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• 1
Article
In: Publications of the Astronomical Society of Japan, 2009, Vol. 61(3), pp.535-541
Description: We present time-resolved CCD photometry of a dwarf nova, NSV 4838 (UMa 8, SDSS J102320.27 $+$ 440509.8), during 2005 June and 2007 February outbursts. The two light curves showed superhumps with mean periods of 0.0699(1)d for the 2005 outburst and 0.069824(83)d for the 2007 outburst. Using its known orbital period of 0.0678 d, we estimated the mass ratio of the system to be $q$ $=$ 0.13 based on an empirical relation. Although the majority of SU UMa-type dwarf novae having similar superhump periods show negative period derivatives, we found that the superhump period increased at $\dot{P}/P_{\rm sh}$ $=$ $+$ 7( $+$ 3, $-$ 4) $\times$ 10 $^{-5}$ during the 2007 superoutburst. We also investigated long-term light curves of NSV 4838, from which we derived 340 d as a supercycle of this system.
Keywords: Accretion, Accretion Disks ; Stars: Dwarf Novae ; Stars: Individual (Nsv 4838) ; Novae, Cataclysmic Variables
ISSN: 0004-6264
E-ISSN: 2053-051X
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• 2
Article
In: Publications of the Astronomical Society of Japan, 2009, Vol. 61(4), pp.659-674
Description: We investigated superhump evolution, analysing optical photometric observations of the 2000 February–March, the 2002 October–November, and the 2006 September superoutbursts of SW UMa. The superhumps evolved in the same way after their appearance during the 2000 and 2002 superoutbursts, and probably during the 2006 one. This indicates that superhump evolution may be governed by invariable binary parameters. We detected a periodicity in the light curve after the end of the 2000 superoutburst without any phase shift, which seems to be the remains of the superhumps. We found QPOs at the end stage of the 2000 and 2002 superoutbursts, but failed to find extraordinarily large-amplitude QPOs, called ‘super-QPOs’, which previously had been observed in SW UMa.
Keywords: Accretion, Accretion Disks ; Stars: Dwarf Novae ; Stars: Individual (Sw Ursae Majoris) ; Stars: Novae, Cataclysmic Variables
ISSN: 0004-6264
E-ISSN: 2053-051X
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• 3
Article
In: Publications of the Astronomical Society of Japan, 2007, Vol. 59(3), pp.643-651
Description: We report on the time-resolved CCD photometry of four outbursts of a short-period SU UMa-type dwarf nova, V844 Herculis. We successfully determined the mean superhump periods to be 0.05584(64) d, and 0.055883(3) d for a 2002 May superoutburst, and a 2006 April–May superoutburst, respectively. During the 2002 October observations, we confirmed that the outburst was a normal outburst, which was the first recorded normal outburst in V844 Her. We also examined superhump period changes during the 2002 May and 2006 April–May superoutbursts, both of which showed an increasing superhump period over the course of the plateau stage. In order to examine the long-term behavior of V844 Her, we analyzed archival data over the past ten years since the discovery of this binary. Although photometry is not satisfactory for some superoutbursts, we found that V844 Her showed no precursors or rebrightenings. Based on the long-term light curve, we further confirmed that V844 Her has shown almost no normal outbursts despite the fact that the supercycle of the system is estimated to be about 300 d. In order to explain the long-term light curves of V844 Her, evaporation in the accretion disk may play a role in the avoidance of several normal outbursts, which does not contradict with the relatively a large X-ray luminosity of V844 Her.
Keywords: Accretion, Accretion Disks ; Stars: Dwarf Novae ; Stars: Individual (V844 Herculis) ; Novae, Cataclysmic Variables ; Stars: Oscillations
ISSN: 0004-6264
E-ISSN: 2053-051X
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• 4
Article
In: Publications of the Astronomical Society of Japan, 2008, Vol. 60(sp1), pp.S351-S360
Description: We observed an X-ray afterglow of GRB060904A with the Swift and Suzaku satellites. We found rapid spectral softening during both the prompt tail phase and the decline phase of an X-ray flare in the Swift/BAT and Swift/XRT data. The observed spectra were fit by power-law photon indices which rapidly changed from $\Gamma =$ 1.51 $^{+0.04}_{-0.03}$ to $\Gamma =$ 5.30 $^{+0.69}_{-0.59}$ within a few hundred seconds. This is one of the steepest X-ray spectra ever observed, making it quite difficult to explain by simple electron-acceleration and synchrotron-radiation processes. We then applied an alternative broken power-law with exponential cutoff (BPEC) model. It is valid to consider the exponential shape is equivalent to a synchrotron cutoff. Since the spectral cutoff appears in the soft X-ray band, we conclude that the electron acceleration must be inefficient in the internal shocks of GRB060904A. These cutoff spectra suddenly disappeared at the end of the prompt tail. After that, typical afterglow spectra with $\Gamma =$ 2.0 have been continuously and preciously monitored by both Swift/XRT and Suzaku/XIS. We could successfully trace the temporal history of two characteristic break energies that have a time dependence of $\propto t^{-3}$ - $t^{-4}$ , while the following afterglow spectra are quite stable. This fact indicates that the emitting material of the prompt tail is due to completely different dynamics from the shallow decay component. Therefore, the emission sites of two distinct phenomena obviously differ from each other.
Keywords: Gamma - Ray Burst: General ; Radiation Mechanisms: Non - Thermal ; Relativistic Jet ; X - Rays: Individual (Grb060904a) ; X - Rays: Stars Acceleration Of Particles
ISSN: 0004-6264
E-ISSN: 2053-051X
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• 5
Article
In: Publications of the Astronomical Society of Japan, 2008, Vol. 60(5), pp.1151-1158
Description: We report on time-resolved CCD photometry of the cataclysmic variable EG Aquarii during a 2006 November outburst. During the outburst, superhumps were unambiguously detected with a mean period of 0.078828(6) d, allowing for the first time to classify the object as an SU UMa-type dwarf nova. It also turned out that the outburst contained a precursor. At the end of the precursor, immature profiles of humps were observed. By a phase analysis of these humps, we interpreted the features as being superhumps. This is the second example that the superhumps were shown during a precursor. Near the maximum stage of the outburst, we discovered an abrupt shift of the superhump period by ${\sim\;}$ 0.002 d. After the supermaximum, the superhump period decreased at a rate of $\dot{P}/P =$ $-$ 8.2 ${\;\times\;}$ 10 $^{-5}$ , which is typical for SU UMa-type dwarf novae. Although the outburst light curve was characteristic of SU UMa-type dwarf novae, long-term monitoring of the variable shows no outbursts over the past decade. We also comment on the basic properties of long period and inactive SU UMa-type dwarf novae.
Keywords: Accretion, Accretion Disks ; Stars: Dwarf Novae ; Stars: Individual (Eg Aquarii) ; Novae, Cataclysmic Variables ; Stars: Oscillations
ISSN: 0004-6264
E-ISSN: 2053-051X
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• 6
Article
Description: We report on time-resolved CCD photometry of four outbursts of a short-period SU UMa-type dwarf nova, V844 Herculis. We successfully determined the mean superhump periods to be 0.05584(64) days, and 0.055883(3) for the 2002 May superoutburst, and the 2006 April-May superoutburst, respectively. During the 2002 October observations, we confirmed that the outburst is a normal outburst, which is the first recorded normal outburst in V844 Her. We also examined superhump period changes during 2002 May and 2006 April-May superoutbursts, both of which showed increasing superhump period over the course of the plateau stage. In order to examine the long-term behavior of V844 Her, we analyzed archival data over the past ten years since the discovery of this binary. Although photometry is not satisfactory in some superoutbursts, we found that V844 Her showed no precursors and rebrightenings. Based on the long-term light curve, we further confirmed V844 Her has shown almost no normal outbursts despite the fact that the supercycle of the system is estimated to be about 300 days. In order to explain the long-term light curves of V844 Her, evaporation in the accretion disk may play a role in the avoidance of several normal outbursts, which does not contradict with the relatively large X-ray luminosity of V844 Her. Comment: 10 pages, 11 figures, accepted for PASJ
Keywords: Astrophysics
Source: Cornell University
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• 7
Article
In: Publications of the Astronomical Society of Japan, 2009, Vol. 61(sp2), pp.S395-S616
Description: We systematically surveyed period variations of superhumps in SU UMa-type dwarf novae based on newly obtained data and past publications. In many systems, the evolution of the superhump period is found to be composed of three distinct stages: an early evolutionary stage with a longer superhump period, a middle stage with systematically varying periods, and a final stage with a shorter, stable superhump period. During the middle stage, many systems with superhump periods of less than 0.08 d show positive period derivatives. We present observational characteristics of these stages and give greatly improved statistics. Contrary to an earlier claim, we found no clear evidence for a variation of period derivatives among different superoutbursts of the same object. We present an interpretation that the lengthening of the superhump period is a result of the outward propagation of an eccentricity wave, which is limited by the radius near the tidal truncation. We interpret that late-stage superhumps are rejuvenated excitation of a 3:1 resonance when superhumps in the outer disk are effectively quenched. The general behavior of the period variation, particularly in systems with short orbital periods, appears to follow a scenario proposed in Kato, Maehara, and Monard ( 2008 , PASJ, 60, L23). We also present an observational summary of WZ Sge-type dwarf novae. Many of them have shown long-enduring superhumps during a post-superoutburst stage having longer periods than those during the main superoutburst. The period derivatives in WZ Sge-type dwarf novae are found to be strongly correlated with the fractional superhump excess, or consequently with the mass ratio. WZ Sge-type dwarf novae with a long-lasting rebrightening or with multiple rebrightenings tend to have smaller period derivatives, and are excellent candidates for those systems around or after the period minimum of evolution of cataclysmic variables.
Keywords: Accretion, Accretion Disks ; Stars: Dwarf Novae ; Novae, Cataclysmic Variables
ISSN: 0004-6264
E-ISSN: 2053-051X
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