355 research outputs found
Discovery of the 2010 Eruption and the Pre-Eruption Light Curve for Recurrent Nova U Scorpii
We report the discovery by B. G. Harris and S. Dvorak on JD 2455224.9385
(2010 Jan 28.4385 UT) of the predicted eruption of the recurrent nova U Scorpii
(U Sco). We also report on 815 magnitudes (and 16 useful limits) on the
pre-eruption light curve in the UBVRI and Sloan r' and i' bands from 2000.4 up
to 9 hours before the peak of the January 2010 eruption. We found no
significant long-term variations, though we did find frequent fast variations
(flickering) with amplitudes up to 0.4 mag. We show that U Sco did not have any
rises or dips with amplitude greater than 0.2 mag on timescales from one day to
one year before the eruption. We find that the peak of this eruption occurred
at JD 2455224.69+-0.07 and the start of the rise was at JD 2455224.32+-0.12.
From our analysis of the average B-band flux between eruptions, we find that
the total mass accreted between eruptions is consistent with being a constant,
in agreement with a strong prediction of nova trigger theory. The date of the
next eruption can be anticipated with an accuracy of +-5 months by following
the average B-band magnitudes for the next ~10 years, although at this time we
can only predict that the next eruption will be in the year 2020+-2.Comment: Astronomical Journal submitted, 36 pages, 3 figures, full table
On Orbital Period Changes in Nova Outbursts
We propose a new mechanism that produces an orbital period change during a
nova outburst. When the ejected material carries away the specific angular
momentum of the white dwarf, the orbital period increases. A magnetic field on
the surface of the secondary star forces a fraction of the ejected material to
corotate with the star, and hence the binary system. The ejected material thus
takes angular momentum from the binary orbit and the orbital period decreases.
We show that for sufficiently strong magnetic fields on the surface of the
secondary star, the total change to the orbital period could even be negative
during a nova outburst, contrary to previous expectations. Accurate
determinations of pre- and post-outburst orbital periods of recurrent nova
systems could test the new mechanism, in addition to providing meaningful
constraints on otherwise difficult to measure physical quantities. We apply our
mechanism to outbursts of the recurrent nova U Sco.Comment: Accepted for publication in MNRA
Gamma-Ray Burster Counterparts: HST Blue and Ultraviolet Data
The surest solution of the Gamma Ray Burst (GRB) mystery is to find an
unambiguous low-energy quiescent counterpart. However, to date no reasonable
candidates have been identified in the x-ray, optical, infrared, or radio
ranges. The Hubble Space Telescope (HST) has now allowed for the first deep
ultraviolet searches for quiescent counterparts. This paper reports on
multiepoch ultraviolet searches of five GRB positions with HST. We found no
sources with significant ultraviolet excesses, variability, parallax, or proper
motion in any of the burst error regions. In particular, we see no sources
similar to that proposed as a counterpart to the GRB970228. While this negative
result is disappointing, it still has good utility for its strict limits on the
no-host-galaxy problem in cosmological models of GRBs. For most cosmological
models (with peak luminosity 6X10^50 erg/s), the absolute B magnitude of any
possible host galaxy must be fainter than -15.5 to -17.4. These smallest boxes
for some of the brightest bursts provide the most critical test, and our limits
are a severe problem for all published cosmological burst models.Comment: 15 pages, 2 ps figures, accepted for publication in the Astrophysical
Journa
Can Light Echoes Account for the Slow Decay of Type IIn Supernovae?
The spectra of type IIn supernovae indicate the presence of apre-existing
slow, dense circumstellar wind (CSW). If the CSW extends sufficiently far from
the progenitor star, then dust formation should occur in the wind. The light
from the supernova explosion will scatter off this dust and produce a light
echo. Continuum emission seen after the peak will have contributions from both
this echo as well as from the shock of the ejecta colliding with the CSW, with
a fundamental question of which source dominates the continuum. We calculate
the brightness of the light echo as a function of time for a range of dust
shell geometries, and use our calculations to fit to the light curves of SN
1988Z and SN 1997ab, the two slowest declining IIn supernovae on record. We
find that the light curves of both objects can be reproduced by the echo model.
However, their rate of decay from peak, color at peak and their observed peak
absolute magnitudes when considered together are inconsistent with the echo
model. Furthermore, when the observed values of M are corrected for the
effects of dust scattering, the values obtained imply that these supernovae
have unrealistically high luminosities. We conclude that light echoes cannot
properly account for the slow decline seen in some IIn's, and that the shock
interaction is likely to dominate the continuum emission.Comment: 15 pages, 9 figure
Superflares on Ordinary Solar-Type Stars
Short duration flares are well known to occur on cool main-sequence stars as
well as on many types of `exotic' stars. Ordinary main-sequence stars are
usually pictured as being static on time scales of millions or billions of
years. Our sun has occasional flares involving up to ergs which
produce optical brightenings too small in amplitude to be detected in
disk-integrated brightness. However, we identify nine cases of superflares
involving to ergs on normal solar-type stars. That is,
these stars are on or near the main-sequence, are of spectral class from F8 to
G8, are single (or in very wide binaries), are not rapid rotators, and are not
exceedingly young in age. This class of stars includes many those recently
discovered to have planets as well as our own Sun, and the consequences for any
life on surrounding planets could be profound. For the case of the Sun,
historical records suggest that no superflares have occurred in the last two
millennia.Comment: 16 pages, accepted for publication in Ap
Macrocerebellum: Neuroimaging and Clinical Features of a Newly Recognized Condition
Other than hamartomatous enlargement of the cerebellum as in Lhermitte-Duclos syndrome, diffuse enlargement of the cerebellum is not clearly described. We report four patients (ages 9 months to 2 years) with diffusely enlarged cerebelli as identified by measurement of the cerebellum and comparison to age appropriate normal values. The cerebellar measurements were determined in absolute numbers and expressed as ratios of cerebellum to whole brain and supratentorial brain. The clinical features of these four children (3 boys, 1 girl) consistently include global developmental delay, tone abnormalities, preserved reflexes, delayed or abnormal maturation of the visual system (oculomotor apraxia), and deficient or delayed myelination of cerebral white matter. The etiology of the macrocerebellum is unknown but we propose that the cerebellum is responding to the elaboration of growth factors intended to augment the slow development of cerebral structures. Regardless of the etiology, the finding of a macrocerebellum appears to allow the clinician to predict the clinical features of the patient and probably represents a marker for disturbed cerebral development. (J Child Neurol 1997;12:365-368).Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline
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
The 2011 Eruption of the Recurrent Nova T Pyxidis; the Discovery, the Pre-eruption Rise, the Pre-eruption Orbital Period, and the Reason for the Long Delay
We report the discovery by M. Linnolt on JD 2455665.7931 (UT 2011 April
14.29) of the sixth eruption of the recurrent nova T Pyxidis. This discovery
was made just as the initial fast rise was starting, so with fast notification
and response by observers worldwide, the entire initial rise was covered (the
first for any nova), and with high time resolution in three filters. The speed
of the rise peaked at 9 mag/day, while the light curve is well fit over only
the first two days by a model with a uniformly expanding sphere. We also report
the discovery by R. Stubbings of a pre-eruption rise starting 18 days before
the eruption, peaking 1.1 mag brighter than its long-time average, and then
fading back towards quiescence 4 days before the eruption. This unique and
mysterious behavior is only the fourth known anticipatory rise closely spaced
before a nova eruption. We present 19 timings of photometric minima from 1986
to February 2011, where the orbital period is fast increasing with
P/dot{P}=313,000 yrs. From 2008-2011, T Pyx had a small change in this rate of
increase, so that the orbital period at the time of eruption was
0.07622950+-0.00000008 days. This strong and steady increase of the orbital
period can only come from mass transfer, for which we calculate a rate of
1.7-3.5x10^-7 Mo/yr. We report 6116 magnitudes between 1890 and 2011, for an
average B=15.59+-0.01 from 1967-2011, which allows for an eruption in 2011 if
the blue flux is nearly proportional to the accretion rate. The
ultraviolet-optical-infrared spectral energy distribution is well fit by a
power law with flux proportional to nu^1.0, although the narrow ultraviolet
region has a tilt with a fit of \nu^{1/3}. We prove that most of the T Pyx
light is not coming from a disk, or any superposition of blackbodies, but
rather is coming from some nonthermal source.Comment: ApJ submitted, 62 pages, 8 figures; much added data, updated analysi
Are superflares on solar analogues caused by extra-solar planets?
Stellar flares with times more energy than the largest solar
flare have been detected from 9 normal F and G main sequence stars (Schaefer,
King & Deliyannis 1999). These superflares have durations of hours to days and
are visible from at least x-ray to optical frequencies. The absence of
world-spanning aurorae in historical records and of anomalous extinctions in
the geological record indicate that our Sun likely does not suffer superflares.
In seeking to explain this new phenomenon, we are struck by its similarity to
large stellar flares on RS Canum Venaticorum binary systems, which are caused
by magnetic reconnection events associated with the tangling of magnetic fields
between the two stars. The superflare stars are certainly not of this class,
although we propose a similar flare mechanism. That is, superflares are caused
by magnetic reconnection between fields of the primary star and a close-in
Jovian planet. Thus, by only invoking known planetary properties and
reconnection scenarios, we can explain the energies, durations, and spectra of
superflares, as well as explain why our Sun does not have such events.Comment: 13 pages, Accepted for publication in Ap
- …