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Register a new userTACOS: TESS AM~CVn Outbursts Survey
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Added by
Manuel Pichardo Marcano
Publication date
2021
fields
Physics
and research's language is
English
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Using TESS we are doing a systematic study of outbursting AM~CVn systems to place some limits on the current outbursts models. We present the TESS light curve (LC) for 9 AM~CVns showing both superoutbursts (SO) and normal outbursts (NO). The continuous coverage of the outbursts with TESS allows us to place stringent limits on the duration and structures of the SO and the NO. We present evidence that in at least some of the systems enhanced mass transfer (EMT) has to be taken into account to explain the observed LC of the SO and rebrighthening phase after the SO. For others, the colour evolution from simultaneous observations in $g$ and $r$ with ZTF differs from previously reported color evolution of longer period AM~CVns where EMT is responsible for the SO. We also find that due to the lack of sufficiently high cadence coverage many of the duration might have been overestimated in previous ground-based surveys and report the SO duration for 6 AM~CVns. We also found that precursors are a common feature of SO in AM~CVns and are seen in the LC of 5 of the 6 reported SO. Finally with the 10-minute and 2-minute cadence LC from TESS also allowed us to find two new candidates orbital periods of AM~CVns, both of which are in reasonably good agreement with the predictions for their periods based on their past outburst histories.
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We present the results of a two and a half year optical photometric monitoring programme covering 16 AM CVn binaries using the Liverpool Telescope on La Palma. We detected outbursts in seven systems, one of which (SDSS J0129) was seen in outburst for the first time. Our study coupled with existing data shows that ~1/3 of these helium-rich accreting compact binaries show outbursts. The orbital period of the outbursting systems lie in the range 24-44 mins and is remarkably consistent with disk-instability predictions. The characteristics of the outbursts seem to be broadly correlated with their orbital period (and hence mass transfer rate). Systems which have short periods (<30 min) tend to exhibit outbursts lasting 1--2 weeks and often show a distinct `dip in flux shortly after the on-set of the burst. We explore the nature of these dips which are also seen in the near-UV. The longer period bursters show higher amplitude events (5 mag) that can last several months. We have made simulations to estimate how many outbursts we are likely to have missed.
We consider initial stage of the evolution of AM CVn type stars with white dwarf donors, which is accompanied by thermonuclear explosions in the layer of accreted He. It is shown that the accretion never results in detonation of He and accretors in AM CVn stars finish their evolution as massive WDs. We found, for the first time, that in the outbursts the synthesis of n-rich isotopes, initiated by the ${mathrm{^{22}{Ne}(alpha,n)^{25}Mg}}$ reaction becomes possible.
We report on the discovery of a dichotomy in the behavior of outbursts in ultracompact accreting white dwarf binaries (AM CVns), using multiwavelength observations of the new AM CVn system ASASSN-21au which has a period of ~58 min. The binary showed a first brightness increase with respect to its quiescent g level of ~0.8 mag which lasted for at least 82 days, followed by an additional 0.5 mag increase which lasted 2 weeks. Afterwards ASASSN-21au went into superoutburst for the first time. Such superoutburst lasted a total of 19 days, showing an amplitude with respect to quiescence of ~7.5 mags in g, with a precursor and an echo outburst. During the superoutburst of ASASSN-21au we also detected the first correlation between the X-rays, UV and optical emission in an AM CVn. The color evolution of ASASSN-21au indicates that during the superoutburst the dominant component was the accretion disk. The short duration, large amplitude and color evolution of the superoutburst agree with expectations from the disk instability model, but they contrast with the long duration (longer than a year), small amplitude and red color evolution of the outbursts of SDSS~J080710+485259 and SDSS J113732+405458, which have periods of ~53 min and ~60 min, respectively. The initial slow brightness increase in the light curve of ASASSN-21au and the behavior after the superoutburst favors a scenario in which changes in the mass-transfer rate led to disk instabilities, while the outburst mechanism of SDSS J080710+485259 and SDSS J113732+405458 has been attributed to enhanced mass-transfer alone. Further observations are needed to understand the origin of this dichotomy.
We present optical and X-ray observations of two tidally distorted, extremely low-mass white dwarfs (WDs) with massive companions. There is no evidence of neutron stars in our Chandra and XMM observations of these objects. SDSS J075141.18$-$014120.9 (J0751) is an eclipsing double WD binary containing a 0.19 Msol WD with a 0.97 Msol companion in a 1.9 h orbit. J0751 becomes the fifth eclipsing double WD system currently known. SDSS J174140.49+652638.7 (J1741) is another binary containing a 0.17 Msol WD with an unseen M > 1.11 Msol WD companion in a 1.5 h orbit. With a mass ratio of ~0.1, J1741 will have stable mass transfer through an accretion disk and turn into an interacting AM Canum Venaticorum (AM CVn) system in the next ~160 Myr. With a mass ratio of 0.2, J0751 is likely to follow a similar evolutionary path. These are the first known AM CVn progenitor binary systems and they provide important constraints on the initial conditions for AM CVn. Theoretical studies suggest that both J0751 and J1741 may create thermonuclear supernovae in ~10^8 yr, either .Ia or Ia. Such explosions can account for ~1% of the Type Ia supernova rate.
We report the discovery of a one magnitude increase in the optical brightness of the 59.63 minute orbital period AM CVn binary SDSS J113732.32+405458.3. Public $g$, $r$, and $i$ band data from the Zwicky Transient Facility (ZTF) exhibit a decline over a 300 day period, while a few data points from commissioning show that the peak was likely seen. Such an outburst is likely due to a change in the state of the accretion disk, making this the longest period AM CVn binary to reveal an unstable accretion disk. The object is now back to its previously observed (by SDSS and PS-1) quiescent brightness that is likely set by the accreting white dwarf. Prior observations of this object also imply that the recurrence times for such outbursts are likely more than 12 years.
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