20 research outputs found
Does The Addition of a Duration Improve the L_iso - E_peak Relation For Gamma-Ray Bursts?
Firmani et al. proposed a new Gamma Ray Burst (GRB) luminosity relation that
showed a significant improvement over the L_iso-E_peak relation. The new
proposed relation simply modifies the E_peak value by multiplying it by a power
of T_0.45, where T_0.45 is a particular measure of the GRB duration. We begin
by reproducing the results of Firmani for his 19 bursts. We then test the
Firmani relation for the same 19 bursts except that we use independently
measured values for L_iso, T_0.45, and E_peak, and we find that the relation
deteriorates substantially. We further test the relation by using 60 GRBs with
measured spectroscopic redshifts, and find a relation that has a comparable
scatter as the original L_iso-E_peak relation. That is, a much larger sample of
bursts does not reproduce the small scatter as reported by Firmani et al.
Finally, we investigate whether the Firmani relation is improved by the use of
any of 32 measures of duration in place of T_0.45. The quality of each
alternative duration measure is evaluated with the root mean square of the
scatter between the observed and fitted logarithmic Liso values. Although we
find some durations yield slightly better results than T_0.45, the differences
between the duration measures are minimal. We find that the addition of a
duration does not add any significant improvement to the L_iso-E_peak relation.
We also present a simple and direct derivation of the Firmani relation from
both the L_iso-E_peak and Amati relations. In all we conclude that the Firmani
relation neither has an independent existence nor does it provide any
significant improvement on previously known relations that are simpler.Comment: ApJ in press, 17 pages, 3 figures, 3 table
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
A significant problem with using the Amati relation for cosmological purposes
We consider the distribution of many samples of gamma-ray bursts when plotted in a diagram with their bolometric fluence (Sbolo) versus the observed photon energy of peak spectral flux (E peak, obs). In this diagram, all bursts that obey the Amati relation (a luminosity relation where the total burst energy has a power-law relation to E peak, obs) must lie above some limiting line, although observational scatter is expected to be substantial. We confirm that early bursts with spectroscopic redshifts are consistent with this Amati limit. But we find that the bursts from BATSE, Swift, Suzaku, and Konus are all greatly in violation of the Amati limit, and this is true whether or not the bursts have measured spectroscopic redshifts. That is, the Amati relation has definitely failed. In the S bolo-E peak, obs diagram, wefind that every satellite has a greatly different distribution. This requires that selection effects are dominating these distributions, which we quantitatively identify. For detector selections, the trigger threshold and the threshold for the burst to obtain a measured E peak, obs combine to make a diagonal cutoff with the position of this cutoff varying greatly detector to detector. For selection effects due to the intrinsic properties of the burst population, the distribution of E peak, obs makes bursts with low and high values rare, while the fluence distribution makes bright bursts relatively uncommon. For a detector with a high threshold, the combination of these selection effects serves to allow only bursts within a region along the Amati limit line to be measured, and these bursts will then appear to follow an Amati relation. Therefore, the Amati relation is an artifact of selection effects within the burst population and the detector. As such, the Amati relation should not be used for cosmological tasks. This failure of the Amati relation is in no way prejudicial against the other luminosity relations. © 2012. The American Astronomical Society. All rights reserved
Generalized Tests for Eight GRB Luminosity Relations
Long duration Gamma-Ray Bursts (GRBs) have eight luminosity relations where
observable burst properties can yield the burst luminosity and hence distance.
This turns GRBs into useful tools of cosmology. Recently, two tests have been
proposed (by Nakar & Piran and by Li) for which one of the eight relations is
claimed to have significant problems. In this paper, we generalize these tests
and apply them to all eight GRB luminosity relations. (a) All eight relations
pass the Nakar & Piran test after accounting for the uncertainties on the data
and the dispersions of the correlations. (b) All eight relations are good when
the GRB redshifts are known, for example for calibration of the relations and
for GRB Hubble diagram purposes. (c) We confirm the earlier results that the
E_gamma,iso - E_peak Amati relation must produce very large error bars whenever
an unknown redshift being sought is >1.4. (d) The E_gamma - E_peak relation of
Ghirlanda et al. must produce very large error bars whenever an unknown
redshift being sought is >3.4. (e) The other six relations have no problem at
all from the ambiguity test of Li.Comment: Ap.J. Letters in press, 11 page
Discovery of a Second Nova Eruption of V2487 Ophiuchi
A directed search for previously-undiscovered nova eruptions was conducted in
the astronomical plate archives at Harvard College Observatory and Sonneberg
Observatory. We found that an eruption of V2487 Oph (Nova Oph 1998) occurred on
1900 June 20. V2487 Oph was previously classified as a classical nova, which we
identified as a probable recurrent nova based on its large expansion velocities
and the presence of high excitation lines in the outburst spectrum. The event
was recorded on Harvard plate AM 505, at a B magnitude of 10.27 +/- 0.11, which
is near peak. The outburst can only be seen on one plate, but the image has a
characteristic dumbbell shape (caused by a double exposure) that is identical
to the other star images on the plate, and thus is not a plate defect. We
conclude that this is in fact a previously-undiscovered nova outburst of V2487
Oph, confirming our prediction that it is a recurrent nova. We also examine the
discovery efficiency for eruptions of the system and conclude that a
randomly-timed outburst has, on average, a 30% chance of being discovered in
the past century. Using this, we deduce a recurrence time for V2487 Oph of
approximately 18 years, which implies that the next eruption is expected around
2016.Comment: 18 pages, 2 figures, to be published in the Astronomical Journa
Novae With Long-Lasting Supersoft Emission That Drive a High Accretion Rate
We identify a new class of novae characterized by the post-eruption quiescent
light curve being more than roughly a factor of ten brighter than the
pre-eruption light curve. Eight novae (V723 Cas, V1500 Cyg, V1974 Cyg, GQ Mus,
CP Pup, T Pyx, V4633 Sgr, and RW UMi) are separated out as being significantly
distinct from other novae. This group shares a suite of uncommon properties,
characterized by the post-eruption magnitude being much brighter than before
eruption, short orbital periods, long-lasting supersoft emission following the
eruption, a highly magnetized white dwarf, and secular declines during the
post-eruption quiescence. We present a basic physical picture which shows why
all five uncommon properties are causally connected. Most novae do not have
adequate accretion for continuous hydrogen burning, but some can achieve this
if the companion star is nearby (with short orbital period) and a magnetic
field channels the matter onto a small area on the white dwarf so as to produce
a locally high accretion rate. The resultant supersoft flux irradiates the
companion star and drives a higher accretion rate (with a brighter
post-eruption phase), which serves to keep the hydrogen burning and the
supersoft flux going. The feedback loop cannot be perfectly self-sustaining, so
the supersoft flux will decline over time, forcing a decline in the accretion
rate and the system brightness. We name this new group after the prototype,
V1500 Cyg. V1500 Cyg stars are definitely not progenitors of Type Ia
supernovae. The V1500 Cyg stars have similar physical mechanisms and
appearances as predicted for nova by the hibernation model, but with this group
accounting for only 14% of novae.Comment: Astronomical Journal, in press, 39 pages, 10 figure