3,038 research outputs found
Sommerfeld Enhancements for Thermal Relic Dark Matter
The annihilation cross section of thermal relic dark matter determines both
its relic density and indirect detection signals. We determine how large
indirect signals may be in scenarios with Sommerfeld-enhanced annihilation,
subject to the constraint that the dark matter has the correct relic density.
This work refines our previous analysis through detailed treatments of resonant
Sommerfeld enhancement and the effect of Sommerfeld enhancement on freeze out.
Sommerfeld enhancements raise many interesting issues in the freeze out
calculation, and we find that the cutoff of resonant enhancement, the
equilibration of force carriers, the temperature of kinetic decoupling, and the
efficiency of self-interactions for preserving thermal velocity distributions
all play a role. These effects may have striking consequences; for example, for
resonantly-enhanced Sommerfeld annihilation, dark matter freezes out but may
then chemically recouple, implying highly suppressed indirect signals, in
contrast to naive expectations. In the minimal scenario with standard
astrophysical assumptions, and tuning all parameters to maximize the signal, we
find that, for force-carrier mass m_phi = 250 MeV and dark matter masses m_X =
0.1, 0.3, and 1 TeV, the maximal Sommerfeld enhancement factors are S_eff = 7,
30, and 90, respectively. Such boosts are too small to explain both the PAMELA
and Fermi excesses. Non-minimal models may require smaller boosts, but the
bounds on S_eff could also be more stringent, and dedicated freeze out analyses
are required. For concreteness, we focus on 4 mu final states, but we also
discuss 4 e and other modes, deviations from standard astrophysical assumptions
and non-minimal particle physics models, and we outline the steps required to
determine if such considerations may lead to a self-consistent explanation of
the PAMELA or Fermi excesses.Comment: 31 pages, published versio
Long gamma-ray bursts without visible supernovae: a case study of redshift estimators and alleged novel objects
There is an ongoing debate on whether or not the observational limits on a supernova (SN) associated with GRB060614 convincingly exclude a SN akin to SN1998bw as its originator, and provide evidence for a new class of long-duration GRBs. We discuss this issue in the contexts of indirect `redshift estimators' and of the fireball and cannonball models of GRBs. The latter explains the unusual properties of GRB060614: at its debated low redshift (0.125) they are predicted, as opposed to exceptional, if the associated SN is of `Pastorello's class'. Long-baseline radio data and deep optical data may test the proposed alternatives
Fireballs and cannonballs confront the afterglow of GRB 991208
Galama et al. have recently reported their follow-up measurements of the radio afterglow (AG) of the Gamma Ray Burst (GRB) 991208, up to 293 days after burst, and their reanalysis of the broad-band AG, in the framework of standard fireball models. They advocate a serious revision of their prior analysis and conclusions, based on optical data and on their earlier observations during the first two weeks of the AG. We comment on their work and fill a lacuna: these authors have overlooked the possibility of comparing their new data to the available predictions of the cannonball (CB) model, based --like their incorrect predictions-- on the first round of data. The new data are in good agreement with these CB-model predictions. This is in spite of the fact that, in comparison to the fireball models, the CB model is much simpler, much more predictive, has many fewer parameters, practically no free choices... and it describes well --on a universal basis-- all the measured AGs of GRBs of known redshift
Origin of the ultrahigh-energy cosmic rays and their spectral break
The energy spectrum, composition and arrival directions of ultrahigh energy cosmic rays (UHECRs) with energy above the cosmic ray ankle, measured by the Pierre Auger Observatory, appear to be in conflict if their origin is assumed to be extragalactic. Their spectrum and composition, however, are those expected from Galactic UHECRs accelerated by highly relativistic jets such as those producing short hard gamma ray bursts (SHBs). If this alternative interpretation is
correct, then the observed break in the energy spectrum of UHECRs around 50 EeV is the energy threshold for free escape of UHE iron nuclei from the Galaxy and not the Greisen-Zatsepin-Kuzmin (GZK) cutoff for protons, and the arrival directions of UHECR nuclei with energy above their UHE breaks must point back to their Galactic sources rather than to active galactic nuclei (AGN) within the GZK horizon
Dark Matter Annihilation Signatures from Electroweak Bremsstrahlung
We examine observational signatures of dark matter annihilation in the Milky
Way arising from electroweak bremsstrahlung contributions to the annihilation
cross section. It has been known for some time that photon bremsstrahlung may
significantly boost DM annihilation yields. Recently, we have shown that
electroweak bremsstrahlung of W and Z gauge bosons can be the dominant
annihilation channel in some popular models with helicity-suppressed 2 --> 2
annihilation. W/Z-bremsstrahlung is particularly interesting because the gauge
bosons produced via annihilation subsequently decay to produce large correlated
fluxes of electrons, positrons, neutrinos, hadrons (including antiprotons) and
gamma rays, which are all of importance in indirect dark matter searches. Here
we calculate the spectra of stable annihilation products produced via
gamma/W/Z-bremsstrahlung. After modifying the fluxes to account for the
propagation through the Galaxy, we set upper bounds on the annihilation cross
section via a comparison with observational data. We show that stringent cosmic
ray antiproton limits preclude a sizable dark matter contribution to observed
cosmic ray positron fluxes in the class of models for which the bremsstrahlung
processes dominate.Comment: 11 pages, 6 figures. Updated to match PRD versio
The supernova associated with GRB 020405
We use the very simple and successful Cannonball (CB) model of gamma ray
bursts (GRBs) and their afterglows (AGs) to analyze the observations of the
mildly extinct optical AG of the relatively nearby GRB 020405. We show that GRB
020405 was associated with a 1998bw-like supernova (SN) at the GRB's redshift
that appeared dimmer and redder than SN1998bw because of extinction in the host
and our Galaxy. The case for the SN/GRB association --advocated in the CB
model-- is becoming indubitable. We discuss the extent to which the GRB/SN
connection is model-dependent.Comment: Accepted for publication in A&A Letter
Common solution of three cosmic puzzles
We show that the observed fluxes, spectra and sky distributions of the diffuse backgrounds of high energy astronomical neutrinos, gamma rays and cosmic ray positrons observed near Earth satisfy the simple relations expected from their common production in hadronic collisions of high energy cosmic ray nuclei with diffuse matter in/near source
On the radio afterglow of gamma ray bursts
We use the cannonball (CB) model of gamma ray bursts (GRBs) to predict the
spectral and temporal behaviour of their radio afterglows (AGs). A single
simple expression describes the AGs at all times and frequencies; its
high-frequency limit reproduces the successful CB model predictions for optical
and X-ray AGs. We analyze all of the observed radio AGs of GRBs with known
redshifts, including those of the exceptionally close-by GRB 980425. We also
study in detail the time-evolution of the AGs' spectral index. The agreement
between theory and observations is excellent, even though the CB model is
extremely frugal in the number of parameters required to explain the radio
observations. We propose to use the scintillations in the radio AGs of GRBs to
verify and measure the hyperluminal speed of their jetted CBs, whose apparent
angular velocity is of the same order of magnitude as that of galactic pulsars,
consistently measured directly, or via scintillations.Comment: Accepted for publication in A&
Challenging GRB models through the broadband dataset of GRB060908
Context: Multiwavelength observations of gamma-ray burst prompt and afterglow
emission are a key tool to disentangle the various possible emission processes
and scenarios proposed to interpret the complex gamma-ray burst phenomenology.
Aims: We collected a large dataset on GRB060908 in order to carry out a
comprehensive analysis of the prompt emission as well as the early and late
afterglow. Methods: Data from Swift-BAT, -XRT and -UVOT together with data from
a number of different ground-based optical/NIR and millimeter telescopes
allowed us to follow the afterglow evolution from about a minute from the
high-energy event down to the host galaxy limit. We discuss the physical
parameters required to model these emissions. Results: The prompt emission of
GRB060908 was characterized by two main periods of activity, spaced by a few
seconds of low intensity, with a tight correlation between activity and
spectral hardness. Observations of the afterglow began less than one minute
after the high-energy event, when it was already in a decaying phase, and it
was characterized by a rather flat optical/NIR spectrum which can be
interpreted as due to a hard energy-distribution of the emitting electrons. On
the other hand, the X-ray spectrum of the afterglow could be fit by a rather
soft electron distribution. Conclusions: GRB060908 is a good example of a
gamma-ray burst with a rich multi-wavelength set of observations. The
availability of this dataset, built thanks to the joint efforts of many
different teams, allowed us to carry out stringent tests for various
interpretative scenarios showing that a satisfactorily modeling of this event
is challenging. In the future, similar efforts will enable us to obtain
optical/NIR coverage comparable in quality and quantity to the X-ray data for
more events, therefore opening new avenues to progress gamma-ray burst
research.Comment: A&A, in press. 11 pages, 5 figure
The short GRB070707 afterglow and its very faint host galaxy
We present the results from an ESO/VLT campaign aimed at studying the
afterglow properties of the short/hard gamma ray burst GRB 070707. Observations
were carried out at ten different epochs from ~0.5 to ~80 days after the event.
The optical flux decayed steeply with a power-law decay index greater than 3,
later levelling off at R~27.3 mag; this is likely the emission level of the
host galaxy, the faintest yet detected for a short GRB. Spectroscopic
observations did not reveal any line features/edges that could unambiguously
pinpoint the GRB redshift, but set a limit z < 3.6. In the range of allowed
redshifts, the host has a low luminosity, comparable to that of long-duration
GRBs. The existence of such faint host galaxies suggests caution when
associating short GRBs with bright, offset galaxies, where the true host might
just be too dim for detection. The steepness of the decay of the optical
afterglow of GRB 070707 challenges external shock models for the optical
afterglow of short/hard GRBs. We argue that this behaviour might results from
prolonged activity of the central engine or require alternative scenarios.Comment: 6 pages, 5 figures, accepted by A&
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