18 research outputs found
Inverse Compton emission from millisecond pulsars in the Galactic bulge
Analyses of Fermi Gamma-Ray Space Telescope data have revealed a source of
excess diffuse gamma rays towards the Galactic center that extends up to
roughly 20 degrees in latitude. The leading theory postulates that this
GeV excess is the aggregate emission from a large number of faint millisecond
pulsars (MSPs). The electrons and positrons () injected by this
population could produce detectable inverse-Compton (IC) emissions by
up-scattering ambient photons to gamma-ray energies. In this work, we calculate
such IC emissions using GALPROP. A triaxial three-dimensional model of the
bulge stars obtained from a fit to infrared data is used as a tracer of the
putative MSP population. This model is compared against one in which the MSPs
are spatially distributed as a Navarro-Frenk-White squared profile. We show
that the resulting spectra for both models are indistinguishable, but that
their spatial morphologies have salient recognizable features. The IC component
above TeV energies carries information on the spatial morphology of the
injected . Such differences could potentially be used by future
high-energy gamma-ray detectors such as the Cherenkov Telescope Array to
provide a viable multiwavelength handle for the MSP origin of the GeV excess.Comment: 13 pages, 8 figures, 3 tables. Match the version published in PR
Evidence for a high-energy tail in the gamma-ray spectra of globular clusters
Millisecond pulsars are very likely the main source of gamma-ray emission
from globular clusters. However, the relative contributions of two separate
emission processes--curvature radiation from millisecond pulsar magnetospheres
vs. inverse Compton emission from relativistic pairs launched into the globular
cluster environment by millisecond pulsars--have long been unclear. To address
this, we search for evidence of inverse Compton emission in 8-year
-LAT data from the directions of 157 Milky Way globular
clusters. We find a mildly statistically significant (3.8) correlation
between the measured globular cluster gamma-ray luminosities and their photon
field energy densities. However, this may also be explained by a hidden
correlation between the photon field densities and the stellar encounter rates
of globular clusters. Analysed , we demonstrate that the
gamma-ray emission of globular clusters can be resolved spectrally into two
components: i) an exponentially cut-off power law and ii) a pure power law. The
latter component--which we uncover at a significance of 8.2--has a
power index of 2.79 0.25. It is most naturally interpreted as inverse
Compton emission by cosmic-ray electrons and positrons injected by millisecond
pulsars. We find the luminosity of this power-law component is comparable to,
or slightly smaller than, the luminosity of the curved component, suggesting
the fraction of millisecond pulsar spin-down luminosity into relativistic
leptons is similar to the fraction of the spin-down luminosity into prompt
magnetospheric radiation.Comment: 16+9 pages, 13+4 figures, 4+1 tables. match the published version in
MNRA
External Inverse-Compton and Proton Synchrotron Emission from the Reverse Shock as the Origin of VHE Gamma-Rays from the Hyper-Bright GRB 221009A
The detection of the hyper-bright gamma-ray burst (GRB) 221009A enables us to
explore the nature of GRB emission and the origin of very-high-energy (VHE)
gamma-rays. We analyze the -LAT data and investigate GeV-TeV
emission in the framework of the external reverse shock model. We show that
early GeV emission can be explained by the external inverse-Compton
mechanism via upscattering MeV gamma-rays by electrons accelerated at the
reverse shock, in addition to the synchrotron self-Compton component. The
predicted early optical flux could have been brighter than the naked-eye GRB
080319B. We also show that proton synchrotron emission from accelerated
ultra-high-energy cosmic rays (UHECRs) is detectable, and could potentially
explain photons detected by LHAASO or UHECR acceleration can
be constrained. Our model suggests that the detection of
photons with energy up to TeV is possible for
reasonable models of the extragalactic background light without invoking new
physics, and predicts anti-correlations between MeV photons and TeV photons,
which can be tested with the LHAASO data.Comment: 9 pages, 4 figures, accepted for publication in ApJL. Results and
figures are updated, and the main conclusions remain unchange
Cherenkov Telescope Array sensitivity to the putative millisecond pulsar population responsible for the Galactic center excess
The leading explanation of the Galactic center -ray
excess is the extended emission from a unresolved population of millisecond
pulsars (MSPs) in the Galactic bulge. Such a population would, along with the
prompt rays, also inject large quantities of electrons/positrons
() into the interstellar medium. These could potentially
inverse-Compton (IC) scatter ambient photons into rays that fall
within the sensitivity range of the upcoming Cherenkov Telescope Array (CTA).
In this article, we examine the detection potential of CTA to this signature by
making a realistic estimation of the systematic uncertainties on the Galactic
diffuse emission model at TeV-scale -ray energies. We forecast that, in
the event that injection spectra are harder than , CTA has the
potential to robustly discover the IC signature of a putative Galactic bulge
MSP population sufficient to explain the GCE for injection efficiencies
in the range , or higher, depending on the level of
mismodeling of the Galactic diffuse emission components. On the other hand, for
spectra softer than , a reliable CTA detection would require an
unphysically large injection efficiency of . However,
even this pessimistic conclusion may be avoided in the plausible event that MSP
observational and/or modeling uncertainties can be reduced. We further find
that, in the event that an IC signal were detected, CTA can successfully
discriminate between an MSP and a dark matter origin for the radiating .Comment: 21+6 pages, 12+5 figures. Updated to match version soon to be
published by MNRA
Twin Sterile Neutrino Dark Matter
We propose that the dark matter of our universe could be sterile neutrinos
which reside within the twin sector of a mirror twin Higgs model. In our
scenario, these particles are produced through a version of the Dodelson-Widrow
mechanism that takes place entirely within the twin sector, yielding a dark
matter candidate that is consistent with X-ray and gamma-ray line constraints.
Furthermore, this scenario can naturally avoid the cosmological problems that
are typically encountered in mirror twin Higgs models. In particular, if the
sterile neutrinos in the Standard Model sector decay out of equilibrium, they
can heat the Standard Model bath and reduce the contributions of the twin
particles to . Such decays also reduce the effective
temperature of the dark matter, thereby relaxing constraints from large-scale
structure. The sterile neutrinos included in this model are compatible with the
seesaw mechanism for generating Standard Model neutrino masses
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Twin sterile neutrino dark matter
We propose that the dark matter of our Universe could be sterile neutrinos which reside within the twin sector of a mirror twin Higgs model. In our scenario, these particles are produced through a version of the Dodelson-Widrow mechanism that takes place entirely within the twin sector, yielding a dark matter candidate that is consistent with x-ray and gamma-ray line constraints. Furthermore, this scenario can naturally avoid the cosmological problems that are typically encountered in mirror twin Higgs models. In particular, if the sterile neutrinos in the Standard Model sector decay out of equilibrium, they can heat the Standard Model bath and reduce the contributions of the twin particles to Neff. Such decays also reduce the effective temperature of the dark matter, thereby relaxing constraints from large-scale structure. The sterile neutrinos included in this model are compatible with the seesaw mechanism for generating Standard Model neutrino masses
Accurate MRCI calculations of the low-lying electronic states of the NCl molecule
Characterization of 22 electronic states of NCl correlating to the two lowest dissociation channels are carried out using high level CASSCF/MRCI calculations with a sextuple-ζ basis set including Davidson modification, core-valence correlation correction and scalar relativistic effects. As far as we know, this radical has never been the preference of theoretical researchers and experimental investigations only concern the ground state and the two low-lying metastable states. Accurate potential energy curves, dissociation energies as well as the equilibrium constants are determined and avoided crossings between the Πsymmetry are studied. Moreover, spin-orbit splitting of several states and transition probabilities and radiative lifetimes for some allowed or forbidden transitions are presented
External Inverse-Compton and Proton Synchrotron Emission from the Reverse Shock as the Origin of VHE Gamma-Rays from the Hyper-Bright GRB 221009A
The detection of the hyper-bright gamma-ray burst (GRB) 221009A enables us to explore the nature of GRB emission and the origin of very-high-energy (VHE) gamma-rays. We analyze the -LAT data and investigate GeV-TeV emission in the framework of the external reverse shock model. We show that early GeV emission can be explained by the external inverse-Compton mechanism via upscattering MeV gamma-rays by electrons accelerated at the reverse shock, in addition to the synchrotron self-Compton component. The predicted early optical flux could have been brighter than the naked-eye GRB 080319B. We also show that proton synchrotron emission from accelerated ultra-high-energy cosmic rays (UHECRs) is detectable, and could potentially explain photons detected by LHAASO or UHECR acceleration can be constrained. Our model suggests that the detection of photons with energy up to TeV is possible for reasonable models of the extragalactic background light without invoking new physics, and predicts anti-correlations between MeV photons and TeV photons, which can be tested with the LHAASO data
Spatio-temporal patterns of Ulva prolifera blooms and the corresponding influence on chlorophyll-a concentration in the Southern Yellow Sea, China
The world's largest macroalgal blooms (MABs) caused by the Mai prolifera outbreaks have occurred every summer since 2007 in the Southern Yellow Sea, China. Accumulating evidence showed that MABs may deteriorate the regional marine environment and influence the growth of some primary producers such as phytoplankton. In this study, we investigated the spatio-temporal patterns of U. prolifera green tides and chlorophyll-a concentration in the Southern Yellow Sea in 2015 using satellite images obtained from HI-1 CCD, MODIS, and GOCI. The correlation between the distributions of U. prolifera abundance and chlorophyll-a concentration was analyzed quantitatively by setting up a series of 5 x 5 km experimental grids, and we also discussed the possible mechanisms about the influence of U. prolifera blooms on the other floating microalgae. The results showed that the development of U. prolifera blooms in the Southern Yellow Sea in 2015 could be featured as "appearance - development - outbreak - decline - disappearance", while the concentration of chlorophyll-a showed "increase - sharp decline - slow recovery - stabilization" from April to August. We also found that the concentration of chlorophyll-a had the following relationships with U. proliferu temporally: (1) the concentration of chlorophyll-a increased with the growth of U. prolifera from April to mid-May; (2) the chlorophyll-a concentration decreased sharply with the dramatically increased coverage of U. prolifera in June; and (3) the chlorophyll-a concentration slowly recovered and finally stabilized as U. prolifera decreased in July. Generally, there was a negative correlation between the occurrence of U. prolifera and chlorophyll-a concentration in the Southern Yellow Sea, China. Our results showed that the outbreak of U. prolifera does have a certain impact on the growth and reproduction of planktonic microalgae, and it suggests that U. pro lifera blooms have potentially altered the ecological balance in the coastal waters of the Southern Yellow Sea. (C) 2018 Elsevier BM. All rights reserved