44 research outputs found
Probing the Intergalactic Magnetic Field with the Anisotropy of the Extragalactic Gamma-ray Background
The intergalactic magnetic field (IGMF) may leave an imprint on the angular
anisotropy of the extragalactic gamma-ray background through its effect on
electromagnetic cascades triggered by interactions between very high energy
photons and the extragalactic background light. A strong IGMF will deflect
secondary particles produced in these cascades and will thus tend to isotropize
lower energy cascade photons, thereby inducing a modulation in the anisotropy
energy spectrum of the gamma-ray background. Here we present a simple,
proof-of-concept calculation of the magnitude of this effect and demonstrate
that current Fermi data already seem to prefer non-negligible IGMF values. The
anisotropy energy spectrum of the Fermi gamma-ray background could thus be used
as a probe of the IGMF strength.Comment: 11 pages, 4 figures, MN LaTeX style; accepted for publication in
MNRA
The Spectral Index Distribution of EGRET Blazars: Prospects for GLAST
The intrinsic distribution of spectral indices in GeV energies of
gamma-ray--loud blazars is a critical input in determining the spectral shape
of the unresolved blazar contribution to the diffuse extragalactic gamma-ray
background, as well as an important test of blazar emission theories. We
present a maximum-likelihood method of determining the intrinsic spectral index
distribution (ISID) of a population of gamma-ray emitters which accounts for
error in measurement of individual spectral indices, and we apply it to EGRET
blazars. We find that the most likely Gaussian ISID for EGRET blazars has a
mean of 2.27 and a standard deviation of 0.20. We additionally find some
indication that FSRQs and BL Lacs may have different ISIDs (with BL Lacs being
harder). We also test for spectral index hardening associated with blazar
variability for which we find no evidence. Finally, we produce simulated GLAST
spectral index datasets and perform the same analyses. With improved statistics
due to the much larger number of resolvable blazars, GLAST data will help us
determine the ISIDs with much improved accuracy. Should any difference exist
between the ISIDs of BL Lacs and FSRQs or between the ISIDs of blazars in the
quiescent and flaring states, GLAST data will be adequate to separate these
ISIDs at a significance better than 3 sigma.Comment: 11 pages, 9 figures, emulateapj; accepted for publication in Ap
New Theoretical Estimates of the Contribution of Unresolved Star-Forming Galaxies to the Extragalactic Gamma-Ray Background (EGB) as Measured by EGRET and the Fermi-LAT
We present new theoretical estimates of the contribution of unresolved star-forming galaxies to the extragalactic gamma-ray background (EGB) as measured by EGRET and the Fermi-LAT. We employ several methods for determining the star-forming galaxy contribution the the EGB, including a method positing a correlation between the gamma-ray luminosity of a galaxy and its rate of star formation as calculated from the total infrared luminosity, and a method that makes use of a model of the evolution of the galaxy gas mass with cosmic time. We find that depending on the model, unresolved star-forming galaxies could contribute significantly to the EGB as measured by the Fermi-LAT at energies between approx. 300 MeV and approx. few GeV. However, the overall spectrum of unresolved star-forming galaxies can explain neither the EGRET EGB spectrum at energies between 50 and 200 MeV nor the Fermi-LAT EGB spectrum at energies above approx. few GeV
The Unresolved Star-Forming Galaxy Component of the Extragalactic Gamma Ray Background
We present new theoretical estimates of the contribution of unresolved star-forming galaxies to the extragalactic gamma-ray background (EGB) as measured by EGRET and the Fermi-LAT. We employ several methods for determining the star-forming galaxy contribution the the EGB, including a method positing a correlation between the gamma-ray luminosity of a galaxy and its rate of star formation as calculated from the total infrared luminosity, and a method that makes use of a model of the evolution of the galaxy gas mass with cosmic time. We find that depending on the model, unresolved star-forming galaxies could contribute significantly to the EGB as measured by the Fermi-LAT at energies between approx. 300 MeV and approx. few GeV. However, the overall spectrum of unresolved star-forming galaxies can explain neither the EGRET EGB spectrum at energies between 50 and 200 MeV nor the Fermi-LAT EGB spectrum at energies above approx. few GeV
Contribution to the Extragalactic Gamma-ray Background from the Cascades of Very-high Energy Gamma Rays
As very-high--energy photons propagate through the extragalactic background
light (EBL), they interact with the soft photons and initiate electromagnetic
cascades of lower energy photons and electrons. The collective intensity of a
cosmological population emitting at very-high energies (VHE) will be attenuated
at the highest energies through interactions with the EBL and enhanced at lower
energies by the resulting cascade. We calculate the cascade radiation created
by VHE photons produced by blazars and investigate the effects of cascades on
the collective intensity of blazars and the resulting effects on the
extragalactic gamma-ray background. We find that cascade radiation greatly
enhances the collective intensity from blazars at high energies before turning
over due to attenuation. The prominence of the resulting features depends on
the blazar gamma-ray luminosity function, spectral index distribution, and the
model of the EBL. We additionally calculate the cascade radiation from the
distinct spectral sub-populations of blazars, BL Lacertae objects (BL Lacs) and
flat-spectrum radio quasars (FSRQs), finding that the collective intensity of
BL Lacs is considerably more enhanced by cascade radiation than that of the
FSRQs due to their harder spectra. As such, studies of the blazar contribution
to the EGRB by Fermi will have profound implications for the nature of the EBL,
the evolution of blazars, and blazar spectra.Comment: 2009 Fermi Symposium, eConf Proceedings C09112
The Effect of Blazar Spectral Breaks on the Blazar Contribution to the Extragalactic Gamma-ray Background
The spectral shapes of the contributions of different classes of unresolved
gamma-ray emitters can provide insight into their relative contributions to the
extragalactic gamma-ray background (EGB) and the natures of their spectra at
GeV energies. We calculate the spectral shapes of the contributions to the EGB
arising from BL Lacertae type objects (BL Lacs) and flat-spectrum radio quasars
(FSRQs) assuming blazar spectra can be described as broken power laws. We fit
the resulting total blazar spectral shape to the Fermi Large Area Telescope
measurements of the EGB, finding that the best-fit shape reproduces well the
shape of the Fermi EGB for various break scenarios. We conclude that a scenario
in which the contribution of blazars is dominant cannot be excluded on spectral
grounds alone, even if spectral breaks are shown to be common among Fermi
blazars. We also find that while the observation of a featureless (within
uncertainties) power-law EGB spectrum by Fermi does not necessarily imply a
single class of contributing unresolved sources with featureless individual
spectra, such an observation and the collective spectra of the separate
contributing populations determine the ratios of their contributions. As such,
a comparison with studies including blazar gamma-ray luminosity functions could
have profound implications for the blazar contribution to the EGB, blazar
evolution, and blazar gamma-ray spectra and emission.Comment: 8 pages, emulateapj format; 5 figures; accepted for publication in
Ap
Overview of the EUSO-SPB2 Target of Opportunity program using the Cherenkov Telescope
During the Extreme Universe Space Observatory on a Super Pressure Balloon 2
(EUSO-SPB2) mission, we planned Target of Opportunity (ToO) operations to
follow up on possible sources of neutrinos. The
original plan before flight was to point the onboard Cherenkov Telescope (CT)
to catch the source's path on the sky just below Earth's horizon. By using the
Earth as a tau-neutrino to tau-lepton converter, the CT would then be able to
look for optical extensive air shower signals induced by tau-lepton decays in
the atmosphere. The CT had a field of view of vertical
horizontal. Possible neutrino source candidates include gamma ray
bursts, tidal disruption events and other bursting or flaring sources. In
addition, follow-ups of binary neutron star mergers would have been possible
after the start of the O4 observation run from LIGO-Virgo-KAGRA. The resulting
exposure is modeled using the NuSpaceSim framework in ToO mode. With the launch
of the EUSO-SPB2 payload on the 13th May 2023, this summarizes the ToO program
status and preliminary data, as available
Components of the Extragalactic Gamma Ray Background
We present new theoretical estimates of the relative contributions of
unresolved blazars and star-forming galaxies to the extragalactic gamma-ray
background (EGB) and discuss constraints on the contributions from alternative
mechanisms such as dark matter annihilation and truly diffuse gamma-ray
production. We find that the Fermi source count data do not rule out a scenario
in which the EGB is dominated by emission from unresolved blazars, though
unresolved star-forming galaxies may also contribute significantly to the
background, within order-of-magnitude uncertainties. In addition, we find that
the spectrum of the unresolved star-forming galaxy contribution cannot explain
the EGB spectrum found by EGRET at energies between 50 and 200 MeV, whereas the
spectrum of unresolved FSRQs, when accounting for the energy-dependent effects
of source confusion, could be consistent with the combined spectrum of the
low-energy EGRET EGB measurements and the Fermi-LAT EGB measurements.Comment: version accepted for publication in the Astrophysical Journa
The Extragalactic Background Light Absorption Feature in the Blazar Component of the Extragalactic Gamma-ray Background
High-energy photons from cosmological emitters suffer attenuation due to pair
production interactions with the extragalactic background light (EBL). The
collective emission of any high-energy emitting cosmological population will
exhibit an absorption feature at the highest energies. We calculate this
absorption feature in the collective emission of blazars for various models of
the blazar gamma-ray luminosity function (GLF) and the EBL. We find that models
of the blazar GLF that predict higher relative contributions of high-redshift
blazars to the blazar collective spectrum result in emission that is more
susceptible to attenuation by the EBL, and hence result in more prominent
absorption features, allowing for better differentiation amongst EBL models. We
thus demonstrate that observations of such an absorption feature will contain
information regarding both the blazar GLF and the EBL, and we discuss tests for
EBL models and the blazar GLF that will become possible with upcoming Fermi
observations.Comment: 9 pages, 2 figures, emulateapj format, to be published in
Astrophysical Journa
Neutrino constraints on long-lived heavy dark sector particle decays in the Earth
Recent theoretical work has explored dark matter accumulation in the Earth
and its drift towards the center of the Earth that, for the current age of the
Earth, does not necessarily result in a concentration of dark matter ()
in the Earth's core. We consider a scenario of long-lived ( s), super heavy ( GeV) dark matter that decays
via or . We show that an IceCube-like
detector over 10 years can constrain a dark matter density that mirrors the
Earth's density or has a uniform density with density fraction
combined with the partial decay width in
the range of s. For , GeV and GeV, the range of constraints is
s.Comment: 9 pages, 9 figure