2,488 research outputs found
Machine-z: Rapid Machine Learned Redshift Indicator for Swift Gamma-ray Bursts
Studies of high-redshift gamma-ray bursts (GRBs) provide important
information about the early Universe such as the rates of stellar collapsars
and mergers, the metallicity content, constraints on the re-ionization period,
and probes of the Hubble expansion. Rapid selection of high-z candidates from
GRB samples reported in real time by dedicated space missions such as Swift is
the key to identifying the most distant bursts before the optical afterglow
becomes too dim to warrant a good spectrum. Here we introduce "machine-z", a
redshift prediction algorithm and a "high-z" classifier for Swift GRBs based on
machine learning. Our method relies exclusively on canonical data commonly
available within the first few hours after the GRB trigger. Using a sample of
284 bursts with measured redshifts, we trained a randomized ensemble of
decision trees (random forest) to perform both regression and classification.
Cross-validated performance studies show that the correlation coefficient
between machine-z predictions and the true redshift is nearly 0.6. At the same
time our high-z classifier can achieve 80% recall of true high-redshift bursts,
while incurring a false positive rate of 20%. With 40% false positive rate the
classifier can achieve ~100% recall. The most reliable selection of
high-redshift GRBs is obtained by combining predictions from both the high-z
classifier and the machine-z regressor.Comment: Accepted to the Monthly Notices of the Royal Astronomical Society
Journal (10 pages, 10 figures, and 3 Tables
The INTEGRAL Core Observing Programme
The Core Programme of the INTEGRAL mission is defined as the portion of the
scientific programme covering the guaranteed time observations for the INTEGRAL
Science Working Team. This paper describes the current status of the Core
Programme preparations and summarizes the key elements of the observing
programme.Comment: Contributed paper, 3rd INTEGRAL Workshop, Taormina/Sicily, Sep 1998,
to be published in Astrophys. Letters & Communications, 199
Optical monitoring of gamma-ray source fields
The three gamma-ray burst source fields GBS1028+46, GBS1205+24, and GBS2252-03 have been monitored for transient optical emission for a combined total of 52 hours. No optical events were seen. The limiting magnitude for the search was M sub V = 15.8 longer and M sub V = 17.0 for 6.0 s or longer
The X-ray flaring properties of Sgr A* during six years of monitoring with Swift
Starting in 2006, Swift has been targeting a region of ~21'X21' around
Sagittarius A* (Sgr A*) with the onboard X-ray telescope. The short,
quasi-daily observations offer an unique view of the long-term X-ray behavior
of the supermassive black hole. We report on the data obtained between 2006
February and 2011 October, which encompasses 715 observations with a total
accumulated exposure time of ~0.8 Ms. A total of six X-ray flares were detected
with Swift, which all had an average 2-10 keV luminosity of Lx (1-4)E35 erg/s
(assuming a distance of 8 kpc). This more than doubles the number of such
bright X-ray flares observed from Sgr A*. One of the Swift-detected flares may
have been softer than the other five, which would indicate that flares of
similar intensity can have different spectral properties. The Swift campaign
allows us to constrain the occurrence rate of bright (Lx > 1E35 erg/s) X-ray
flares to be ~0.1-0.2 per day, which is in line with previous estimates. This
analysis of the occurrence rate and properties of the X-ray flares seen with
Swift offers an important calibration point to asses whether the flaring
behavior of Sgr A* changes as a result of its interaction with the gas cloud
that is projected to make a close passage in 2013.Comment: 8 pages, 5 figures, 3 tables. Shortened, accepted to Ap
Analysis of the X-ray Emission of Nine Swift Afterglows
The X-ray light-curves of 9 Swift XRT afterglows (050126, 050128, 050219A,
050315, 050318, 050319, 050401, 050408, 050505) display a complex behaviour: a
steep t^{-3.0 \pm 0.3} decay until ~400 s, followed by a significantly slower
t^{-0.65+/-0.20} fall-off, which at 0.2--2 d after the burst evolves into a
t^{-1.7+/-0.5} decay. We consider three possible models for the geometry of
relativistic blast-waves (spherical outflows, non-spreading jets, and spreading
jets), two possible dynamical regimes for the forward shock (adiabatic and
fully radiative), and we take into account a possible angular structure of the
outflow and delayed energy injection in the blast-wave, to identify the models
which reconcile the X-ray light-curve decay with the slope of the X-ray
continuum for each of the above three afterglow phases. By piecing together the
various models for each phase in a way that makes physical sense, we identify
possible models for the entire X-ray afterglow. The major conclusion of this
work is that a long-lived episode of energy injection in the blast-wave, during
which the shock energy increases at t^{1.0+/-0.5}, is required for five
afterglows and could be at work in the other four as well. Optical observations
in conjunction with the X-ray can distinguish among these various models. Our
simple tests allow the determination of the location of the cooling frequency
relative to the X-ray domain and, thus, of the index of the electron power-law
distribution with energy in the blast-wave. The resulting indices are clearly
inconsistent with an universal value.Comment: 10 pages, minor changes, to be published in the MNRA
Photospheric Emission in the Joint GBM and Konus Prompt Spectra of GRB 120323A
GRB 120323A is a very intense short Gamma Ray Burst (GRB) detected
simultaneously during its prompt gamma-ray emission phase with the Gamma-ray
Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope and the Konus
experiment on board the Wind satellite. GBM and Konus operate in the keV--MeV
regime, however, the GBM range is broader both toward the low and the high
parts of the gamma-ray spectrum. Analysis of such bright events provide a
unique opportunity to check the consistency of the data analysis as well as
cross-calibrate the two instruments. We performed time-integrated and coarse
time-resolved spectral analysis of GRB 120323A prompt emission. We conclude
that the analyses of GBM and Konus data are only consistent when using a
double-hump spectral shape for both data sets; in contrast, the single-hump of
the empirical Band function, traditionally used to fit GRB prompt emission
spectra, leads to significant discrepancies between GBM and Konus analysis
results. Our two-hump model is a combination of a thermal-like and a
non-thermal component. We interpret the first component as a natural
manifestation of the jet photospheric emission.Comment: 7 pages of article (3 figures and 1 table) + 3 pages of Appendix (3
figures). Submitted to ApJ on 2017 March 2
Indication of Two Classes in the Swift Short Gamma-Ray Bursts from the XRT X-Ray Afterglow Light Curves
We present the discovery of two distinct classes in the Swift short duration
gamma-ray bursts (S-GRBs) from the X-Ray Telescope (XRT) X-ray afterglow light
curve. We find that about 40% of the Swift S-GRBs have an X-ray afterglow light
curves which only lasts less than 10000 seconds after the burst trigger
(hereafter short-lived S-GRBs). On the other hand, another 60% of S-GRBs have a
long lasting X-ray afterglow light curve which resembles the long duration
gamma-ray bursts. We also find that none of the short-lived S-GRBs shows the
extended emission in the Burst Alert Telescope (BAT) energy range. We compare
the burst properties for both the prompt emission and the afterglow, and
discuss the possibility of different progenitors for the Swift short GRBs.Comment: 3 pages, 6 figures, Submitted to Proceedings of Huntsville 2008
Symposium on GRB
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