20,846 research outputs found
Gamma-ray emission from the globular clusters Liller 1, M80, NGC 6139, NGC 6541, NGC 6624, and NGC 6752
Globular clusters (GCs) are emerging as a new class of gamma-ray emitters,
thanks to the data obtained from the Fermi Gamma-ray Space Telescope. By now,
eight GCs are known to emit gamma-rays at energies >100~MeV. Based on the
stellar encounter rate of the GCs, we identify potential gamma-ray emitting GCs
out of all known GCs that have not been studied in details before. In this
paper, we report the discovery of a number of new gamma-ray GCs: Liller 1, NGC
6624, and NGC 6752, and evidence for gamma-ray emission from M80, NGC 6139, and
NGC 6541, in which gamma-rays were found within the GC tidal radius. With one
of the highest metallicity among all GCs in the Milky Way, the gamma-ray
luminosity of Liller 1 is found to be the highest of all known gamma-ray GCs.
In addition, we confirm a previous report of significant gamma-ray emitting
region next to NGC 6441. We briefly discuss the observed offset of gamma-rays
from some GC cores. The increasing number of known gamma-ray GCs at distances
out to ~10 kpc is important for us to understand the gamma-ray emitting
mechanism and provides an alternative probe to the underlying millisecond
pulsar populations of the GCs.Comment: 22 pages, 7 figures, 2 tables; ApJ, in pres
Recommended from our members
Research on VCSEL interference analysis and elimination method
Laser methane gas sensors have been increasingly accepted in coal mine safety monitoring. Most laser spectroscopic methane gas sensors are based in BFB lasers at around 1650nm. However, they suffer from high power consumption and high cost due to temperature control is required for laser diode operation at constant temperature. VCSEL lasers have offered low operation current and low power consumption when operating at non-TEC mode. However, it is found that the interference noise is critical for laser methane detection. This paper report typical results of the laser diode ripple characterization method and methods of noise reduction methods are discussed
A NuSTAR Observation of the Gamma-ray Emitting Millisecond Pulsar PSR J1723-2837
We report on the first NuSTAR observation of the gamma-ray emitting
millisecond pulsar binary PSR J1723-2837. X-ray radiation up to 79 keV is
clearly detected and the simultaneous NuSTAR and Swift spectrum is well
described by an absorbed power-law with a photon index of ~1.3. We also find
X-ray modulations in the 3-10 keV, 10-20 keV, 20-79 keV, and 3-79 keV bands at
the 14.8-hr binary orbital period. All these are entirely consistent with
previous X-ray observations below 10 keV. This new hard X-ray observation of
PSR J1723-2837 provides strong evidence that the X-rays are from the
intrabinary shock via an interaction between the pulsar wind and the outflow
from the companion star. We discuss how the NuSTAR observation constrains the
physical parameters of the intrabinary shock model.Comment: Accepted for publication in ApJ. 5 pages, 3 figure
The Discovery of an X-ray/UV Stellar Flare from the Late-K/Early-M Dwarf LMC 335
We report the discovery of an X-ray/UV stellar flare from the source LMC 335,
captured by XMM-Newton in the field of the Large Magellanic Cloud. The flare
event was recorded continuously in X-ray for its first 10 hours from the
precursor to the late decay phases. The observed fluxes increased by more than
two orders of magnitude at its peak in X-ray and at least one in the UV as
compared to quiescence. The peak 0.1-7.0 keV X-ray flux is derived from the
two-temperature APEC model to be ~(8.4 +/- 0.6) x 10^-12 erg cm-2 s-1.
Combining astrometric information from multiple X-ray observations in the
quiescent and flare states, we identify the NIR counterpart of LMC 335 as the
2MASS source J05414534-6921512. The NIR color relations and spectroscopic
parallax characterize the source as a Galactic K7-M4 dwarf at a foreground
distance of (100 - 264) pc, implying a total energy output of the entire event
of ~(0.4 - 2.9) x 10^35 erg. This report comprises detailed analyses of this
late-K / early-M dwarf flare event that has the longest time coverage yet
reported in the literature. The flare decay can be modeled with two exponential
components with timescales of ~28 min and ~4 hours, with a single component
decay firmly ruled out. The X-ray spectra during flare can be described by two
components, a dominant high temperature component of ~40-60MK and a low
temperature component of ~10MK, with a flare loop length of about 1.1-1.3
stellar radius.Comment: 35 pages, 6 figures, 5 tables, accepted for publication in Ap
Swift, XMM-Newton, and NuSTAR observations of PSR J2032+4127/MT91 213
We report our recent Swift, NuSTAR, and XMM-Newton X-ray and Lijiang optical
observations on PSR J2032+4127/MT91 213, the gamma-ray binary candidate with a
period of 45-50 years. The coming periastron of the system was predicted to be
in November 2017, around which high-energy flares from keV to TeV are expected.
Recent studies with Chandra and Swift X-ray observations taken in 2015/16
showed that its X-ray emission has been brighter by a factors of ~10 than that
before 2013, probably revealing some on-going activities between the pulsar
wind and the stellar wind. Our new Swift/XRT lightcurve shows no strong
evidence of a single vigorous brightening trend, but rather several strong
X-ray flares on weekly to monthly timescales with a slowly brightening
baseline, namely the low state. The NuSTAR and XMM-Newton observations taken
during the flaring and the low states, respectively, show a denser environment
and a softer power-law index during the flaring state, implying that the pulsar
wind interacted with stronger stellar winds of the companion to produce the
flares. These precursors would be crucial in studying the predicted giant
outburst from this extreme gamma-ray binary during the periastron passage in
late 2017.Comment: 6 pages, including 3 figures and 2 tables. Accepted for publication
in Ap
Discovery of {\gamma}-ray pulsation and X-ray emission from the black widow pulsar PSR J2051-0827
We report the discovery of pulsed {\gamma}-ray emission and X-ray emission
from the black widow millisecond pulsar PSR J2051-0827 by using the data from
the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope and
the Advanced CCD Imaging Spectrometer array (ACIS-S) on the Chandra X-ray
Observatory. Using 3 years of LAT data, PSR J2051-0827 is clearly detected in
{\gamma}-ray with a signicance of \sim 8{\sigma} in the 0.2 - 20 GeV band. The
200 MeV - 20 GeV {\gamma}-ray spectrum of PSR J2051-0827 can be modeled by a
simple power- law with a photon index of 2.46 \pm 0.15. Significant (\sim
5{\sigma}) {\gamma}-ray pulsations at the radio period were detected. PSR
J2051-0827 was also detected in soft (0.3-7 keV) X-ray with Chandra. By
comparing the observed {\gamma}-rays and X-rays with theoretical models, we
suggest that the {\gamma}-ray emission is from the outer gap while the X-rays
can be from intra-binary shock and pulsar magnetospheric synchrotron emissions.Comment: 10 pages, 4 figures, accepted by ApJ on Jan 28, 201
Radiative transfer theory for polarimetric remote sensing of pine forest
The radiative transfer theory is applied to interpret polarimetric radar backscatter from pine forest with clustered vegetation structures. To take into account the clustered structures with the radiative transfer theory, the scattering function of each cluster is calculated by incorporating the phase interference of scattered fields from each component. Subsequently, the resulting phase matrix is used in the radiative transfer equations to evaluate the polarimetric backscattering coefficients from random medium layers embedded with vegetation clusters. Upon including the multi-scale structures, namely, trunks, primary and secondary branches, as well as needles, we interpret and simulate the polarimetric radar responses from pine forest for different frequencies and looking angles. The preliminary results are shown to be in good agreement with the measured backscattering coefficients at the Landes maritime pine forest during the MAESTRO-1 experiment
Discovery of gamma-ray emission from the supernova remnant Kes 17 with Fermi Large Area Telescope
We report the discovery of GeV emission at the position of supernova remnant
Kes 17 by using the data from the Large Area Telescope on board the Fermi
Gamma-ray Space Telescope. Kes 17 can be clearly detected with a significance
of ~12 sigma in the 1 - 20 GeV range. Moreover, a number of gamma-ray sources
were detected in its vicinity. The gamma-ray spectrum of Kes 17 can be well
described by a simple power-law with a photon index of ~ 2.4. Together with the
multi-wavelength evidence for its interactions with the nearby molecular cloud,
the gamma-ray detection suggests that Kes 17 is a candidate acceleration site
for cosmic-rays.Comment: 13 pages, 3 figures, 1 table, accepted for publication in ApJ Lette
- …