Scintillation noise due to the Earth's turbulent atmosphere can be a dominant noise source in high-precision astronomical photometry when observing bright targets from the ground. Here we describe ...the phenomenon of scintillation from its physical origins to its effect on photometry. We show that Young's scintillation-noise approximation used by many astronomers tends to underestimate the median scintillation noise at several major observatories around the world. We show that using median atmospheric optical turbulence profiles, which are now available for most sites, provides a better estimate of the expected scintillation noise and that real-time turbulence profiles can be used to precisely characterize the scintillation-noise component of contemporaneous photometric measurements. This will enable a better understanding and calibration of photometric noise sources and the effectiveness of scintillation correction techniques. We also provide new equations for calculating scintillation noise, including for extremely large telescopes where the scintillation noise will actually be lower than previously thought. These equations highlight the fact that scintillation noise and shot noise have the same dependence on exposure time and so if an observation is scintillation limited, it will be scintillation limited for all exposure times. The ratio of scintillation noise to shot noise is also only weakly dependent on telescope diameter and so a bigger telescope may not yield a reduction in fractional scintillation noise.
Abstract
We present a search for optical bursts from the repeating fast radio burst FRB 121102 using simultaneous observations with the high-speed optical camera ULTRASPEC on the 2.4-m Thai National ...Telescope and radio observations with the 100-m Effelsberg Radio Telescope. A total of 13 radio bursts were detected, but we found no evidence for corresponding optical bursts in our 70.7-ms frames. The 5σ upper limit to the optical flux density during our observations is 0.33 mJy at 767 nm. This gives an upper limit for the optical burst fluence of 0.046 Jy ms, which constrains the broad-band spectral index of the burst emission to α ≤ −0.2. Two of the radio pulses are separated by just 34 ms, which may represent an upper limit on a possible underlying periodicity (a rotation period typical of pulsars), or these pulses may have come from a single emission window that is a small fraction of a possible period.
The scatter of the M dwarf mass–radius relationship Parsons, S G; Gänsicke, B T; Marsh, T R ...
Monthly notices of the Royal Astronomical Society,
11/2018, Volume:
481, Issue:
1
Journal Article, Publication
Peer reviewed
Open access
M dwarfs are prime targets in the hunt for habitable worlds around other stars. This
is due to their abundance as well as their small radii and low masses and tempera-
tures, which facilitate the ...detection of temperate, rocky planets in orbit around them.
However, the fundamental properties of M dwarfs are difficult to constrain, often lim-
iting our ability to characterise the planets they host. Here we test several theoretical
relationships for M dwarfs by measuring 23 high precision, model-independent masses
and radii for M dwarfs in binaries with white dwarfs. We find a large scatter in the
radii of these low-mass stars, with 25 per cent having radii consistent with theoret-
ical models while the rest are up to 12 per cent over-inflated. This scatter is seen
in both partially- and fully-convective M dwarfs. No clear trend is seen between the
over-inflation and age or metallicity, but there are indications that the radii of slowly
rotating M dwarfs are more consistent with predictions, albeit with a similar amount of
scatter in the measurements compared to more rapidly rotating M dwarfs. The sample
of M dwarfs in close binaries with white dwarfs appears indistinguishable from other
M dwarf samples, implying that common envelope evolution has a negligible impact on
their structure. We conclude that theoretical and empirical mass-radius relationships
lack the precision and accuracy required to measure the fundamental parameters of
M dwarfs well enough to determine the internal structure and bulk composition of the
planets they host.
Peer Reviewed
ABSTRACT
This article reports quasi-continuous transiting events towards WD 1054–226 at d = 36.2 pc and V = 16.0 mag, based on simultaneous, high-cadence, multiwavelength imaging photometry using ...ULTRACAM over 18 nights from 2019 to 2020 March. The predominant period is 25.02 h and corresponds to a circular orbit with blackbody Teq = 323 K, where a planetary surface can nominally support liquid water. The light curves reveal remarkable night-to-night similarity, with changes on longer time-scales, and lack any transit-free segments of unocculted starlight. The most pronounced dimming components occur every 23.1 min – exactly the 65th harmonic of the fundamental period – with depths of up to several per cent, and no evident colour dependence. Myriad additional harmonics are present, as well as at least two transiting features with independent periods. High-resolution optical spectra are consistent with stable, photospheric absorption by multiple, refractory metal species, with no indication of circumstellar gas. Spitzer observations demonstrate a lack of detectable dust emission, suggesting that the otherwise hidden circumstellar disc orbiting WD 1054–226 may be typical of polluted white dwarfs, and detected only via favourable geometry. Future observations are required to constrain the orbital eccentricity, but even if periastron is near the Roche limit, sublimation cannot drive mass loss in refractory parent bodies, and collisional disintegration is necessary for dust production.
ABSTRACT
We report on our second campaign to search for old nova shells around cataclysmic variables (CVs). Our aim was to test the theory that nova eruptions cause cycles in the mass transfer rates ...of CVs. These mass transfer cycles change the behaviour of CVs during their intereruption periods. We examined Hα images of 47 objects and found no new shells around any of the targets. Combining our latest results with those of our previous campaign (Sahman et al.), and the searches by Schmidtobreick et al. and Pagnotta & Zurek, we estimate that the nova-like phase of the mass transfer cycle lasts ∼3000 yr.
ABSTRACT We obtained high-speed photometry of the disintegrating planetesimals orbiting the white dwarf WD 1145+017, spanning a period of four weeks. The light curves show a dramatic evolution of the ...system since the first observations obtained about seven months ago. Multiple transit events are detected in every light curve, which have varying durations ( 3-12 minutes) and depths ( 10%-60%). The time-averaged extinction is 11%, much higher than at the time of the Kepler observations. The shortest-duration transits require that the occulting cloud of debris has a few times the size of the white dwarf, longer events are often resolved into the superposition of several individual transits. The transits evolve on timescales of days, both in shape and in depth, with most of them gradually appearing and disappearing over the course of the observing campaign. Several transits can be tracked across multiple nights, all of them recur on periods of 4.49 hr, indicating multiple planetary debris fragments on nearly identical orbits. Identifying the specific origin of these bodies within this planetary system, and the evolution leading to their current orbits remains a challenging problem.
The next generation of adaptive optics systems will require tomographic reconstruction techniques to map the optical refractive index fluctuations, generated by the atmospheric turbulence, along the ...line of sight to the astronomical target. These systems can be enhanced with data from an external atmospheric profiler. This is important for Extremely Large Telescope scale tomography. Here we propose a new instrument which utilizes the generalized Scintillation Detection And Ranging (SCIDAR) technique to allow high sensitivity vertical profiles of the atmospheric optical turbulence and wind velocity profile above astronomical observatories. The new approach, which we refer to as 'stereo-SCIDAR', uses a stereoscopic system with the scintillation pattern from each star of a double-star target incident on a separate detector. Separating the pupil images for each star has several advantages including increased magnitude difference tolerance for the target stars; negating the need for re-calibration due to the normalization errors usually associated with SCIDAR; an increase of at least a factor of 2 in the signal-to-noise ratio of the cross-covariance function and hence the profile for equal magnitude target stars and up to a factor of 16 improvement for targets of 3 mag difference and easier real-time reconstruction of the wind-velocity profile. Theoretical response functions are calculated for the instrument, and the performance is investigated using a Monte Carlo simulation. The technique is demonstrated using data recorded at the 2.5-m Nordic Optical Telescope and the 1.0-m Jacobus Kapteyn Telescope, both on La Palma.
We present time-resolved optical photometry of the binary millisecond ‘redback’ pulsar PSR J1023+0038 (=AY Sex) during its low-mass X-ray binary phase. The light curves taken between 2014 January and ...April show an underlying sinusoidal modulation due to the irradiated secondary star and accretion disc. We also observe superimposed rapid flaring on time-scales as short as ∼20 s with amplitudes of ∼0.1–0.5 mag and additional large flare events on time-scales of ∼5–60 min with amplitudes of ∼0.5–1.0 mag. The power density spectrum of the optical flare light curves is dominated by a red-noise component, typical of aperiodic activity in X-ray binaries. Simultaneous X-ray and UV observations by the Swift satellite reveal strong correlations that are consistent with X-ray reprocessing of the UV light, most likely in the outer regions of the accretion disc. On some nights we also observe sharp-edged, rectangular, flat-bottomed dips randomly distributed in orbital phase, with a median duration of ∼250 s and a median ingress/egress time of ∼20 s. These rectangular dips are similar to the mode-switching behaviour between disc ‘active’ and ‘passive’ luminosity states, observed in the X-ray light curves of other redback millisecond pulsars. This is the first time that the optical analogue of the X-ray mode-switching has been observed. The properties of the passive- and active-state light curves can be explained in terms of clumpy accretion from a trapped inner accretion disc near the corotation radius, resulting in rectangular, flat-bottomed optical and X-ray light curves.
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