Secular changes in the quiescence of WZ Sge: the development of a cavity in the inner disk

We find a dimming during optical quiescence of the cataclysmic variable WZ Sge by about half a magnitude between superoutbursts. We connect the dimming with the development of a cavity in the inner part of the accretion disk. We suggest that, when the cavity is big enough, accretion of material is governed by the magnetic field of the white dwarf and pulsations from the weakly magnetic white dwarf appear. The time scale of forming the cavity is about a decade, and it persists throughout the whole quiescent phase. Such a cavity can be accommodated well by the proposed magnetic propeller model for WZ Sge, where during quiescence mass is being expelled by the magnetic white dwarf from the inner regions of the accretion disk to larger radii.Comment: 10 pages, 4 figures, accepted for publication in Astronomy and Astrophysics; following referee report, many textual changes, figures improved, more historic data added, conclusions unchange

Rapid radio flaring during an anomalous outburst of SS Cyg

The connection between accretion and jet production in accreting white dwarf binary systems, especially dwarf novae, is not well understood. Radio wavelengths provide key insights into the mechanisms responsible for accelerating electrons, including jets and outflows. Here, we present densely sampled radio coverage, obtained with the Arcminute MicroKelvin Imager Large Array, of the dwarf nova SS Cyg during its 2016 February anomalous outburst. The outburst displayed a slower rise (3 dmag-1) in the optical than typical ones and lasted for more than three weeks. Rapid radio flaring on time-scales <1 h was seen throughout the outburst. The most intriguing behaviour in the radio was towards the end of the outburst where a fast, luminous ('giant'), flare peaking at ~20 mJy and lasting for 15 min was observed. This is the first time that such a flare has been observed in SS Cyg and insufficient coverage could explain its non-detection in previous outbursts. These data, together with past radio observations, are consistent with synchrotron emission from plasma ejection events as being the origin of the radio flares. However, the production of the giant flare during the declining accretion rate phase remains unexplained within the standard accretion-jet framework and appears to be markedly different to similar patterns of behaviour in X-ray binaries

Secular Evolution in Mira Variable Pulsations

Stellar evolution theory predicts that asymptotic giant branch stars undergo a series of short thermal pulses that significantly change their luminosity and mass on timescales of hundreds to thousands of years. Secular changes in these stars resulting from thermal pulses can be detected as measurable changes in period if the star is undergoing Mira pulsations. The American Association of Variable Star Observers (AAVSO) International Database currently contains visual data for over 1500 Mira variables. Light curves for these stars span nearly a century in some cases, making it possible to study the secular evolution of the pulsation behavior on these timescales. In this paper, we present the results of our study of period change in 547 Mira variables using data from the AAVSO. We find non-zero rates of period change, dlnP/dt, at the 2-sigma significance level in 57 of the 547 stars, at the 3-sigma level in 21 stars, and at the level of 6-sigma or greater in eight of the 547. The latter eight stars have been previously noted in the literature, and our derived rates of period changes largely agree with published values. The largest and most statistically significant dlnP/dt are consistent with the rates of period change expected during thermal pulses on the AGB. A number of other stars exhibit non-monotonic period changes on decades-long timescales, the cause of which is not yet known.Comment: 37 pages, with 9 figures and 1 table. The complete electronic version of Table 1 is available from the authors upon request. Accepted for publication in The Astronomical Journa

The reproducible radio outbursts of SS Cygni

We present the results of our intensive radio observing campaign of the dwarf nova SS Cyg during its 2010 April outburst. We argue that the observed radio emission was produced by synchrotron emission from a transient radio jet. Comparing the radio light curves from previous and subsequent outbursts of this system (including high-resolution observations from outbursts in 2011 and 2012) shows that the typical long and short outbursts of this system exhibit reproducible radio outbursts that do not vary significantly between outbursts, which is consistent with the similarity of the observed optical, ultraviolet and X-ray light curves. Contemporaneous optical and X-ray observations show that the radio emission appears to have been triggered at the same time as the initial X-ray flare, which occurs as disc material first reaches the boundary layer. This raises the possibility that the boundary region may be involved in jet production in accreting white dwarf systems. Our high spatial resolution monitoring shows that the compact jet remained active throughout the outburst with no radio quenching

The First Post-Kepler Brightness Dips of KIC 8462852

We present a photometric detection of the first brightness dips of the unique variable star KIC 8462852 since the end of the Kepler space mission in 2013 May. Our regular photometric surveillance started in October 2015, and a sequence of dipping began in 2017 May continuing on through the end of 2017, when the star was no longer visible from Earth. We distinguish four main 1-2.5% dips, named "Elsie," "Celeste," "Skara Brae," and "Angkor", which persist on timescales from several days to weeks. Our main results so far are: (i) there are no apparent changes of the stellar spectrum or polarization during the dips; (ii) the multiband photometry of the dips shows differential reddening favoring non-grey extinction. Therefore, our data are inconsistent with dip models that invoke optically thick material, but rather they are in-line with predictions for an occulter consisting primarily of ordinary dust, where much of the material must be optically thin with a size scale <<1um, and may also be consistent with models invoking variations intrinsic to the stellar photosphere. Notably, our data do not place constraints on the color of the longer-term "secular" dimming, which may be caused by independent processes, or probe different regimes of a single process

Astrophysics in 2005

We bring you, as usual, the Sun and Moon and stars, plus some galaxies and a new section on astrobiology. Some highlights are short (the newly identified class of gamma-ray bursts, and the Deep Impact on Comet 9P/ Tempel 1), some long (the age of the universe, which will be found to have the Earth at its center), and a few metonymic, for instance the term "down-sizing" to describe the evolution of star formation rates with redshift

An Accurate Geometric Distance to the Compact Binary SS Cygni Vindicates Accretion Disc Theory

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A new early Smithian ammonoid fauna from the Salt Range (Pakistan)

Recent extensive investigations in the Salt Range (Pakistan) yielded abundant, well-preserved ammonoid faunas of earliest to latest Smithian age that provided the basis for a major revision of Smithian ammonoid taxonomy and for the establishment of a high-resolution biostratigraphic sequence. Here, an additional new ammonoid fauna of typical early Smithian affinity from the uppermost part of the Ceratite Sandstone of the Nammal Gorge section is described. The new fauna, termed Euflemingites cirratus beds, is bracketed between the underlying early Smithian 1CFlemingites flemingianus beds 1D, here renamed Clypeoceras superbum beds, and the overlying middle Smithian Brayardites compressus beds. Comparison with a recently published high-resolution biochronological scheme for the Smithian of the NIM (northern Indian Margin) based on the Salt Range, Spiti (Himachal Pradesh, northern India) and Tulong (South Tibet) basins shows that the Euflemigites cirratus fauna correlates with the Dieneroceras beds from Spiti based on the common occurrence of the ammonoid species Kraffticeras pseudoplanulatum. The trans-panthalassic biogeographical distribution of Euflemingites cirratus allows correlating the new ammonoid fauna with part of the Meekoceras gracilitatis ammonoid zone of western USA. Three new species (Kashmirites weisserti, Arctoceras schalteggeri and Vercherites wyleri) are described