77 research outputs found
The Total Errors In Measuring Epeak for Gamma-Ray Bursts
While Epeak has been extensively used in the past, for example with
luminosity indicators, it has not been thoroughly examined for possible sources
of scatter. In the literature, the reported error bars for Epeak are the simple
Poisson statistical errors. Additional uncertainties arise due to the choices
made by analysts in determining Epeak (e.g., the start and stop times of
integration), imperfect knowledge of the response of the detector, different
energy ranges for various detectors, and differences in models used to fit the
spectra. We examine the size of these individual sources of scatter by
comparing many independent pairs of published Epeak values for the same bursts.
Indeed, the observed scatter in multiple reports of the same burst (often with
the same data) is greatly larger than the published statistical error bars. We
measure that the one-sigma uncertainty associated with the analyst's choices is
28%, i.e., 0.12 in Log10(Epeak), with the resultant errors always being
present. The errors associated with the detector response are negligibly small.
The variations caused by commonly-used alternative definitions of Epeak (such
as present in all papers and in all compiled burst lists) is typically 23%-46%,
although this varies substantially with the application. The implications of
this are: (1) Even the very best measured Epeak values will have systematic
uncertainties of 28%. (2) Thus, GRBs have a limitation in accuracy for a single
event, with this being reducible by averaging many bursts. (3) The typical
one-sigma total uncertainty for collections of bursts is 55%. (4) We also find
that the width of the distribution for Epeak in the burst frame must be near
zero, implying that some mechanism must exist to thermostat GRBs. (5) Our
community can only improve on this situation by using collections of bursts
which all have identical definitions for the Epeak calculation.Comment: 25 pages, 2 figures, ApJ accepte
Variability of Optical Counterparts in the Chandra Galactic Bulge Survey
We present optical lightcurves of variable stars consistent with the
positions of X-ray sources identified with the Chandra X-ray Observatory for
the Chandra Galactic Bulge Survey. Using data from the Mosaic-II instrument on
the Blanco 4m Telescope at CTIO, we gathered time-resolved photometric data on
timescales from hr to 8 days over the of the X-ray survey
containing sources from the initial GBS catalog. Among the lightcurve
morphologies we identify are flickering in interacting binaries, eclipsing
sources, dwarf nova outbursts, ellipsoidal variations, long period variables,
spotted stars, and flare stars. of X-ray sources have at least one
potential optical counterpart. of these candidate counterparts are
detectably variable; a much greater fraction than expected for randomly
selected field stars, which suggests that most of these variables are real
counterparts. We discuss individual sources of interest, provide variability
information on candidate counterparts, and discuss the characteristics of the
variable population.Comment: Accepted for publication in the Astrophysical Journal Supplement
The First Pulse of the Extremely Bright GRB 130427A: A Test Lab for Synchrotron Shocks
Gamma-ray burst (GRB) 130427A is one of the most energetic GRBs ever observed. The initial pulse up to 2.5 s is possibly the brightest well-isolated pulse observed to date. A fine time resolution spectral analysis shows power-law decays of the peak energy from the onset of the pulse, consistent with models of internal synchrotron shock pulses. However, a strongly correlated power-law behavior is observed between the luminosity and the spectral peak energy that is inconsistent with curvature effects arising in the relativistic outflow. It is difficult for any of the existing models to account for all of the observed spectral and temporal behaviors simultaneously
The 3rd Fermi GBM Gamma-Ray Burst Catalog: The First Six Years
Since its launch in 2008, the Fermi Gamma-ray Burst Monitor (GBM) has
triggered and located on average approximately two gamma-ray bursts (GRB) every
three days. Here we present the third of a series of catalogs of GRBs detected
by GBM, extending the second catalog by two more years, through the middle of
July 2014. The resulting list includes 1405 triggers identified as GRBs. The
intention of the GBM GRB catalog is to provide information to the community on
the most important observables of the GBM detected GRBs. For each GRB the
location and main characteristics of the prompt emission, the duration, peak
flux and fluence are derived. The latter two quantities are calculated for the
50-300~keV energy band, where the maximum energy release of GRBs in the
instrument reference system is observed, and also for a broader energy band
from 10-1000 keV, exploiting the full energy range of GBM's low-energy NaI(Tl)
detectors. Using statistical methods to assess clustering, we find that the
hardness and duration of GRBs are better fitted by a two-component model with
short-hard and long-soft bursts, than by a model with three components.
Furthermore, information is provided on the settings and modifications of the
triggering criteria and exceptional operational conditions during years five
and six in the mission. This third catalog is an official product of the Fermi
GBM science team, and the data files containing the complete results are
available from the High-Energy Astrophysics Science Archive Research Center
(HEASARC).Comment: 225 pages, 13 figures and 8 tables. Accepted for publication in
Astrophysical Journal Supplement 201
The Behavior of Novae Light Curves Before Eruption
In 1975, E. R. Robinson conducted the hallmark study of the behavior of
classical nova light curves before eruption, and this work has now become part
of the standard knowledge of novae. He made three points; that 5 out of 11
novae showed pre-eruption rises in the years before eruption, that one nova
(V446 Her) showed drastic changes in the variability across eruptions, and that
all but one of the novae (excepting BT Mon) have the same quiescent magnitudes
before and after the outburst. This work has not been tested since it came out.
We have now tested these results by going back to the original archival
photographic plates and measuring large numbers of pre-eruption magnitudes for
many novae using comparison stars on a modern magnitude scale. We find in
particular that four out of five claimed pre-eruption rises are due to simple
mistakes in the old literature, that V446 Her has the same amplitude of
variations across its 1960 eruption, and that BT Mon has essentially unchanged
brightness across its 1939 eruption. Out of 22 nova eruptions, we find two
confirmed cases of significant pre-eruption rises (for V533 Her and V1500 Cyg),
while T CrB has a deep pre-eruption dip. These events are a challenge to
theorists. We find no significant cases of changes in variability across 27
nova eruptions beyond what is expected due to the usual fluctuations seen in
novae away from eruptions. For 30 classical novae plus 19 eruptions from 6
recurrent novae, we find that the average change in magnitude from before the
eruption to long after the eruption is 0.0 mag. However, we do find five novae
(V723 Cas, V1500 Cyg, V1974 Cyg, V4633 Sgr, and RW UMi) that have significantly
large changes, in that the post-eruption quiescent brightness level is over ten
times brighter than the pre-eruption level.Comment: 91 pages (preprint), AJ accepte
The nature of the late achromatic bump in GRB 120326A
The long gamma-ray burst GRB 120326A at redshift exhibited a multi-band light curve with a striking feature: a late-time, long-lasting achromatic rebrightening, rarely seen in such events. Peaking in optical and X-ray bands ks ( ks in the GRB rest frame) after the 70-s GRB prompt burst, the feature brightens nearly two orders of magnitude above the underlying optical power-law decay. Modelling the multiwavelength light curves, we investigate possible causes of the rebrightening in the context of the standard fireball model. We exclude a range of scenarios for the origin of this feature: reverse-shock flash, late-time forward shock peak due to the passage of the maximal synchrotron frequency through the optical band, late central engine optical/X-ray flares, interaction between the expanding blast wave and a density enhancement in the circumburst medium and gravitational microlensing. Instead we conclude that the achromatic rebrightening may be caused by a refreshed forward shock or a geometrical effect. In addition, we identify an additional component after the end of the prompt emission, that shapes the observed X-ray and optical light curves differently, ruling out a single overall emission component to explain the observed early time emission
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