We present the results of an analysis of Kepler data covering 1.5 yr of the dwarf nova V447 Lyr. We detect eclipses of the accretion disc by the mass donating secondary star every 3.74 h which is the ...binary orbital period. V447 Lyr is therefore the first dwarf nova in the Kepler field to show eclipses.We also detect five long outbursts and six short outbursts showing V447 Lyr is a U Gem-type dwarf nova. We show that the orbital phase of the mid-eclipse occurs earlier during outbursts compared to quiescence and that the width of the eclipse is greater during outburst. This suggests that the bright spot is more prominent during quiescence and that the disc is larger during outburst than quiescence. This is consistent with an expansion of the outer disc radius due to the presence of high viscosity material associated with the outburst, followed by a contraction in quiescence due to the accretion of low angular momentum material. We note that the long outbursts appear to be triggered by a short outburst, which is also observed in the super-outbursts of SU UMa dwarf novae as observed using Kepler.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
High-energy transients Gehrels, Neil; Cannizzo, John K.
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
06/2013, Volume:
371, Issue:
1992
Journal Article
Peer reviewed
Open access
We present an overview of high-energy transients in astrophysics, highlighting important advances over the past 50 years. We begin with early discoveries of γ-ray transients, and then delve into ...physical details associated with a variety of phenomena. We discuss some of the unexpected transients found by Fermi and Swift, many of which are not easily classifiable or in some way challenge conventional wisdom. These objects are important insofar as they underscore the necessity of future, more detailed studies.
Full text
Available for:
BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
We search the American Association of Variable Star Observers (AAVSO) archives of the two best-studied dwarf novae in an attempt to find light curves for long outbursts that are extremely well ...characterized. The systems are U Gem and SS Cyg. Our goal is to search for embedded precursors such as those that have been found recently in the high-fidelity Kepler data for superoutbursts (SOs) of some members of the SU UMa subclass of dwarf novae. For the vast majority of AAVSO data, the combination of low data cadence and large errors associated with individual measurements precludes one from making any strong statement about the shape of the long outbursts. However, for a small number of outbursts, extensive long-term monitoring with digital photometry yields high-fidelity light curves. We report the discovery of embedded precursors in two of three candidate long outbursts. This is the first time that such embedded precursors have been found in dwarf novae above the period gap in other than kepler data, and reinforces van Paradijs' finding that long outbursts in dwarf novae above the period gap and SOs in systems below the period gap constitute a unified class. The thermal-tidal instability to account for SOs in the SU UMa stars predicts embedded precursors only for short orbital period dwarf novae, therefore the presence of embedded precursors in long orbital period systems-U Gem and SS Cyg-argues for a more general mechanism to explain long outbursts.
Ozone Depletion from Nearby Supernovae Gehrels, Neil; Laird, Claude M; Jackman, Charles H ...
Astrophysical journal/The Astrophysical journal,
03/2003, Volume:
585, Issue:
2
Journal Article
The timing capabilities and sensitivity of Kepler, NASA's observatory to find Earth-sized planets within the habitable zone of stars, are well matched to the timescales and amplitudes of accretion ...disk variability in cataclysmic variables. This instrumental combination provides an unprecedented opportunity to test and refine stellar accretion paradigms with high-precision, uniform data, and containing none of the diurnal or season gaps that limit ground-based observations. We present a 3 month, 1 minute cadence Kepler light curve of V344 Lyr, a faint, little-studied dwarf nova within the Kepler field. The light curve samples V344 Lyr during five full normal outbursts and one superoutburst. Surprisingly, the superhumps found during the superoutburst continue to be detected during the following quiescent state and normal outburst. The fractional excess of the superhump period over the presumed orbital period suggests a relatively high binary mass ratio in a system where the radius of the accretion disk must vary by less than 2% in order to maintain tidal precession throughout the extended episode of superhumping. The disk radius is less restricted if the quiescent signal identified tentatively as the orbital period is a negative superhump, generated by a retrograde-precessing accretion disk, tilted with respect to the binary orbital plane.
BOKS 45906 was found to be a blue source in the Burrell-Optical-Kepler Survey which showed a 3 mag outburst lasting ∼5 d. We present the Kepler light curve of this source which covers nearly 3 years. ...We find that it is in a faint optical state for approximately half the time and shows a series of outbursts separated by distinct dips in flux. Using data with 1 min sampling, we find clear evidence that in its low state BOKS 45906 shows a flux variability on a period of 56.5574 ± 0.0014 min and a semi-amplitude of ∼3 per cent. Since we can phase all the 1 min cadence data on a common ephemeris using this period, it is probable that 56.56 min is the binary orbital period. Optical spectra of BOKS 45906 show the presence of Balmer lines in emission indicating it is not an AM CVn (pure Helium) binary. Swift data show that it is a weak X-ray source and is weakly detected in the bluest of the UVOT filters. We conclude that BOKS 45906 is a cataclysmic variable with a period shorter than the 'period-bounce' systems and therefore BOKS 45906 could be the first helium-rich cataclysmic variable detected in the Kepler field.
We present the first unrestricted, three-dimensional relativistic hydrodynamical calculations of the blob of gas associated with the jet that produces a gamma-ray burst. We investigate the ...deceleration phase of the blob, which corresponds to the time when afterglow radiation is produced, concentrating on the transition in which the relativistic beaming gamma super(-1) goes from being less than theta , where gamma is the bulk Lorentz factor and theta is the angular width of the jet, to gamma super(-1) being greater than theta . We study the time-dependent evolution of the physical parameters associated with the jet, both parallel to the direction of motion and perpendicular to it. We calculate light curves for observers at varying angles with respect to the velocity vector of the blob, assuming optically thin emission that scales with the local pressure. Our main findings are that (1) gas ahead of the advancing blob does not accrete onto and merge with the blob material but rather flows around the blob, (2) the decay light curve steepens at a time corresponding roughly to gamma super(-1) ~ theta (in accord with earlier studies), and (3) the rate of decrease of the forward component of momentum in the blob is well fitted by a simple model in which the gas in front of the blob exerts a drag force on the blob and the cross-sectional area of the blob increases quadratically with laboratory time (or distance).
Neil Gehrels 1952–2017 Cannizzo, John K
Astronomy & geophysics : the journal of the Royal Astronomical Society,
06/2017, Volume:
58, Issue:
3
Journal Article
Peer reviewed
Neil Gehrels.
The high-energy astrophysicist, musician, adventurer and Swift PI is remembered by John K Cannizzo.
PDF
