ABSTRACT On 2014 April 23, the Swift satellite responded to a hard X-ray transient detected by its Burst Alert Telescope, which turned out to be a stellar flare from a nearby, young M dwarf binary DG ...CVn. We utilize observations at X-ray, UV, optical, and radio wavelengths to infer the properties of two large flares. The X-ray spectrum of the primary outburst can be described over the 0.3-100 keV bandpass by either a single very high-temperature plasma or a nonthermal thick-target bremsstrahlung model, and we rule out the nonthermal model based on energetic grounds. The temperatures were the highest seen spectroscopically in a stellar flare, at TX of 290 MK. The first event was followed by a comparably energetic event almost a day later. We constrain the photospheric area involved in each of the two flares to be >1020 cm2, and find evidence from flux ratios in the second event of contributions to the white light flare emission in addition to the usual hot, T ∼ 104 K blackbody emission seen in the impulsive phase of flares. The radiated energy in X-rays and white light reveal these events to be the two most energetic X-ray flares observed from an M dwarf, with X-ray radiated energies in the 0.3-10 keV bandpass of 4 × 1035 and 9 × 1035 erg, and optical flare energies at EV of 2.8 × 1034 and 5.2 × 1034 erg, respectively. The results presented here should be integrated into updated modeling of the astrophysical impact of large stellar flares on close-in exoplanetary atmospheres.
We report precise measures of the orbital and superhump period in 20 more dwarf novae. For 10 stars, we report new and confirmed spectroscopic periods—signifying the orbital period
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—as well as the superhump period
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. These are GX Cas, HO Del, HS Vir, BC UMa, RZ Leo, KV Dra, KS UMa, TU Crt, QW Ser, and RZ Sge. For the remaining 10, we report a medley of
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and
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measurements from photometry; most are new, with some confirmations of previous values. These are KV And, LL And, WX Cet, MM Hya, AO Oct, V2051 Oph, NY Ser, KK Tel, HV Vir, and RX J1155.4−5641.
Periods, as usual, can be measured to high accuracy, and these are of special interest since they carry dynamical information about the binary. We still have not quite learned how to read the music, but a few things are clear. The fractional superhump excess
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varies smoothly with
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. The scatter of the points about that smooth curve is quite low, and can be used to limit the intrinsic scatter in
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, the white dwarf mass, and the mass‐radius relation of the secondary. The dispersion in
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does not exceed 24%, and the secondary‐star radii scatter by no more than 11% from a fixed mass‐radius relation. For the well‐behaved part of
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space, we estimate from superhump theory that the secondaries are
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larger than theoretical ZAMS stars. This affects some other testable predictions about the secondaries: at a fixed
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, it suggests that the secondaries are (compared with ZAMS predictions)
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less massive,
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smaller,
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cooler, and less luminous by a factor of 2.5(7). The presence of a well‐defined mass‐radius relation, reflected in a well‐defined
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We report on two superoutbursts of the AM CVn-type object CR Boo in 2014 April–March and 2015 May–June. A precursor outburst accompanied both of these superoutbursts. During the rising branch of the ...main superoutburst in 2014, we detected growing superhumps (stage A superhumps) whose period was 0.017669(24) d. Assuming that this period reflects the dynamical precession rate at the radius of the 3:1 resonance, we could estimate the mass ratio (q = M
2/M
1) of 0.101(4) by using the stage A superhump period and the orbital period of 0.0170290(6) d. This mass ratio is consistent with that expected from the theoretical evolutionary model of AM CVn-type objects. The detection of precursor outbursts and stage A superhumps is the second case in AM CVn-type objects. There are two interpretations of the outbursts of AM CVn-type objects. One is a dwarf nova (DN) outbursts analogy, which suggets that the outbursts are caused by thermal and tidal instabilities. Another is the VY Scl-type variation, which suggests that the outbursts are caused by the variation of the mass-transfer rate of the secondary.This detection of the superhump variations strongly supports the former interpretation.
Our mainly R band photometry of V1493 Aql (1999) during its outburst, indicates periodic variations with a period of 0.156 days, seen only 5 days after the maximum in V. This period can be orbital. ...The light curve indicates in addition, the presence of a faint eclipse like feature. We try to explain its extremely low amlitude, as being due to the eclipse of an expanded white dwarf component of the binary below an optically thick wind. We discuss what conditions would be required for such an interpretation to work.
Abstract
We report on a superoutburst of the AM CVn-type object SDSS J090221.35+381941.9 J0902; orbital period 0.03355(6) d in 2014 March–April. The entire superoutburst consisted of a precursor ...outburst and the main superoutburst, followed by a short rebrightening. During the rising phase of the main superoutburst, we detected growing superhumps (stage A superhumps) with a period of 0.03409(1) d. During the plateau phase of the superoutburst, superhumps with a shorter period (stage B superhumps) were observed. Using the orbital period and the period of stage A superhumps, we were able to measure the dynamical precession rate of the accretion disk at the 3:1 resonance, and obtained a mass ratio (q) of 0.041(7). This is the first successful measurement of the mass ratio in an AM CVn-type object accomplished by the recently developed stage A superhump method. The value is generally in agreement with that based on the theoretical evolutionary model. The orbital period of J0902 is the longest among those of the outbursting AM CVn-type objects, and a period on the borderline between the outbursting system and the system with a stable cool disk appears to be longer than one supposed.
Continuing the project described by Kato et al. (2009, PASJ, 61, S395), we collected times of superhump maxima for 102 SU UMa-type dwarf novae, observed mainly during the 2014–2015 season, and ...characterized these objects. Our project has greatly improved the statistics of the distribution of orbital periods, which is a good approximation of the distribution of cataclysmic variables at the terminal evolutionary stage, and has confirmed the presence of a period minimum at a period of 0.053 d and a period spike just above this period. The number density monotonically decreased toward the longer period and there was no strong indication of a period gap. We detected possible negative superhumps in Z Cha. It is possible that normal outbursts are also suppressed by the presence of a disk tilt in this system. There was no indication of enhanced orbital humps just preceding the superoutburst, and this result favors the thermal–tidal disk instability as the origin of superoutbursts. We detected superhumps in three AM CVn-type dwarf novae. Our observations and recent other detections suggest that 8% of objects showing dwarf nova-type outbursts are AM CVn-type objects. AM CVn-type objects and EI Psc-type objects may be more abundant than previously recognized. OT J213806, a WZ Sge-type object, exhibited remarkably different features between the 2010 and 2014 superoutbursts. Although the 2014 superoutburst was much fainter, the plateau phase was shorter than the 2010 one, and the course of the rebrightening phase was similar. This object indicates that the O − C diagrams of superhumps can indeed be variable, at least in WZ Sge-type objects. Four deeply eclipsing SU UMa-type dwarf novae (ASASSN-13cx, ASASSN-14ag, ASASSN-15bu, and NSV 4618) were identified. We studied long-term trends in supercycles in MM Hya and CY UMa and found systematic variations of supercycles of ∼20%.
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.
Continued from Kato et al. (2009, PASJ, 61, S395), we collected the times of superhump maxima for 68 SU UMa-type dwarf novae, mainly observed during the 2009–2010 season. The newly obtained data ...confirmed the basic findings reported in Kato et al. (ibid.): the presence of stages A–C and the predominance of positive period derivatives during stage B in systems with superhump periods shorter than 0.07 d. There was a systematic difference in the period derivatives for the systems with superhump periods longer than 0.075 d between this study and Kato et al. (ibid.). We suggest that this difference was possibly caused by a relative lack of frequently outbursting SU UMa-type dwarf novae in this period regime in the present study. We recorded a strong beat phenomenon during the 2009 superoutburst of IY UMa. A close correlation between the beat period and the superhump period suggests that the changing angular velocity of the apsidal motion of the elliptical disk is responsible for the variation of the superhump periods. We also described three new WZ Sge-type objects with established early superhumps and one with likely early superhumps. We suggest that two systems, VX For and EL UMa, are WZ Sge-type dwarf novae with multiple rebrightenings. The
$O-C$
variation in OT J213806.6
$+$
261957 suggests that the frequent absence of rebrightenings in very short-
$P_{\rm orb}$
objects can be the result of a sustained superoutburst plateau at the epoch when usual SU UMa-type dwarf novae return to quiescence, preceding a rebrightening. We also present a formulation for a variety of Bayesian extensions to traditional period analyses.
We report on the results of the QW Ser campaign, which has been continued from 2000 to 2003 by the VSNET collaboration team. Four long outbursts and many short ones were caught during this period. ...Our intensive photometric observations revealed superhumps with a period of
$0.07700(\pm 0.00004) \,\mathrm{d}$
during all four superoutbursts, proving the SU UMa nature of this star. The recurrence cycles of the normal outbursts and the superoutbursts were measured to be
$\sim 50$
days and
$240(\pm 30)$
days, respectively. The change rate of the superhump period was
$-5.8 \times 10^{-5}$
. The distance and the X-ray luminosity in the range of 0.5–2.4 keV are estimated to be
$380(\pm60) \,\mathrm{pc}$
and
$\log L_\mathrm{X} = 31.0 \pm 0.1 \,\mathrm{erg} \,\mathrm{s}^{-1}$
. These properties have typical values for an SU UMa-type dwarf nova with this superhump period.
Abstract Continuing the project undertaken by Kato et al. (2009), we collected times of superhump maxima for 56 SU UMa-type dwarf novae mainly observed during the 2013–2014 season and characterized ...these objects. We detected negative superhumps in VW Hyi and indicated that the low number of normal outbursts in some supercycles can be interpreted as a result of disk tilt. This finding, combined with the Kepler observation of V1504 Cyg and V344 Lyr, suggests that disk tilt is responsible for modulating the outburst pattern in SU UMa-type dwarf novae. We also studied the deeply eclipsing WZ Sge-type dwarf nova MASTER OT J005740.99+443101.5 and found evidence of a sharp eclipse during the phase of early superhumps. The profile can be reproduced by a combination of the eclipse of the axisymmetric disk and the uneclipsed light source of early superhumps. This finding shows the lack of evidence for a greatly enhanced hot spot during the early stage of WZ Sge-type outburst. We detected growing (stage A) superhumps in MN Dra and give a suggestion that some of SU UMa-type dwarf novae situated near the critical condition of tidal instability may show long-lasting stage A superhumps. The large negative period derivatives reported in such systems can be understood as a result of the combination of stage A and B superhumps. Two WZ Sge-type dwarf novae, AL Com and ASASSN-13ck, showed a long-lasting (plateau-type) rebrightening. In the early phase of their rebrightenings, both objects showed a precursor-like outburst, suggesting that the long-lasting rebrightening is triggered by a precursor outburst.
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