331 research outputs found
The Connection Between Thermal and Non-Thermal Emission in Gamma-ray Bursts: General Considerations and GRB090902B as a Case Study
Photospheric (thermal) emission is inherent to the gamma-ray burst (GRB)
"fireball" model. We show here, that inclusion of this component in the
analysis of the GRB prompt emission phase naturally explains some of the prompt
GRB spectra seen by the Fermi satellite over its entire energy band. The
sub-MeV peak is explained as multi-color black body emission, and the high
energy tail, extending up to the GeV band, results from roughly similar
contributions of synchrotron emission, synchrotron self Compton(SSC) and
Comptonization of the thermal photons by energetic electrons originating after
dissipation of the kinetic energy above the photosphere. We show how this
analysis method results in a complete, self consistent picture of the physical
conditions at both emission sites of the thermal and non-thermal radiation. We
study the connection between the thermal and non-thermal parts of the spectrum,
and show how the values of the free model parameters are deduced from the data.
We demonstrate our analysis method on GRB090902B: We deduce a Lorentz factor in
the range 920 <= \eta <= 1070, photospheric radius r_{ph} ~ 7.2 - 8.4 * 10^{11}
cm and dissipation radius r_\gamma >= 3.5 - 4.1 * 10^{15} cm. By comparison to
afterglow data, we deduce that a large fraction, epsilon_d ~85% - 95% of the
kinetic energy is dissipated, and that large fraction, ~equipartition of this
energy is carried by the electrons and the magnetic field. This high value of
epsilon_d questions the "internal shock" scenario as the main energy
dissipation mechanism for this GRB.Comment: 15 pages, 5 figures; minor revisions, typos corrected. Accepted for
publication in MNRA
(Semi-)Predictive Discretization During Model Selection
In this paper, we present an approach to discretizing multivariate continuous data while learning the structure of a graphical model. We derive the joint scoring function from the principle of predictive accuracy, which inherently ensures the optimal trade-off between goodness of fit and model complexity (including the number of discretization levels). Using the so-called finest grid implied by the data, our scoring function depends only on the number of data points in the various discretization levels. Not only can it be computed efficiently, but it is also independent of the metric used in the continuous space. Our experiments with gene expression data show that discretization plays a crucial role regarding the resulting network structure
Photoassociation adiabatic passage of ultracold Rb atoms to form ultracold Rb_2 molecules
We theoretically explore photoassociation by Adiabatic Passage of two
colliding cold ^{85}Rb atoms in an atomic trap to form an ultracold Rb_2
molecule. We consider the incoherent thermal nature of the scattering process
in a trap and show that coherent manipulations of the atomic ensemble, such as
adiabatic passage, are feasible if performed within the coherence time window
dictated by the temperature, which is relatively long for cold atoms. We show
that a sequence of ~2*10^7 pulses of moderate intensities, each lasting ~750
ns, can photoassociate a large fraction of the atomic ensemble at temperature
of 100 microkelvin and density of 10^{11} atoms/cm^3. Use of multiple pulse
sequences makes it possible to populate the ground vibrational state. Employing
spontaneous decay from a selected excited state, one can accumulate the
molecules in a narrow distribution of vibrational states in the ground
electronic potential. Alternatively, by removing the created molecules from the
beam path between pulse sets, one can create a low-density ensemble of
molecules in their ground ro-vibrational state.Comment: RevTex, 23 pages, 9 figure
Coherence as ultrashort pulse train generator
Intense, well-controlled regular light pulse trains start to play a crucial
role in many fields of physics. We theoretically demonstrate a very simple and
robust technique for generating such periodic ultrashort pulses from a
continuous probe wave which propagates in a dispersive thermal gas media
The Correlation of Spectral Lag Evolution with Prompt Optical Emission in GRB 080319B
We report on observations of correlated behavior between the prompt gamma-ray
and optical emission from GRB 080319B, which confirm that (i) they occurred
within the same astrophysical source region and (ii) their respective radiation
mechanisms were dynamically coupled. Our results, based upon a new CCF
methodology for determining the time-resolved spectral lag, are summarized as
follows. First, the evolution in the arrival offset of prompt gamma-ray photon
counts between Swift-BAT 15-25 keV and 50-100 keV energy bands (intrinsic
gamma-ray spectral lag) appears to be anti-correlated with the arrival offset
between prompt 15-350 keV gamma-rays and the optical emission observed by
TORTORA (extrinsic optical/gamma-ray spectral lag), thus effectively
partitioning the burst into two main episodes at ~T+28+/-2 sec. Second, the
rise and decline of prompt optical emission at ~T+10+/-1 sec and ~T+50+/-1 sec,
respectively, both coincide with discontinuities in the hard to soft evolution
of the photon index for a power law fit to 15-150 keV Swift-BAT data at
~T+8+/-2 sec and ~T+48+/-1 sec. These spectral energy changes also coincide
with intervals whose time-resolved spectral lag values are consistent with
zero, at ~T+12+/-2 sec and ~T+50+/-2 sec. These results, which are robust
across heuristic permutations of Swift-BAT energy channels and varying temporal
bin resolution, have also been corroborated via independent analysis of
Konus-Wind data. This potential discovery may provide the first observational
evidence for an implicit connection between spectral lags and GRB emission
mechanisms in the context of canonical fireball phenomenology. Future work
includes exploring a subset of bursts with prompt optical emission to probe the
unique or ubiquitous nature of this result.Comment: 6 pages, 3 figures. Contributed to the Proceedings of the Sixth
Huntsville GRB Symposium. Edited by C.A. Meegan, N. Gehrels, and C.
Kouvelioto
Ultracold dense gas of deeply bound heteronuclear molecules
Recently, the quest for an ultracold and dense ensemble of polar molecules
has attracted strong interest. Polar molecules have bright prospects for novel
quantum gases with long-range and anisotropic interactions, for quantum
information science, and for precision measurements. However, high-density
clouds of ultracold polar molecules have so far not been produced. Here, we
report a key step towards this goal. Starting from an ultracold dense gas of
heteronuclear 40K-87Rb Feshbach molecules with typical binding energies of a
few hundred kHz and a negligible dipole moment, we coherently transfer these
molecules into a vibrational level of the ground-state molecular potential
bound by >10 GHz. We thereby increase the binding energy and the expected
dipole moment of the 40K-87Rb molecules by more than four orders of magnitude
in a single transfer step. Starting with a single initial state prepared with
Feshbach association, we achieve a transfer efficiency of 84%. While dipolar
effects are not yet observable, the presented technique can be extended to
access much more deeply bound vibrational levels and ultimately those
exhibiting a significant dipole moment. The preparation of an ultracold quantum
gas of polar molecules might therefore come within experimental reach.Comment: 5 pages, 5 figure
Modeling the high-energy emission in GRB 110721A and implications on the early multiwavelength and polarimetric observations
GRB 110721A was detected by the Gamma-ray Burst Monitor and the Large Area
Telescope (LAT) onboard the Fermi satellite and the Gamma-ray Burst Polarimeter
onboard the IKAROS solar mission. Previous analysis done of this burst showed:
i) a linear polarization signal with position angle stable () and high degree of , ii) an extreme peak
energy of a record-breaking at 152 MeV, and iii) a subdominant prompt
thermal component observed right after the onset of this burst. In this paper,
the LAT data around the reported position of GRB 110721A are analysed with the
most recent software and then, the LAT light curve above 100 MeV was obtained.
The LAT light curve is modelled in terms of adiabatic early-afterglow external
shocks when the outflow propagates into a stellar wind. Additionally, we
discuss the possible origins and also study the implications of the
early-afterglow external shocks on the extreme peak energy observed at 152
MeV, the polarization observations and the subdominant prompt thermal
component.Comment: 9 pages and one figure. Accepted for publication in Ap
GRB afterglow light curves from realistic density profiles
The afterglow emission that follows gamma-ray bursts (GRBs) contains valuable
information about the circumburst medium and, therefore, about the GRB
progenitor. Theoretical studies of GRB blast waves, however, are often limited
to simple density profiles for the external medium (mostly constant density and
power-law R^{-k} ones). We argue that a large fraction of long-duration GRBs
should take place in massive stellar clusters where the circumburst medium is
much more complicated. As a case study, we simulate the propagation of a GRB
blast wave in a medium shaped by the collision of the winds of O and Wolf-Rayet
stars, the typical distance of which is d /sim 0.1 - 1 pc. Assuming a spherical
blast wave, the afterglow light curve shows a flattening followed by a shallow
break on a timescale from hours up to a week after the burst, which is a result
of the propagation of the blast wave through the shocked wind region. If the
blast wave is collimated, the jet break may, in some cases, become very
pronounced with the post break decline of the light curve as steep as t-5.
Inverse Compton scattering of ultra-violet photons from the nearby star off
energetic electrons in the blast wave leads to a bright \simGeV afterglow flare
that may be detectable by Fermi.Comment: 7 pages, 7 figures, submitted to MNRA
Quantum-inspired interferometry with chirped laser pulses
We introduce and implement an interferometric technique based on chirped
femtosecond laser pulses and nonlinear optics. The interference manifests as a
high-visibility (> 85%) phase-insensitive dip in the intensity of an optical
beam when the two interferometer arms are equal to within the coherence length
of the light. This signature is unique in classical interferometry, but is a
direct analogue to Hong-Ou-Mandel quantum interference. Our technique exhibits
all the metrological advantages of the quantum interferometer, but with signals
at least 10^7 times greater. In particular we demonstrate enhanced resolution,
robustness against loss, and automatic dispersion cancellation. Our
interferometer offers significant advantages over previous technologies, both
quantum and classical, in precision time delay measurements and biomedical
imaging.Comment: 6 pages, 4 figure
GeV emission from short Gamma-Ray Bursts: the case of GRB 081024B
We investigate whether the high energy tail detected by the Fermi/LAT for the
short GRB 081024B can be caused by synchrotron and self-Compton emission in the
context of either the internal or external shock models. For the internal shock
scenario, we explore the possibility of generating the high energy photons
directly by means of the synchrotron process, or inverse Compton emission in
which target photons are synchrotron photons produced in internal shocks taking
place in a lately emitted shell (delayed internal shocks). In the external
shock scenario, we test whether the high energy tail can be an extension of the
afterglow synchrotron emission, or alternatively the inverse Compton component
associated with the afterglow synchrotron photons. For the internal shock
scenario, we conclude that only an inverse Compton component from delayed
internal shocks can explain the high energy tail that extends to the GeV range.
In the external shock scenario, we show that the high energy tail may be
interpreted as synchrotron afterglow emission, if the slow cooling phase starts
as early as a few seconds after the trigger. On the other hand, the observed
high energy tail is consistent with an inverse Compton component of the
afterglow in the fast cooling regime.Comment: 8 pages, 3 figures; new version addresses Referee's comments;
accepted for publication in Astronomy and Astrophysic
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