A Year-long Superoutburst from an Ultracompact White Dwarf Binary Reveals the Importance of Donor Star Irradiation

Monthly Notices of the Royal Astronomical Society

et al. 2021

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 (SOs) and normal outbursts (NOs). The continuous coverage of the outbursts with TESS allows us to place stringent limits on the duration and structures of the SOs and the NOs. 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 SOs and rebrighthening phase after the SOs. For others, the colour evolution from simultaneous observations in g and r with ZTF differs from previously reported colour 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 the duration of many systems might have been overestimated in previous ground-based surveys. We report the SO duration for 6 AM CVns. We also found that precursors are a common feature of SOs in AM CVns and are seen in the LC of 5 of the 6 reported SOs. Finally the 10-minute and 2-minute cadence LCs from TESS also allowed us to find two new candidate 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.

Section: Ptf1 J0719+4858: Evidence For Enhanced Mass Transfermentioning

confidence: 90%

TACOS: TESS AM CVn Outbursts Survey

Marcano

Sandoval

Monthly Notices of the Royal Astronomical Society

et al. 2021

“…Such outbursts are likely to be qualitatively different in nature than standard outbursts due to the ionization instability, because a minimum amount of mass needs to be involved to trigger the ionization instability. In recent years, a few accreting white dwarfs have shown long, faint outbursts in which the optically thick components of their accretion discs never reached high enough temperatures for the disc to become ionized (Rivera Sandoval, Maccarone & Pichardo Marcano 2020 ;Rivera Sandoval et al 2021 ;Sunny Wong et al 2021 ), and these are likely due to changes in the mass transfer rate from the donor stars, rather than due to disc processes (Rivera Sandoval et al 2021 ). This is likely to happen in X-ray binaries as well, and such long term, low amplitude variability is already well-noted in the optical bands in one black hole X-ray binary (Cantrell et al 2008 ), and in the X-ray band in at least one accreting NS (Cackett et al 2010 ) and one black hole (Bernardini & Cackett 2014 ).…”

Section: F U T U R E P Ro S P E C T Smentioning

confidence: 99%

On the recurrence times of neutron star X-ray binary transients and the nature of the Galactic Centre quiescent X-ray binaries

Monthly Notices of the Royal Astronomical Society

Degenaar

Tetarenko

et al. 2022

The presence of some X-ray sources in the Galactic Centre region which show variability, but do not show outbursts in over a decade of monitoring has been used to argue for the presence of a large population of stellar mass black holes in this region. A core element of the arguments that these objects are accreting black holes is the claim that neutron stars (NSs) in low mass X-ray binaries (LMXBs) do not have long transient recurrence times. We demonstrate in this paper that about half of the known transient LMXBs with clear signatures for NS primaries have recurrence times in excess of a decade for outbursts at the sensitivity of MAXI. We furthermore show that, in order to reconcile the expected total population of NS LMXBs with the observed one and with the millisecond radio pulsar (MSRP) population of the Galaxy, systems with recurrence times well in excess of a century for outbursts detectable by instruments like MAXI must be the dominant population of NS LMXBs, and that few of these systems have yet to be discovered.

The outburst of a 60 min AM CVn reveals peculiar colour evolution: implications for outbursts in long-period double white dwarfs

Sandoval

Monthly Notices of the Royal Astronomical Society

Cavecchi

et al. 2021

We report on multi-wavelength observations during quiescence and of the first detected outburst of the ≈60 min orbital period AM CVn SDSS J113732+405458. Using X-ray and UV observations we determined an upper limit duration of the event of about one year. The amplitude of the outburst was remarkably small, of around one magnitude in r and 0.5 magnitudes in g. We have also investigated the colour variations of SDSS J113732+405458 and other long period AM CVns in outbursts and identified a track on the colour-magnitude diagram that is not compatible with the predictions of the disk instability model, suggesting that some outbursts in long period AM CVns are caused by enhanced mass-transfer. To our knowledge, these are the first studies of the colour evolution in AM CVns. During quiescence we measured an X-ray luminosity for SDSS J113732+405458 of ≈3 × 1029 erg s−1 in the 0.5-10 keV band. This indicates a very low accretion rate, in agreement with the disk instability model for long period systems. However, such a model predicts stable disks at somewhat long periods. The discovery of this system outburst, along with similarities to the long period system SDSS J080710+485259 with a comparably long, weak outburst, indicates that these enhanced mass-transfer events may be more common in long period AM CVns. A larger sample would be needed to determine empirically at what period, if any, the disk instability stops functioning entirely. Finally, we identified an infrared excess in the quiescence spectrum attributable to the donor. This makes SDSS J113732+405458 the second AM CVn to have a directly detected donor.