373 research outputs found
A dynamic model of filament eruptions and two ribbon flares
Two basically different models for the filament equilibrium by Kippenhahn and Schluter (1957) and Kuperus and Raadu (1974) have appeared in the literature. A further analyses by van Tend and Kuperus (1978) added the force due to the horizontal component of the background field to the Kuperus and Raadu model. In order to obtain a better model which actually describes these phenomena, the evolution of the filament has to be considered in detail. A first attempt was recently presented by Kaastra. Kaastra did not formulate the precise energy balance equations for the problem, as is done in the present work. In the present model not only the force balance, but also the energy balance of the filament is taken into account. Thus a fully closed system of equations is obtained, that describes the evolution of the filament, first in force equilibrium during the current build-up phase, then in the non-equilibrium phase before the eruption, and the eruption itself. A neutral point appears above the photospheric surface in the non-equilibrium phase, but long before the eruption. It was found that although the filament itself may be in non-equilibrium, the evolution may still be slow up to the height where the eruption takes place. The eruption of the filament itself causes a large induced electric field at the neutral point which leads to the observed flare phenomena
The measurement errors in the Swift-UVOT and XMM-OM
The probability of photon measurement in some photon counting
instrumentation, such as the Optical Monitor on the XMM-Newton satellite, and
the UVOT on the Swift satellite, does not follow a Poisson distribution due to
the detector characteristics, but a Binomial distribution. For a single-pixel
approximation, an expression was derived for the incident countrate as a
function of the measured count rate by Fordham, Moorhead and Galbraith (2000).
We show that the measured countrate error is binomial, and extend their
formalism to derive the error in the incident count rate. The error on the
incident count rate at large count rates is larger than the Poisson-error of
the incident count rate.Comment: 4 pages, 2 postscript figures, submitted to MNRA
Swift UVOT Grism Observations of Nearby Type Ia Supernovae - I. Observations and Data Reduction
Ultraviolet (UV) observations of Type Ia supernovae (SNe Ia) are useful tools
for understanding progenitor systems and explosion physics. In particular, UV
spectra of SNe Ia, which probe the outermost layers, are strongly affected by
the progenitor metallicity. In this work, we present 120 Neil Gehrels Swift
Observatory UV spectra of 39 nearby SNe Ia. This sample is the largest UV
(lambda < 2900 A) spectroscopic sample of SNe Ia to date, doubling the number
of UV spectra and tripling the number of SNe with UV spectra. The sample spans
nearly the full range of SN Ia light-curve shapes (delta m(B) ~ 0.6-1.8 mag).
The fast turnaround of Swift allows us to obtain UV spectra at very early
times, with 13 out of 39 SNe having their first spectra observed >~ 1 week
before peak brightness and the earliest epoch being 16.5 days before peak
brightness. The slitless design of the Swift UV grism complicates the data
reduction, which requires separating SN light from underlying host-galaxy light
and occasional overlapping stellar light. We present a new data-reduction
procedure to mitigate these issues, producing spectra that are significantly
improved over those of standard methods. For a subset of the spectra we have
nearly simultaneous Hubble Space Telescope UV spectra; the Swift spectra are
consistent with these comparison data.Comment: Accepted for publication in MNRA
An Updated Ultraviolet Calibration for the Swift/UVOT
We present an updated calibration of the Swift/UVOT broadband ultraviolet
(uvw1, uvm2, and uvw2) filters. The new calibration accounts for the ~1% per
year decline in the UVOT sensitivity observed in all filters, and makes use of
additional calibration sources with a wider range of colours and with HST
spectrophotometry. In this paper we present the new effective area curves and
instrumental photometric zeropoints and compare with the previous calibration.Comment: 4 pages, 3 figures, 2 tables. Presented at GRB 2010 symposium,
Annapolis, November 2010 to be published in American Institute of Physics
Conference Serie
A panchromatic analysis of starburst galaxy M82: Probing the dust properties
(Abridged) We combine NUV, optical and IR imaging of the nearby starburst
galaxy M82 to explore the properties of the dust both in the interstellar
medium of the galaxy and the dust entrained in the superwind. The three NUV
filters of Swift/UVOT enable us to probe in detail the properties of the
extinction curve in the region around the 2175A bump. The NUV colour-colour
diagram strongly rules out a Calzetti-type law, which can either reflect
intrinsic changes in the dust properties or in the star formation history
compared to starbursts well represented by such an attenuation law. We
emphasize that it is mainly in the NUV region where a standard Milky-Way-type
law is preferred over a Calzetti law. The age and dust distribution of the
stellar populations is consistent with the scenario of an encounter with M81 in
the recent 400 Myr. The radial gradients of the NUV and optical colours in the
superwind region support the hypothesis that the emission in the wind cone is
driven by scattering from dust grains entrained in the ejecta. The observed
wavelength dependence reveals either a grain size distribution , where is the size of the grain, or a flatter distribution with a
maximum size cutoff, suggesting that only small grains are entrained in the
supernovae-driven wind.Comment: 12 pages, 12 figures, 3 tables, MNRAS, in pres
Increasing activity in T CrB suggests nova eruption is impending
Estimates of the accretion rate in symbiotic recurrent novae (RNe) often fall
short of theoretical expectations by orders of magnitude. This apparent
discrepancy can be resolved if the accumulation of mass by the white dwarf (WD)
is highly sporadic, and most observations are performed during low states. Here
we use a reanalysis of archival data from the Digital Access to a Sky Century
@Harvard (DASCH) survey to argue that the most recent nova eruption in
symbiotic RN T CrB, in 1946, occurred during -- and was therefore triggered by
-- a transient accretion high state. Based on similarities in the optical light
curve around 1946 and the time of the prior eruption, in 1866, we suggest that
the WD in T CrB accumulates most of the fuel needed to ignite the thermonuclear
runaways (TNRs) during accretion high states. A natural origin for such states
is dwarf-nova like accretion-disk instabilities, which are expected in the
presumably large disks in symbiotic binaries. The timing of the TNRs in
symbiotic RNe could thus be set by the stability properties of their accretion
disks. T CrB is in the midst of an accretion high state like the ones we posit
led to the past two nova eruptions. Combined with the approach of the time at
which a TNR would be expected based on the 80-year interval between the prior
two novae (3), the current accretion high state increases the
likelihood of a TNR occurring in T CrB in the next few years.Comment: Accepted in ApJ
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