Outburst Duration Research Articles

1993 Superoutburst of LL Andromedae

Abstract We present time-resolved CCD photometry of LL And during its 1993 outburst. The observation revealed the presence of superhumps with a period of 0.05697(3)d. This period is one of the smallest among the hydrogen-rich dwarf novae. Although LL And has been proposed to be a WZ Sge-type dwarf nova based on its low outburst frequency, our new analysis indicates that the outburst amplitude ($\sim 5 \,\mathrm{mag}$) and outburst duration ($9\pm 2 \,\mathrm{d}$) are much smaller and shorter than in typical WZ Sge-type dwarf novae. We suspect that the unusual outburst properties of LL And might be explained by assuming a “leaky disk” in quiescence, which was originally proposed to explain the prototypical WZ Sge-type outbursts. By combining with the recent suggestion of the orbital period, the fractional superhump excess is found to be 3.5(1)%, which is unusually large for this short-period system. LL And may be an object filling the gap in the evolutionary track, which has recently been proposed to explain the unusual ultracompact binaries with an evolved mass donor.

Radiation Pressure Instability Driven Variability in the Accreting Black Holes

The time-dependent evolution of the accretion disk around the black hole is computed. The classical description of the α-viscosity is adopted so the evolution is driven by the instability operating in the innermost radiation pressure-dominated part of the accretion disk. We assume that the optically thick disk always extends down to the marginally stable orbit, so it is never evacuated completely. We include the effect of the advection, coronal dissipation, and vertical outflow. We show that the presence of the corona and/or the outflow reduces the amplitude of the outburst. If only about half of the energy is dissipated in the disk (with the other half dissipated in the corona and carried away by the outflow), the outburst amplitude and duration are consistent with observations of the microquasar GRS 1915+105. Viscous evolution explains in a natural way the lack of direct transitions from the state C to the state B in the color-color diagram of this source. Further reduction of the fraction of energy dissipated in the optically thick disk switches off the outbursts, which may explain why they are not seen in all high accretion rate sources being in the very high state.

Statistical analysis of the long-term visual light curve parameters of dwarf novae

The long-term visual light curve parameters of 36 dwarf novae are measured. The data is anayzed in search of likely relationships among the light curve parameters and system parameters. New relations of the orbital period with the mean light curve parameters are given. Our ndings for the correlation analysis of individual systems agree with some of the results in previous studies. Almost all of the systems in this study have a bimodal frequency distribution of the outburst duration. By comparing the observational correlations with their theoretical counterparts, we nd that the value of the viscosity parameter of the disk instability model is hot 0:2.

Accretional Heating by Periodic Dwarf Nova Events

We have carried out the first quasi-static evolutionary model sequence with time-variable accretion in which the combined heating effect of boundary layer irradiation (including stellar rotation) and compressional heating on the underlying white dwarf accretor has been included. In the present study, we focus mainly on the behavior of the surface observables of the accreting white dwarf. This initial exploration followed the thermal evolution of a 0.6 M☉ white dwarf in a dwarf nova over many dwarf nova accretion cycles. Accretion rates of ≈10-8 M☉ yr-1, for outburst duration of days to weeks, are followed by a shutoff of the radial infall during dwarf nova quiescence. The matter is assumed to accrete softly with the same entropy as the white dwarf outer layers, but a fraction of the energy liberated in the boundary layer is assumed to be absorbed by the outer layer of the star (boundary layer irradiation). Accretion is resumed and shut off repeatedly at intervals of months to simulate the thermal evolution of the white dwarf in typical dwarf novae. The timescale of the white dwarf cooling is such that after a complete (outburst + quiescence) cycle, the surface of the white dwarf has not yet completely cooled down: the star has not yet readjusted and does not reach thermal equilibrium. When the evolution of the white dwarf is followed for 125 cycles (about 8 yr), the effective temperature (Teff) of the underlying white dwarf increases as a function of time (t) as log(Teff) = a log(t), where a > 0. We find that a itself decreases with increasing temperature. The inclusion of the boundary layer irradiation has no detectable effect on this particular result over the timescale studied here. When the boundary layer irradiation is taken into account, the effective surface temperature of the accreting white dwarf can increase by more than ≈20,000 K (in addition to the increase due to compressional heating) during outburst. During quiescence the outermost layer of the star quickly radiates away the energy absorbed during outburst. The overall fraction of the outer layer of the star thermally affected by the compressional heating is of the order of the mass accreted, namely, ≈10-8 M☉ during a complete run. These results indicate that after a long evolution time of many accretion cycles, the effective surface temperature of the white dwarf will increase substantially. We discuss the application of the sequence to HST studies of the observed heating of white dwarfs in dwarf novae. We also examine the effect of compressional heating on the outer envelope structure, which directly demonstrates that there is a significant heating effect. We suggest that the envelope thermal structure resulting from compression and irradiation should be a crucial component in understanding the envelope structure of a prenova white dwarf.

A qualitative probabilistic model of microbial outbursts in foods

AbstractSequences of counts of potentially harmful organisms in foods usually exhibit an irregular fluctuating pattern. The counts are determined by the interplay of numerous random factors that tend to promote or inhibit the organisms' growth. The counts can be recorded as zero, indicating that either the organism is not present or is below a minimum detectable level, or they can fluctuate randomly within characteristic bounds. An outburst is said to occur when the population surpasses a specified threshold determined by safety or quality considerations. The growth pattern in this ‘explosive’ mode is also governed by a combination of random mechanisms that determine the growth rate and eventual decline of the population. This paper presents a probabilistic model for such scenarios. The model parameters represent the underlying distribution of the fluctuations, the detection and explosion thresholds and the probability of continued growth after an outburst has begun. A simplified version of the model was used to simulate examples of microbial histories that resemble those of sensitive foods. It is also used to elucidate how the frequency, intensity and duration of outbursts are affected by the parameters of the model. In addition, we demonstrate how to estimate the model's parameters from actual records and illustrate the efficacy of the estimation method with simulated data. The utility of such models for risk assessment will depend on the availability of long records of microbial counts that include outbursts in order to test their predictive ability. Because the presence of a harmful agent is not always sufficient to cause food poisoning, models of this kind can only estimate the expected frequency of outbursts but not the frequency of actual food‐poisoning outbreaks.© 2001 Society of Chemical Industry

Solar-type cycles of the secondary stars in cataclysmic variables

We found cyclical variations in the quiescent magnitudes and outburst intervals of 21 dwarf novae, 1 nova and 1 nova-like star produced by the solar-type cycles of the late-type secondary components. More than 70% of individual systems in this work are rst studied by Fourier periodogram analysis. It seems that the mean cycle amplitudes of the quiescent magnitudes are considerably larger for SU UMa stars. Cyclical modulations of quiescent magnitudes found for the stars, which have orbital periods below the period gap, support the hypothesis that even almost fully convective secondaries may possess activity cycles. The estimated probability density functions show peaks at 9.7, 7.9 and 8.6 yrs for cataclysmic variables (CVs), single main-sequence stars (MS) and all stars (CVs+MS), respectively. Using a larger set of data, we conrm that there is no correlation of the cycle period with the rotational regime of the secondary star. It is also conrmed that cycle periods of CVs are in the range of those of single main-sequence stars. We note that the observational cycle parameters (i.e. cycle period, Pcyc, and cycle amplitude of quiescent magnitudes, m) do not seem in relation with any other parameter such as masses of primary and secondary stars, mean outburst interval, mean outburst duration, mean decline and rise rate of outbursts, absolute magnitudes in quiescent and outburst states etc.

Rossi X‐Ray Timing ExplorerObservations of an Outburst of Recurrent X‐Ray Nova GS 1354−644

We present the results of Rossi X-Ray Timing Explorer (RXTE) observations of GS 1354–644 during a modest outburst in 1997-1998. The source is one of a handful of black hole X-ray transients that are confirmed to be recurrent in X-rays. A 1987 outburst of the same source observed by Ginga was much brighter and showed a high/soft spectral state. In contrast, the 1997-1998 outburst showed a low/hard spectral state. Both states are typical of black hole binaries. The RXTE All-Sky Monitor observed an outburst duration of 150-200 days. PCA and HEXTE observations covered ~70 days near the maximum of the light curve and during the flux decline. Throughout the observations, the spectrum can be approximated by Compton upscattering of soft photons by energetic electrons. The hot electron cloud has a temperature kT ~ 30 keV and optical depth τ ~ 4-5. To fit the data well, an additional iron fluorescent line and reflection component are required, which indicates the presence of optically thick cool material, most probably in the outer part of the accretion disk. Dramatic fast variability was observed and has been analyzed in the context of a shot noise model. The spectrum appeared to be softest at the peaks of the shot noise variability. The shape of the power spectrum was typical for black hole systems in a low/hard state. We note a qualitative difference in the shape of the dependence of fractional variability on energy when we compare systems with black holes and with neutron stars. Since it is difficult to discriminate these systems on spectral grounds, at least in their low/hard states, this new difference might be important.

Can FU Orionis Outbursts Regulate the Rotation Rates of T Tauri Stars?

We propose that FU Orionis outbursts may play an important role in maintaining the slow rotation of classical disk-accreting T Tauri stars. Current estimates for the frequency and duration of FU Orionis outbursts and the mass accretion rates of T Tauri and FU Orionis stars suggest that more mass may be accreted during the outbursts than during the T Tauri phases. If this is the case, then the outbursts should also dominate the accretion of angular momentum. During the outbursts, the accretion rate is so high that the magnetic field of the star should not disrupt the disk, and the disk will extend all the way in to the stellar surface. Standard thin disk models then predict that the star should accrete large amounts of angular momentum, which will produce a secular spinup of the star. We present boundary layer solutions for FU Orionis parameters which show that the angular momentum accretion rate drops below zero for stellar rotation rates which are always substantially below breakup, but depend on the mass accretion rate and on the adopted definition of the stellar radius. When the angular momentum accretion rate drops close to zero, the star will stop spinning up. Faster stellar rotation rates will produce negative angular momentum accretion which will spin the star down. Therefore, FU Orionis outbursts can keep the stellar rotation rate close to some equilibrium value for which the angular momentum accretion rate is small. We show that this equilibrium rotation rate may be similar to the observed rotation rates of T Tauri stars; thus we propose that FU Orionis outbursts may be responsible for the observed slow rotation of T Tauri stars. This mechanism is independent of whether the disk is disrupted by the stellar magnetic field during the T Tauri phase.

The evolutionary thermal response of a white dwarf to compressional heating by periodic dwarf nova accretion events

view Abstract Citations (71) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Evolutionary Thermal Response of a White Dwarf to Compressional Heating by Periodic Dwarf Nova Accretion Events Sion, Edward M. Abstract The thermal response of the underlying white dwarf in a cataclysmic variable to the periodic deposition of mass by a dwarf nova accretion event is simulated with a quasi-static stellar evolution code incorporating time variable accretion. After accretion at rates typical of dwarf nova outbursts (approx. 10-7 to 10-8/yr) for outburst durations of days to 2 weeks, the radial infall is shut off and the evolution of the white dwarf is followed during dwarf nova quiescence. The matter is assumed to accrete softly with the same entropy as the white dwarf outer layers. In some sequences accretion is resumed and shut off repeatedly at intervals of months to simulate the thermal evolution of the white dwarf in typical dwarf novae such as SS Cygni and U Geminorum, between successive dwarf nova outbursts. Thermal timescales for white dwarf cooling following a given dwarf nova outburst depend upon the accretion rate, outburst duration, and white dwarf mass; they are nominally in the range 0.2-0.8 years for parameters typical of dwarf novae (white dwarf masses in the range 1.2-0.6 solar mass, outburst accretion rates in the range 1 x 10-7-10-8 solar mass/yr, outburst durations in the range 7-14, days and quiescent intervals of 30-300 days). If the e-folding timescale of the white dwarf cooling is shorter than the quiescent interval bewteen outbursts, then the effect of compressional heating is too small to be observationally detectable. Publication: The Astrophysical Journal Pub Date: January 1995 DOI: 10.1086/175129 Bibcode: 1995ApJ...438..876S Keywords: Accretion Disks; Cataclysmic Variables; Heating; Novae; Stellar Evolution; Stellar Mass Accretion; White Dwarf Stars; Stellar Mass Ejection; Stellar Physics; Stellar Temperature; Astrophysics; ACCRETION; ACCRETION DISKS; STARS: NOVAE; CATACLYSMIC VARIABLES; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (3)

Structure and evolution of irradiated accretion disks. I - Static thermal equilibrium structure. II - Dynamical evolution of a thermally unstable torus

The thermal equilibrum structure and dynamical behavior of externally irradiated accretion disks are investigated. For radiative disks only the surface layer is heated, while for convective disks the heat penetrates deeply into the disk. For sufficiently strong radiation and given irradiation flux F(irr), the disk is completely stabilized against thermal instabilities of the sort invoked to explain dwarf novae. For moderately strong irradiation there is still an unstable branch in the thermal equilibrium curve. In typical soft X-ray transients, the disk is unstable against the dwarf-nova type instability. Fixed F(irr) on accretion disk annuli reduces the amplitude and the quiescent times and increases the outburst duration of the resultant light curves. Varying F(irr) in proportion to the mass accretion rate at the disks's inner edge results in light curves with a plateau in the decay from outbursts. In the case when irradiation is suddenly switched on, a temperature inversion results which leads to the formation of an accretion corona.

The nova type outburst of the symbiotic star AS 296

AbstractThe photometric and spectroscopic evolutions, displayed by AS 296 since the June 1988 outburst ([1]), are presented and discussed. The main features of the model outlined by [2], [3] and [4], are confirmed and further developped. An orbital period of about 3 years is inferred from Hα modulation (see [5]).The outburst originated from a TNR event in the accreted envelope of a WD. The IUE and optical spectroscopic evolution agrees with the expected scenario for degenerate conditions in the accreted material, while the high quiescence luminosity of the WD would indicate nondegenerate conditions.The late type giant passed unchanged the outburst. Also the region of Hα formation was not touched by the eruption.After one year the system has not yet reached the quiescence again. The photometric evolution displayed by AS 296 up to June 15, 1989 is presented in Fig.1.In Tab.1, the main features exhibited by symbiotic stars that have experienced a TNR event are summarized. The first 8 objects in the table are usually collectively called "symbiotic novae". They distinguish themselves for the very long outburst duration. At present, AS 296 appears to be a borderline case of such class, and a firm understanding needs to wait for the end of current active phase.

Flares of radio line emission H2O in Ori A and W49N

The results of observations of H2O maser sources Ori A and W49N at 1.35cm made the RATAN-600 radio telescope from June 1981 till December 1988 are presented. The light curves of these sources showed several outbursts. Duration of the strongest outbursts for the Ori A and W49N are approximately 1 and 1.5yr respectively. The maximum of the total luminosity (assuming isotropic emission) of these sources for the velocity range ±8km.s-1 is egual to 1×1047 ph.s-1 and 2×1049 ph.s-1 respectively. The time behaviour of the water maser source in W49N during more than 7yr is monitored for the first time.

An analysis and reduction of disruptive behavior on school buses.

Thousands of children are injured or killed each year in school bus accidents. A significant number of these tragic incidents is precipitated by disruptive child behavior that distracts the drivers from their difficult task. Two experiments were conducted which addressed this problem. For both experiments an automated sound recording device (referred to as a Noise Guard) selectively responsive to frequencies above 500 Hz (i.e., unresponsive to bus drone) recorded both the duration and frequency of noise outbursts above a tolerable threshold. Additionally, an observer made in situ measurements of other disruptions including roughhousing and getting-out-of-seat. In the first experiment, following baseline measurements of these behaviors, middle-school students received feedback for noise outbursts. That is, when "Noise Guard" was activated, it in turn operated one of several lights on a panel visible to all passengers. Each day students were allowed to listen to high-appeal taped music while riding the bus and to participate in a raffle for prizes, provided the number of outbursts on the preceding day remained below a specified criterion indicated on the light panel. This intervention resulted in drastic reductions of noise outbursts with a concomitant reduction in other disruptive behaviors. Comparable results were obtained in the second experiment which eliminated the raffle from the intervention.