolution. The survey's velocity coverage is -5 to 135 km/s for Galactic
longitudes l 40 deg. At
the velocity resolution of 0.21 km/s, the typical rms sensitivity is
sigma(TA*)~0.13 K. The survey comprises a total of 1,993,522 spectra. We show
integrated intensity images (zeroth moment maps), channel maps,
position-velocity diagrams, and an average spectrum of the completed survey
dataset. We also discuss the telescope and instrumental parameters, the
observing modes, the data reduction processes, and the emission and noise
characteristics of the dataset. The Galactic Ring Survey data are available to
the community at www.bu.edu/galacticring or in DVD form by request.Comment: Accepted ApJ. 21 pages (ApJ emulate style). 12 figure
Constraining gamma-ray pulsar gap models with a simulated pulsar population
With the large sample of young gamma-ray pulsars discovered by the Fermi
Large Area Telescope (LAT), population synthesis has become a powerful tool for
comparing their collective properties with model predictions. We synthesised a
pulsar population based on a radio emission model and four gamma-ray gap models
(Polar Cap, Slot Gap, Outer Gap, and One Pole Caustic) normalizing to the
number of detected radio pulsars in select group of surveys. The luminosity and
the wide beams from the outer gaps can easily account for the number of Fermi
detections in 2 years of observations. The wide slot-gap beams requires an
increase by a factor of ~10 of the predicted luminosity to produce a reasonable
number of gamma-ray pulsars. Such large increases in the luminosity may be
accommodated by implementing offset polar caps. The narrow polar-cap beams
contribute at most only a handful of LAT pulsars. Standard distributions in
birth location and pulsar spin-down power (Edot) fail to reproduce the LAT
findings: all models under-predict the number of LAT pulsars with high Edot,
and they cannot explain the high probability of detecting both the radio and
gamma-ray beams at high Edot. The beaming factor remains close to 1 over 4
decades in Edot evolution for the slot gap whereas it significantly decreases
with increasing age for the outer gaps. The evolution of the slot-gap
luminosity with Edot is compatible with the large dispersion of gamma-ray
luminosity seen in the LAT data. The stronger evolution predicted for the outer
gap, which is linked to the polar cap heating by the return current, is
apparently not supported by the LAT data. The LAT sample of gamma-ray pulsars
therefore provides a fresh perspective on the early evolution of the luminosity
and beam width of the gamma-ray emission from young pulsars, calling for thin
and more luminous gaps.Comment: 23 pages, 21 figures, accepted for publication in A&
Eight gamma-ray pulsars discovered in blind frequency searches of Fermi LAT data
We report the discovery of eight gamma-ray pulsars in blind frequency
searches using the LAT, onboard the Fermi Gamma-ray Space Telescope. Five of
the eight pulsars are young (tau_c10^36 erg/s), and
located within the Galactic plane (|b|<3 deg). The remaining three are older,
less energetic, and located off the plane. Five pulsars are associated with
sources included in the LAT bright gamma-ray source list, but only one, PSR
J1413-6205, is clearly associated with an EGRET source. PSR J1023-5746 has the
smallest characteristic age (tau_c=4.6 kyr) and is the most energetic
(Edot=1.1E37 erg/s) of all gamma-ray pulsars discovered so far in blind
searches. PSRs J1957+5033 and J2055+25 have the largest characteristic ages
(tau_c~1 Myr) and are the least energetic (Edot~5E33 erg/s) of the
newly-discovered pulsars. We present the timing models, light curves, and
detailed spectral parameters of the new pulsars. We used recent XMM
observations to identify the counterpart of PSR J2055+25 as XMMU
J205549.4+253959. In addition, publicly available archival Chandra X-ray data
allowed us to identify the likely counterpart of PSR J1023-5746 as a faint,
highly absorbed source, CXOU J102302.8-574606. The large X-ray absorption
indicates that this could be among the most distant gamma-ray pulsars detected
so far. PSR J1023-5746 is positionally coincident with the TeV source HESS
J1023-575, located near the young stellar cluster Westerlund 2, while PSR
J1954+2836 is coincident with a 4.3 sigma excess reported by Milagro at a
median energy of 35 TeV. Deep radio follow-up observations of the eight pulsars
resulted in no detections of pulsations and upper limits comparable to the
faintest known radio pulsars, indicating that these can be included among the
growing population of radio-quiet pulsars in our Galaxy being uncovered by the
LAT, and currently numbering more than 20.Comment: Submitted to Ap
Detection of 16 Gamma-Ray Pulsars Through Blind Frequency Searches Using the Fermi LAT
Pulsars are rapidly-rotating, highly-magnetized neutron stars emitting
radiation across the electromagnetic spectrum. Although there are more than
1800 known radio pulsars, until recently, only seven were observed to pulse in
gamma rays and these were all discovered at other wavelengths. The Fermi Large
Area Telescope makes it possible to pinpoint neutron stars through their
gamma-ray pulsations. We report the detection of 16 gamma-ray pulsars in blind
frequency searches using the LAT. Most of these pulsars are coincident with
previously unidentified gamma-ray sources, and many are associated with
supernova remnants. Direct detection of gamma-ray pulsars enables studies of
emission mechanisms, population statistics and the energetics of pulsar wind
nebulae and supernova remnants.Comment: Corresponding authors: Michael Dormody, Paul S. Ray, Pablo M. Saz
Parkinson, Marcus Ziegle
Search for gamma-ray emission from magnetars with the Fermi Large Area Telescope
We report on the search for 0.1-10 GeV emission from magnetars in 17 months
of Fermi Large Area Telescope (LAT) observations. No significant evidence for
gamma-ray emission from any of the currently-known magnetars is found. The most
stringent upper limits to date on their persistent emission in the Fermi-LAT
energy range are estimated between ~10^{-12}-10^{-10} erg/s/cm2, depending on
the source. We also searched for gamma-ray pulsations and possible outbursts,
also with no significant detection. The upper limits derived support the
presence of a cut-off at an energy below a few MeV in the persistent emission
of magnetars. They also show the likely need for a revision of current models
of outer gap emission from strongly magnetized pulsars, which, in some
realizations, predict detectable GeV emission from magnetars at flux levels
exceeding the upper limits identified here using the Fermi-LAT observations.Comment: ApJ Letters in press; Corresponding authors: Caliandro G. A., Hadasch
D., Rea N., Burnett
Fermi Large Area Telescope observations of the Vela-X Pulsar Wind Nebula
We report on gamma-ray observations in the off-pulse window of the Vela
pulsar PSR B0833-45, using 11 months of survey data from the Fermi Large Area
Telescope (LAT). This pulsar is located in the 8 degree diameter Vela supernova
remnant, which contains several regions of non-thermal emission detected in the
radio, X-ray and gamma-ray bands. The gamma-ray emission detected by the LAT
lies within one of these regions, the 2*3 degrees area south of the pulsar
known as Vela-X. The LAT flux is signicantly spatially extended with a best-fit
radius of 0.88 +/- 0.12 degrees for an assumed radially symmetric uniform disk.
The 200 MeV to 20 GeV LAT spectrum of this source is well described by a
power-law with a spectral index of 2.41 +/- 0.09 +/- 0.15 and integral flux
above 100 MeV of (4.73 +/- 0.63 +/- 1.32) * 10^{-7} cm^{-2} s^{-1}. The first
errors represent the statistical error on the fit parameters, while the second
ones are the systematic uncertainties. Detailed morphological and spectral
analyses give strong constraints on the energetics and magnetic field of the
pulsar wind nebula (PWN) system and favor a scenario with two distinct electron
populations.Comment: 21 pages, 5 figures, accepted for publication in Astrophysical
Journa
Fermi Large Area Telescope Measurements of the Diffuse Gamma-Ray Emission at Intermediate Galactic Latitudes
The diffuse Galactic gamma-ray emission is produced by cosmic rays (CRs)
interacting with the interstellar gas and radiation field. Measurements by the
Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton
Gamma-Ray Observatory indicated excess gamma-ray emission > 1 GeV relative to
diffuse Galactic gamma-ray emission models consistent with directly measured CR
spectra (the so-called ``EGRET GeV excess''). The excess emission was observed
in all directions on the sky, and a variety of explanations have been proposed,
including beyond-the-Standard-Model scenarios like annihilating or decaying
dark matter. The Large Area Telescope (LAT) instrument on the Fermi Gamma-ray
Space Telescope has measured the diffuse gamma-ray emission with improved
sensitivity and resolution compared to EGRET. We report on LAT measurements of
the diffuse gamma-ray emission for energies 100 MeV to 10 GeV and Galactic
latitudes 10 deg. <= |b| <= 20 deg. The LAT spectrum for this region of the sky
is well reproduced by a diffuse Galactic gamma-ray emission model that is
consistent with local CR spectra and inconsistent with the EGRET GeV excess.Comment: 2 figures, 1 table, accepted by Physical Review Letters, available
online Dec. 18th, 200
Fermi LAT Observations of LS I +61 303: First detection of an orbital modulation in GeV Gamma Rays
This Letter presents the first results from the observations of LSI +61 303
using Large Area Telescope data from the Fermi Gamma-Ray Space Telescope
between 2008 August and 2009 March. Our results indicate variability that is
consistent with the binary period, with the emission being modulated at 26.6
+/- 0.5 days. This constitutes the first detection of orbital periodicity in
high-energy gamma rays (20 MeV-100 GeV, HE). The light curve is characterized
by a broad peak after periastron, as well as a smaller peak just before
apastron. The spectrum is best represented by a power law with an exponential
cutoff, yielding an overall flux above 100 MeV of 0.82 +/- 0.03(stat) +/-
0.07(syst) 10^{-6} ph cm^{-2} s^{-1}, with a cutoff at 6.3 +/- 1.1(stat) +/-
0.4(syst) GeV and photon index Gamma = 2.21 +/- 0.04(stat) +/- 0.06(syst).
There is no significant spectral change with orbital phase. The phase of
maximum emission, close to periastron, hints at inverse Compton scattering as
the main radiation mechanism. However, previous very high-energy gamma ray
(>100 GeV, VHE) observations by MAGIC and VERITAS show peak emission close to
apastron. This and the energy cutoff seen with Fermi suggest the link between
HE and VHE gamma rays is nontrivial.Comment: 7 pages, 5 figures, accepted for publication in ApJ Letters 21 July
200
Fermi LAT observations of the Geminga pulsar
We report on the \textit{Fermi}-LAT observations of the Geminga pulsar, the
second brightest non-variable GeV source in the -ray sky and the first
example of a radio-quiet -ray pulsar. The observations cover one year,
from the launch of the satellite through 2009 June 15. A data sample of
over 60,000 photons enabled us to build a timing solution based solely on
rays. Timing analysis shows two prominent peaks, separated by = 0.497 0.004 in phase, which narrow with increasing energy. Pulsed
rays are observed beyond 18 GeV, precluding emission below 2.7 stellar
radii because of magnetic absorption. The phase-averaged spectrum was fitted
with a power law with exponential cut-off of spectral index = (1.30
0.01 0.04), cut-off energy = (2.46 0.04 0.17)
GeV and an integral photon flux above 0.1 GeV of (4.14 0.02 0.32)
10 cm s. The first uncertainties are statistical
and the second are systematic. The phase-resolved spectroscopy shows a clear
evolution of the spectral parameters, with the spectral index reaching a
minimum value just before the leading peak and the cut-off energy having maxima
around the peaks. Phase-resolved spectroscopy reveals that pulsar emission is
present at all rotational phases. The spectral shape, broad pulse profile, and
maximum photon energy favor the outer magnetospheric emission scenarios.Comment: 32 pages, 12 figures, 3 tables. Accepted for publication in The
Astrophysical Journal. Corresponding authors: Denis Dumora
([email protected]), Fabio Gargano ([email protected]),
Massimiliano Razzano ([email protected]
- …