105 research outputs found
Multi-Wavelength Observations of Short-Duration Gamma-Ray Bursts: Recent Results
The number of detections as well as significantly deep non-detections of
optical/NIR afterglows of Type I (short-duration population) Gamma-Ray Bursts
(GRBs) has become large enough that statistically meaningful samples can now be
constructed. I present within some recent results on the luminosity
distribution of Type I GRB afterglows in comparison to those of Type II GRBs
(collapsar population), the issue of the existence of jet breaks in Type I GRB
afterglows, and the discovery of dark Type I GRBs.Comment: 10 pages, 3 figures, based on an invited talk, to appear in the
proceedings of the Gamma-Ray Burst Symposium 2012- IAA-CSIC - Marbella,
editors: Castro-Tirado, A. J., Gorosabel, J. and Park, I. H; v2: accepted,
slightly expanded, minor changes after referee repor
Afterglows of Gamma-Ray Bursts: Short vs. Long GRBs
We compiled a large sample of Swift-era photometric data on long (Type II)
and short (Type I) GRB afterglows. We compare the luminosity and energetics of
the different samples to each other and to the afterglows of the pre-Swift era.
Here, we present the first results of these studies.Comment: Conference Proceedings, "Gamma-Ray Bursts 2007", Santa Fe, shortened
poster presentation; 4 pages, 3 figures; for full updated papers, go here to
arXiv:0712.2186 and also here to arXiv:0804.195
The afterglows of swift-era short and long gamma-ray bursts
The phenomenon of Gamma-Ray Bursts (GRBs) has been a great mystery since their discovery four decades ago. Even today, over a decade into the "afterglow age", many questions are still unanswered. The canonical picture which satisfies most of the data is that GRBs are produced when a massive celestial body (either a post-main sequence star or merging compact objects) at cosmological distances collapses to a rapidly rotating compact object (a black hole, for example) which launches ultra-relativistic polar jets. The internal dissipation of energy within the jets leads to collimated non-thermal high-energy emission (the actual GRB), whereas shocks created from the interactions of the jets with the interstellar medium create a long-lasting fading afterglow. The collected physical processes describing this emission are called the standard fireball model. GRBs have been found to be the most luminous electromagnetic sources in the universe for short time periods. In this Thesis, I present my study of the afterglows of Type I (not associated with massive star formation, probably due to the merger of compact objects) and Type II (associated with massive star formation, the late optical emission includes a component due to a Type Ic supernova with high expansions speeds) GRBs, and compare them with each other, especially in terms of host-galaxy dust extinction and the luminosity distribution. To accomplish this, I have collected the largest sample of photometric afterglow data available worldwide, and from this, selected GRBs with data sufficient for a more detailed analysis.Das Phänomen der Gammastrahlenausbrüche (Gamma-Ray Bursts, GRBs) war auch lange nach ihrer Entdeckung vor über vier Jahrzehnten ein großes Rätsel. Selbst heute, über ein Jahrzehnt seit Beginn der "Ära der Nachglühen" (engl.: Afterglows) sind noch viele Fragen unbeantwortet. Das akzeptierte Bild, welches einen Großteil der Daten erklären kann ist, dass GRBs erzeugt werden, wenn ein massereicher Himmelskörper (entweder ein Stern, der die Hauptreihe verlassen hat, oder miteinander verschmelzende kompakte Objekte) in kosmologischer Distanz zu einem schnell rotierenden Objekt kollabiert (z.B. ein Schwarzes Loch), welches ultrarelativistische Materieauswürfe ("Jets") entlang der Polachse ausschleudert. Die interne Dissipation von Energie in dem Jet führt zu kollimierter nicht-thermischer Strahlung bei hohen Energien (der GRB), während Schockfronten, die bei der Interaktion des Jets mit der interstellaren Materie erzeugt werden, zu einem langlebigen abklingenden Afterglow führen. Die gesammelten physikalischen Prozesse, die die GRB-Emission beschreiben, werden als das Standard-Feuerballmodell bezeichnet. GRBs sind für kurze Zeiträume nachweislich die leuchtkräftigsten elektromagnetischen Quellen des Universums. In dieser Dissertation präsentiere ich meine Untersuchungen zu den Afterglows von Typ I (nicht mit massereicher Sternentstehung verknüpft, vermutlich durch die Verschmelzung kompakter Objekte ausgelöst) und Typ II (mit massereicher Sternentstehung verknüpft, die optische Emission zu späten Zeiten enthält eine Komponente, die einer Typ Ic Supernova mit hoher Ausbreitungsgeschwindigkeit zuzuschreiben ist) GRBs, und vergleiche sie miteinander, insbesondere, was die Extinktion durch Staub in den Muttergalaxien sowie die Leuchtkraftverteilung angeht. Um dies zu ermöglichen, habe ich das weltweit größte Archiv an photometrischen Daten zu Afterglows zusammengestellt und habe aus diesem GRBs selektiert, die ausreichende Daten für eine weitergehende Analyse boten
Gamma-Ray Burst/Supernova Associations: Energy Partition and the Case of a Magnetar Central Engine
The favored progenitor model for Gamma-ray Bursts (GRBs) with Supernova (SN) association is the core collapse of massive stars. One possible outcome of such a collapse is a rapidly spinning, strongly magnetized neutron star ( magnetar ). We systematically analyze the multi-wavelength data of GRB/SN associations detected by several instruments before 2017 June. Twenty GRB/SN systems have been confirmed via direct spectroscopic evidence or a clear light curve bump, as well as some spectroscopic evidence resembling a GRB-SN. We derive/collect the basic physical parameters of the GRBs and the SNe, and look for correlations among these parameters. We find that the peak brightness, 56Ni mass, and explosion energy of SNe associated with GRBs are statistically higher than other Type Ib/c SNe. A statistically significant relation between the peak energy of GRBs and the peak brightness of their associated SNe is confirmed. No significant correlations are found between the GRB energies (either isotropic or beaming-corrected) and the supernova energy. We investigate the energy partition within these systems and find that the beaming-corrected GRB energy of most systems is smaller than the SN energy, with less than 30% of the total energy distributed in the relativistic jet. The total energy of the systems is typically smaller than the maximum available energy of a millisecond magnetar (2 × 1052 erg), especially if aspherical SN explosions are considered. The data are consistent with—although not proof of—the hypothesis that most, but not all, GRB/SN systems are powered by millisecond magnetars
GRB 180325A: dust grain-size distribution and interstellar iron nanoparticles contribution
We modelled dust grain-size distributions for carbonaceous and silicates
dust, as well as for free-flying iron nanoparticles in the environment of a
-ray burst (GRB) afterglow, GRB 180325A. This GRB, at , has
an unambiguous detection of the 2175 \r{A} extinction feature with
and . In addition to silicates, polycyclic aromatic hydrocarbons
(PAH), and graphite, we used iron nanoparticles grain-size distributions for
the first time to model the observed extinction curve of GRB 180325A. We fit
the observed extinction for four model permutations, using 232 sets of
silicates, graphite, carbon abundance in hydrocarbon molecules (), and
fraction of iron abundance in free-flying nanoparticles ().
These four different permutations were chosen to test iron nanoparticles
significance and carbon abundance in hydrocarbons. Our results indicate that
iron nanoparticles contribution is insignificant and there is a degeneracy of
carbon abundances, with the range
providing the best-fit to the observed extinction curve of GRB 180325A. We
therefore favour the simplest model of silicates and polycyclic aromatic
hydrocarbons. The silicates are dominant and contribute to the entire
wavelength range of the GRB extinction curve while graphite contributes towards
both the 2175 \r{A} bump and the UV extinction. The afterglow peak luminosity
( ergs/s) indicates dust destruction may have taken place. We
conclude that further investigations into other potential contributors of
extinction are warranted, particularly for steep UV extinction.Comment: 13 pages, 7 figures, accepted for publication in MNRA
Combined X-ray and optical analysis to probe the origin of the plateau emission in -ray bursts afterglows
A large fraction of gamma-ray bursts (GRBs) shows a plateau phase during the
X-ray afterglow emission, whose physical origin is still debated. In this work
we define a sample of 30 GRBs with simultaneous X-ray and optical data during
and after the plateau phase. Through a time-resolved spectral analysis of the
X-ray plateaus, we test the consistency of the unabsorbed optical fluxes with
those obtained via X-ray-to-optical spectral extrapolation by assuming a
synchrotron spectrum. Combining X-ray with optical data, we find that 63%
(19/30) GRBs are compatible with a single synchrotron spectrum thus suggesting
that both the optical and X-ray radiations are produced from a single emitting
region. For these GRBs we derive the temporal evolution of the break frequency
and we compare it with the expectations predicted by several models. For 11/30
GRBs the optical emission is above the predicted range of values extrapolated
from the X-rays in at least one temporal bin of the light curve. These GRBs may
not be explained with a single zone emission, indicating the necessity of
invoking two cooperating processes in order to explain the broad band spectral
behaviour during X-ray plateaus. We discuss our findings in the framework of
different scenarios invoked to explain the plateau feature, including the
energy injection from a spinning-down magnetar and the high latitude emission
from a structured jet.Comment: submitted to A&
GRB 091127: The cooling break race on magnetic fuel
Using high-quality, broad-band afterglow data for GRB 091127, we investigate
the validity of the synchrotron fireball model for gamma-ray bursts, and infer
physical parameters of the ultra-relativistic outflow. We used multi-wavelength
follow-up observations obtained with GROND and the XRT onboard the Swift
satellite. The resulting afterglow light curve is of excellent accuracy, and
the spectral energy distribution is well-sampled over 5 decades in energy.
These data present one of the most comprehensive observing campaigns for a
single GRB afterglow and allow us to test several proposed emission models and
outflow characteristics in unprecedented detail. Both the multi-color light
curve and the broad-band SED of the afterglow of GRB 091127 show evidence of a
cooling break moving from high to lower energies. The early light curve is well
described by a broken power-law, where the initial decay in the optical/NIR
wavelength range is considerably flatter than at X-rays. Detailed fitting of
the time-resolved SED shows that the break is very smooth with a sharpness
index of 2.2 +- 0.2, and evolves towards lower frequencies as a power-law with
index -1.23 +- 0.06. These are the first accurate and contemporaneous
measurements of both the sharpness of the spectral break and its time
evolution. The measured evolution of the cooling break (nu_c propto t^-1.2) is
not consistent with the predictions of the standard model, wherein nu_c propto
t^-0.5 is expected. A possible explanation for the observed behavior is a time
dependence of the microphysical parameters, in particular the fraction of the
total energy in the magnetic field epsilon_B. This conclusion provides further
evidence that the standard fireball model is too simplistic, and time-dependent
micro-physical parameters may be required to model the growing number of
well-sampled afterglow light curves.Comment: accepted to A&A, 13 pages, 5 figure
The Rapidly Flaring Afterglow of the Very Bright and Energetic GRB 070125
We report on multi-wavelength observations, ranging from the X-ray to radio
wave bands, of the IPN-localized gamma-ray burst GRB 070125. Spectroscopic
observations reveal the presence of absorption lines due to O I, Si II, and C
IV, implying a likely redshift of z = 1.547. The well-sampled light curves, in
particular from 0.5 to 4 days after the burst, suggest a jet break at 3.7 days,
corresponding to a jet opening angle of ~7.0 degrees, and implying an intrinsic
GRB energy in the 1 - 10,000 keV band of around E = (6.3 - 6.9)x 10^(51) erg
(based on the fluences measured by the gamma-ray detectors of the IPN network).
GRB 070125 is among the brightest afterglows observed to date. The spectral
energy distribution implies a host extinction of Av < 0.9 mag. Two
rebrightening episodes are observed, one with excellent time coverage, showing
an increase in flux of 56% in ~8000 seconds. The evolution of the afterglow
light curve is achromatic at all times. Late-time observations of the afterglow
do not show evidence for emission from an underlying host galaxy or supernova.
Any host galaxy would be subluminous, consistent with current GRB host-galaxy
samples. Evidence for strong Mg II absorption features is not found, which is
perhaps surprising in view of the relatively high redshift of this burst and
the high likelihood for such features along GRB-selected lines of sight.Comment: 50 pages, 9 figures, 5 tables Accepted to the Astrophysical Journa
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Single-cell RNA-seq highlights intratumoral heterogeneity in primary glioblastoma
Human cancers are complex ecosystems composed of cells with distinct phenotypes, genotypes and epigenetic states, but current models do not adequately reflect tumor composition in patients. We used single cell RNA-seq to profile 430 cells from five primary glioblastomas, which we found to be inherently variable in their expression of diverse transcriptional programs related to oncogenic signaling, proliferation, complement/immune response and hypoxia. We also observed a continuum of stemness-related expression states that enabled us to identify putative regulators of stemness in vivo. Finally, we show that established glioblastoma subtype classifiers are variably expressed across individual cells within a tumor and demonstrate the potential prognostic implications of such intratumoral heterogeneity. Thus, we reveal previously unappreciated heterogeneity in diverse regulatory programs central to glioblastoma biology, prognosis, and therapy
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