The LIGO detection of gravitational waves （GW） from merging black holes in 2015 marked the beginning of a new era in observational astronomy. The detection of an electromagnetic signal from a GW ...source is the critical next step to explore in detail the physics involved. The Antarctic Survey Telescopes （AST3）, located at Dome A, Antarctica, is uniquely situated for rapid response time-domain astronomy with its continuous night-time coverage during the austral winter. We report optical observations of the GW source （GW 170817） in the nearby galaxy NGC 4993 using AST3. The data show a rapidly fading transient at around 1 day after the GW trigger, with the/-band magnitude declining from 17.23 - 0.13 magnitude to 17.72 - 0.09 magnitude in - 1.8 h. The brightness and time evolution of the optical transient associ- ated with GW 170817 are broadly consistent with the predictions of models involving merging binary neutron stars. We infer from our data that the merging process ejected about -10-2 solar mass of radioactive material at a speed of up to 30% the speed of light.
Seeing-the angular size of stellar images blurred by atmospheric turbulence-is a critical parameter used to assess the quality of astronomical sites at optical/infrared wavelengths. Median values at ...the best mid-latitude sites are generally in the range of 0.6-0.8 arcseconds
. Sites on the Antarctic plateau are characterized by comparatively weak turbulence in the free atmosphere above a strong but thin boundary layer
. The median seeing at Dome C is estimated to be 0.23-0.36 arcseconds
above a boundary layer that has a typical height of 30 metres
. At Domes A and F, the only previous seeing measurements have been made during daytime
. Here we report measurements of night-time seeing at Dome A, using a differential image motion monitor
. Located at a height of just 8 metres, it recorded seeing as low as 0.13 arcseconds, and provided seeing statistics that are comparable to those at a height of 20 metres at Dome C. This indicates that the boundary layer was below 8 metres for 31 per cent of the time, with median seeing of 0.31 arcseconds, consistent with free-atmosphere seeing. The seeing and boundary-layer thickness are found to be strongly correlated with the near-surface temperature gradient. The correlation confirms a median thickness of approximately 14 metres for the boundary layer at Dome A, as found from a sonic radar
. The thinner boundary layer makes it less challenging to locate a telescope above it, thereby giving greater access to the free atmosphere.
Abstract
We study the evolution of the FU Ori object V960 Mon since its outburst, using available multiwavelength photometric time series over 8 yr, complemented by several epochs of ...moderate-dispersion spectrophotometry. We find that the source fading can be well-described by a decrease in the temperature of the inner disk, which results from a combination of decreasing accretion rate and increasing inner disk radius. We model the system with a disk atmosphere model that produces the observed variations in multiband photometry (this paper) and high-resolution spectral lines (a companion paper).
Palomar Gattini-IR is a new wide-field, near-infrared (NIR) robotic time domain survey operating at Palomar Observatory. Using a 30 cm telescope mounted with a H2RG detector, Gattini-IR achieves a ...field of view (FOV) of 25 sq. deg. with a pixel scale of 8 7 in J-band. Here, we describe the system design, survey operations, data processing system and on-sky performance of Palomar Gattini-IR. As a part of the nominal survey, Gattini-IR scans 7500 square degrees of the sky every night to a median 5 depth of 15.7 AB mag outside the Galactic plane. The survey covers 15,000 square degrees of the sky visible from Palomar with a median cadence of 2 days. A real-time data processing system produces stacked science images from dithered raw images taken on sky, together with point-spread function (PSF)-fit source catalogs and transient candidates identified from subtractions within a median delay of 4 hr from the time of observation. The calibrated data products achieve an astrometric accuracy (rms) of 0 7 with respect to Gaia DR2 for sources with signal-to-noise ratio > 10, and better than 0 35 for sources brighter than 12 Vega mag. The photometric accuracy (rms) achieved in the PSF-fit source catalogs is better than 3% for sources brighter than 12 Vega mag and fainter than the saturation magnitude of 8.5 Vega mag, as calibrated against the Two Micron All Sky Survey catalog. The detection efficiency of transient candidates injected into the images is better than 90% for sources brighter than the 5 limiting magnitude. The photometric recovery precision of injected sources is 3% for sources brighter than 13 mag, and the astrometric recovery rms is 0 9. Reference images generated by stacking several field visits achieve depths of 16.5 AB mag over 60% of the sky, while it is limited by confusion in the Galactic plane. With a FOV 40× larger than any other existing NIR imaging instrument, Gattini-IR is probing the reddest and dustiest transients in the local universe such as dust obscured supernovae in nearby galaxies, novae behind large columns of extinction within the galaxy, reddened microlensing events in the Galactic plane and variability from cool and dust obscured stars. We present results from transients and variables identified since the start of the commissioning period.
One of the most important considerations when planning the next generation of ground-based optical astronomical telescopes is to choose a site that has excellent 'seeing'-the jitter in the apparent ...position of a star that is caused by light bending as it passes through regions of differing refractive index in the Earth's atmosphere. The best mid-latitude sites have a median seeing ranging from 0.5 to 1.0 arcsec (refs 1-5). Sites on the Antarctic plateau have unique atmospheric properties that make them worth investigating as potential observatory locations. Previous testing at the US Amundsen-Scott South Pole Station has, however, demonstrated poor seeing, averaging 1.8 arcsec (refs 6, 7). Here we report observations of the wintertime seeing from Dome C (ref. 8), a high point on the Antarctic plateau at a latitude of 75° S. The results are remarkable: the median seeing is 0.27 arcsec, and below 0.15 arcsec 25 per cent of the time. A telescope placed at Dome C would compete with one that is 2 to 3 times larger at the best mid-latitude observatories, and an interferometer based at this site could work on projects that would otherwise require a space mission.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
ABSTRACT
Dome A in Antarctica has many characteristics that make it an excellent site for astronomical observations, from the optical to the terahertz. Quantitative site testing is still needed to ...confirm the site’s properties. In this paper, we present a statistical analysis of cloud cover and aurora contamination from the Kunlun Cloud and Aurora Monitor (KLCAM). KLCAM is an automatic unattended all-sky camera aiming for long-term monitoring of the usable observing time and optical sky background at Dome A. It was installed at Dome A in January 2017, worked through the austral winter, and collected over 47 000 images over 490 d. A semi-quantitative visual data analysis of cloud cover and auroral contamination was carried out by five individuals. The analysis shows that the night sky was free of clouds for 83 per cent of the time, which ranks Dome A highly in a comparison with other observatory sites. Although aurorae were detected somewhere on an image for nearly 45 per cent of the time, the chance of a point on the sky being affected by an aurora is small. The strongest auroral emission lines can be filtered out with customized filters.
ABSTRACT
The 0.5-m Antarctic Survey Telescopes (AST3) were designed for time-domain optical/infrared astronomy. They are located in Dome A, Antarctica, where they can take advantage of the continuous ...dark time during winter. Since the site is unattended in winter, everything for the operation, from observing to data reduction, had to be fully automated. Here, we present a brief overview of the AST3 project and some of its unique characteristics due to its location in Antarctica. We summarize the various components of the survey, including the customized hardware and software, that make complete automation possible.
Abstract
The nova rate in the Milky Way remains largely uncertain, despite its vital importance in constraining models of Galactic chemical evolution as well as understanding progenitor channels for ...Type Ia supernovae. The rate has been previously estimated to be in the range of ≈10–300 yr
−1
, either based on extrapolations from a handful of very bright optical novae or the nova rates in nearby galaxies; both methods are subject to debatable assumptions. The total discovery rate of optical novae remains much smaller (≈5–10 yr
−1
) than these estimates, even with the advent of all-sky optical time-domain surveys. Here, we present a systematic sample of 12 spectroscopically confirmed Galactic novae detected in the first 17 months of Palomar Gattini-IR (PGIR), a wide-field near-infrared time-domain survey. Operating in the
J
band (≈1.2
μ
m), which is significantly less affected by dust extinction compared to optical bands, the extinction distribution of the PGIR sample is highly skewed to a large extinction values (>50% of events obscured by
A
V
≳ 5 mag). Using recent estimates for the distribution of Galactic mass and dust, we show that the extinction distribution of the PGIR sample is commensurate with dust models. The PGIR extinction distribution is inconsistent with that reported in previous optical searches (null-hypothesis probability <0.01%), suggesting that a large population of highly obscured novae have been systematically missed in previous optical searches. We perform the first quantitative simulation of a 3
π
time-domain survey to estimate the Galactic nova rate using PGIR, and derive a rate of
≈
43.7
−
8.7
+
19.5
yr
−1
. Our results suggest that all-sky near-infrared time-domain surveys are well poised to uncover the Galactic nova population.
Abstract
We report the discovery of a likely outbursting Class I young stellar object, associated with the star-forming region NGC 281-W (distance ∼2.8 kpc). The source is currently seen only at ...infrared wavelengths, appearing in both the Palomar Gattini InfraRed (1.2
μ
m) and the Near-Earth Object Wide-field Infrared Survey Explorer (3.4 and 4.6
μ
m) photometric time-domain surveys. Recent near-infrared imaging reveals a new, extended scattered light nebula. Recent near-infrared spectroscopy confirms the similarity of PGIR 20dci to FU Ori–type sources, based on strong molecular absorption in CO, H
2
O, and OH; weak absorption in several atomic lines; and a warm wind/outflow as indicated by a P Cygni profile in the He
i
λ
10830 line. This is a rare case of an FU Ori star with a well-measured long-term photometric rise before a sharper outburst, and the second instance of an FU Ori star with a documented two-step brightening in the mid-infrared.
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