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Example of a negative parallax arising from the astrometric data processing. Solid blue lines, true path of the object; red dots, the individual measurements of the source position on the sky; dashed orange lines, the source path according to the least-squares astrometric solution, which here features a negative parallax. Left: Path on the sky showing the effect of proper motion (linear trend) and parallax (loops). Right: Right ascension and declination of the source as a function of time. In the fitted solution the negative parallax effect is equivalent to a yearly motion of the star in the opposite direction of the true parallactic motion (which gives a phase-shift of π in the sinusoidal curves in the right panels). The error bars indicate a measurement uncertainty of 0.7 mas, the uncertainties on ∆α * and ∆δ are assumed to be uncorrelated.
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Context. The second Gaia data release ( Gaia DR2) provides precise five-parameter astrometric data (positions, proper motions, and parallaxes) for an unprecedented number of sources (more than 1.3 billion, mostly stars). This new wealth of data will enable the undertaking of statistical analysis of many astrophysical problems that were previously i...
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... The latter option is fairly draconian, although it will only affect saturating stars which are brighter than 14.7, 15.7, 15.8, 15.8, 15.3 and 13.9 magnitude in the , , , , , and bands respectively [8]. These should number somewhere in the order of 100 million total in the sky [9], or half that in the southern sky that LSST will map. A point source will be imaged over ∼10 pixels on the detector. ...
LSST Camera CCDs produced by the manufacturer e2v exhibit strong and novel residual charge images when exposed to bright sources. These manifest in images following bright exposures both in the same pixel areas as the bright source, and in the pixels trailing between the source and the serial register. Both of these pose systematic challenges to the Rubin Observatory Legacy Survey of Space and Time instrument signature removal. The latter trail region is especially impactful as it affects a much larger pixel area in a less well defined position. In our study of this effect at UC Davis, we imaged bright spots to characterize these residual charge effects. We find a strong dependence of the residual charge on the parallel clocking scheme, including the relative levels of the clocking voltages, and the timing of gate phase transition during the parallel transfer. Our study points to independent causes of residual charge in the bright spot region and trail region. We propose potential causes in both regions and suggest methodologies for minimizing residual charge. We consider the trade-offs to these methods including decreasing the camera's full well and dynamic range at the high end. Some of these results and suggestions have been reviewed by the camera commissioning team and may result in changes made to the clocking voltage scheme on the LSST Camera.
... The ZP correction reduces velocity only by ≲ 2% in our selected data. The analysis and discussion of distance extraction from parallax is thoroughly conducted in [34][35][36]. In Section 3.1, we will study the dependence of the mean values ⟨v l ⟩, ⟨v b ⟩ on the distance and direction from the Sun. ...
The aim of the analysis of data from the Gaia Space Observatory is to obtain kinematic parameters of the collective motion of stars in a part of our galaxy. This research is based on a statistical analysis of the motion of 55,038,539 stars selected in different directions from the Sun up to a distance of 3–6 kpc. We developed statistical methods for the analysis working with input data represented by the full astrometric solution (five parameters). Using the proposed statistical methods, we obtained the local velocity of the Sun U⊙,V⊙,W⊙=(9.58,16.25,7.33)±(0.05,0.04,0.02)stat±(0.7,0.9,0.1)syst km/s and the rotation velocity of the galaxy at different radii. For the Sun’s orbit radius, we obtained the velocity of the galaxy rotationVc≈234±4 km/s. Collective rotation slows down in the region under study linearly with distance from the disk plane: ΔV/ΔZ≅33.5kms−1kpc−1. We showed that the different kinematic characteristics and distributions, which depend on the position in the galaxy, can be well described in the studied 3D region by a simple Monte Carlo simulation model, representing an axisymmetric approximation of the galaxy kinematics. The optimal values of the six free parameters were tuned by comparison with the data.
... Only ∼ 10 8 stars in Gaia DR3 satisfy this condition, and about twice as many pass a weaker cut ϖ/ϵ ϖ ≳ 5, which is borderline acceptable for subsequent analysis. The problem with making signal-to-noise cuts on parallax, highlighted by Luri et al. (2018), is that the remaining sample becomes biased: the selection is made on the measured value of ϖ, which is equally likely to be higher or lower than the true value ϖ (t) (indeed, the distribution of measurement errors normalised by the estimated uncertainty ϵ ϖ is close to standard normal for most stars with reliable astrometric solution, as quantified by the ruwe parameter). However, if ϖ < ϖ (t) , the star may be excluded from the sample defined by a signal-to-noise cut, whereas it would have passed the cut if ϖ > ϖ (t) ; the true distance distribution of the resulting sample is therefore skewed to larger values than the distribution of 1/ϖ. ...
The Gaia mission has triggered major developments in the field of Galactic dynamics in recent years, which we discuss in this review. The structure and kinematics of all Galactic components - disc, bar/bulge and halo - are now mapped in great detail not only in the Solar neighbourhood, but across a large part of the Milky Way. The dramatic improvements in the coverage and precision of observations revealed various disequilibrium processes, such as perturbations in the Galactic disc and the deformations of the outer halo, which are partly attributed to the interaction with satellite galaxies. The knowledge of the gravitational potential at all scales has also advanced considerably, but we are still far from having a consistent view on the key properties of the Galaxy, such as the bar pattern speed or the mass profile and shape of the dark halo. The complexity and interplay of several dynamical processes makes the interpretation of observational data challenging, and it is fair to say that more theoretical effort is needed to fully reap the fruit of the Gaia revolution.
... We can, however, resort to the known distances from Gaia's parallax measurements (Bailer-Jones et al. 2018). The reported mean uncertainty in parallax measurements is ∼40 µas (Luri et al. 2018), which corresponds to ∼4% at a 1 kpc distance. At the same time, both Kepler and TESS data are given in e − /s, which (in conjunction with extinction maps) can be converted to absolute fluxes using dedicated tools (Ridden-Harper et al. 2021), with the reported precision of ∼1%. ...
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... To demonstrate the procedure, we used a database of 131 LCs of Monson and Pierce (2011) in our analysis. The apparent J,H,K colors of our program stars are given by Monson and Pierce (2011), and their parallaxes are available in the Gaia DR2 archive (Luri et al., 2018). We corrected the interstellar extinction by comparing the observed J-H, and K-H color indices with their intrinsic values given by Bessell and Brett (1988). ...
Recent space-borne and ground-based observations provide photometric measurements as time series. The effect of interstellar dust extinction in the near-infrared range is only 10% of that measured in the V band. However, the sensitivity of the light curve shape to the physical parameters in the near-infrared is much lower. So, interpreting these types of data sets requires new approaches like the different large-scale surveys, which create similar problems with big data. Using a selected data set, we provide a method for applying routines implemented in R to extract most information of measurements to determine physical parameters, which can also be used in automatic classification schemes and pipeline processing. We made a multivariate classification of 131 Cepheid light curves (LC) in J, H, and K colors, where all the LCs were represented in 20D parameter space in these colors separately. Performing a Principal Component Analysis (PCA), we got an orthogonal coordinate system and squared Euclidean distances between LCs, with 6 significant eigenvalues, reducing the 20-dimension to 6. We also estimated the optimal number of partitions of similar objects and found it to be equal to 7 in each color; their dependence on the period, absolute magnitude, amplitude, and metallicity are also discussed. We computed the Spearman rank correlations, showing that periods and absolute magnitudes correlate with the first three PCs significantly. The first two PC are also found to have a relationship with the amplitude, but the metallicity effects are only marginal. The method shown can be generalized and implemented in unsupervised classification schemes and analysis of mixed and biased samples. The analysis of our Classical Cepheid near-infrared LC sample showed that the J, H, K curves are insufficient for determination of stellar metallicity, with mass being the key factor shaping them.
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Compact binary mergers detectable in gravitational waves can be accompanied by a kilonova, an electromagnetic transient powered by radioactive decay of newly synthesised r-process elements. A few kilonova candidates have been observed during short gamma-ray burst follow-up, and one found associated with a gravitational wave detection, GW170817. However, robust kilonova candidates are yet to be found in un-triggered, wide-field optical surveys, that is, a search not requiring an initial gravitational wave or gamma-ray burst trigger. Here we present the first observing run for the Kilonova and Transients Program (KNTraP) using the Dark Energy Camera. The first KNTraP run ran for 11 nights, covering 31 fields at a nightly cadence in two filters. The program can detect transients beyond the LIGO/Virgo/KAGRA horizon, be agnostic to the merger orientation, avoid the Sun and/or Galactic plane, and produces high cadence multi-wavelength light curves. The data were processed nightly in real-time for rapid identification of transient candidates, allowing for follow-up of interesting candidates before they faded away. Three fast-rising candidates were identified in real-time, however none had the characteristics of the kilonova AT2017gfo associated with GW170817 or with the expected evolution for kilonovae from our fade-rate models. After the run, the data were reprocessed, then subjected to stringent filtering and model fitting to search for kilonovae offline. Multiple KNTraP runs (3+) are expected to detect kilonovae via this optical-only search method. No kilonovae were detected in this first KNTraP run using our selection criteria, constraining the KN rate to Gpc yr.
... The right panel of the same figure shows the position of CVs in the Gaia colour-magnitude diagram, showing that they occupy a relatively less contaminated region. It is important to keep in mind that the use of the Gaia parallax as distance is not always possible and one should approach this database with a grain of salt, following the prescription provided by the Gaia team, in particular [8]. The spectral energy distributions of the 10 brightest CVs detected in J-PLUS are shown in Figure 3. ...
... Typical noise levels are 0.1 K rms of brightness temperature in an integrated velocity of 1 km s −1 . Based on the Arecibo 21 cm H I data, we found an atomic gas excess at V LSR ∼ 66 km/s, which corresponds to a distance of ∼ 4.1 ± 0.7 kpc, consistent with that of SS 433 33 . The atomic gas excess is coincident with the gamma-ray emission above 100 TeV (see Extended Data Figure 1). ...
Black holes (BH), one of the most intriguing objects in the universe, can manifest themselves through electromagnetic radiations initiated by the accretion flow. Some stellar-mass BHs drive relativistic jets when accreting matter from their companion stars, called microquasars. Non-thermal emission from the radio to tera-electronvolt (TeV) gamma-ray band has been observed from microquasars, indicating acceleration of relativistic particles by the system. Here we report detection of ultrahigh-energy (UHE, photon energy TeV) gamma-ray associated with 5 out of 12 microquasars harboring BHs visible by the Large High Altitude Air Shower Observatory, namely, SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070, and Cygnus X-1. In the central region of SS 433, extended UHE emission is detected in spatial coincidence with a giant gas cloud, suggesting the hadronic origin of the emission. An elongated source is discovered from V4641 Sgr with the spectrum continuing up to 800 TeV. GRS 1915+105, MAXI J1820+070 and Cygnus X-1 are detected with quasi-stable radiations up to 100 TeV. The detection of UHE gamma-rays demonstrates that accreting BHs and their environments can operate as extremely efficient accelerators of particles out of 1 peta-electronvolt (PeV), thus contributing to Galactic cosmic rays especially around the `knee' region.
... While it may seem nonphysical, Gaia observed parallax measurements can be negative (Lindegren et al. 2021a), but it is important to remember that the observed parallax values define the mean of a distribution whose width/uncertainty usually places a large amount of probability in positive parallaxes; in this way, we must treat the Gaia astrometric measurements as distributions and not individual points. Luri et al. (2018), for example, explain how the definitions of motion on the sky (both by Gaia as well as in this work) technically allow for negative observed parallax values, which is especially likely to occur when the position uncertainty is relatively large compared to the size of the parallax motion (e.g., see their Section 3 and Figure 2); as a result, they remind the reader that the Gaia observed parallaxes should not be thought of as a direct measurement of distance, and instead, distances need to be estimated by proper statistical modeling of the information contained in the astrometric solution distributions. ...
We present a hierarchical Bayesian pipeline, BP3M , that measures positions, parallaxes, and proper motions (PMs) for cross-matched sources between Hubble Space Telescope (HST) images and Gaia—even for sparse fields ( N * < 10 per image)—expanding from the recent GaiaHub tool. This technique uses Gaia-measured astrometry as priors to predict the locations of sources in HST images, and is therefore able to put the HST images onto a global reference frame without the use of background galaxies/QSOs. Testing our publicly available code in the Fornax and Draco dwarf spheroidal galaxies, we measure PMs that are a median of 8–13 times more precise than Gaia DR3 alone for 20.5 < G < 21 mag. We are able to explore the effect of observation strategies on BP3M astrometry using synthetic data, finding an optimal strategy to improve parallax and position precision at no cost to the PM uncertainty. Using 1619 HST images in the sparse COSMOS field (median nine Gaia sources per HST image), we measure BP3M PMs for 2640 unique sources in the 16 < G < 21.5 mag range, 25% of which have no Gaia PMs; the median BP3M PM uncertainty for 20.25 < G < 20.75 mag sources is 0.44 mas yr ⁻¹ compared to 1.03 mas yr ⁻¹ from Gaia, while the median BP3M PM uncertainty for sources without Gaia-measured PMs (20.75 < G < 21.5 mag) is 1.16 mas yr ⁻¹ . The statistics that underpin the BP3M pipeline are a generalized way of combining position measurements from different images, epochs, and telescopes, which allows information to be shared between surveys and archives to achieve higher astrometric precision than that from each catalog alone.
... Figure 2 shows the cumulative number of systems satisfying the above conditions as a function of the probed volume. To take into account the Lutz-Kelker bias (Lutz & Kelker 1973;Luri et al. 2018), causing the mean observed parallax to be systematically larger than its true value, the distance to each system is taken as the median value of 1/ϖ calculated based on 10,000 realizations of the parallax drawn from a Gaussian distribution defined using the observed Gaia parallax value and uncertainty. ...
The third data release of Gaia introduced a large catalog of astrometric binaries, out of which about 3200 are likely main-sequence stars with a white dwarf (WD) companion. These binaries are typically found with orbital separations of ∼1 au, a separation range that was largely unexplored due to observational challenges. Such systems are likely to have undergone a phase of stable mass transfer while the WD progenitor was on the asymptotic giant branch. Here we study the WD mass distribution of a volume-complete sample of binaries with K/M dwarf primaries and orbital separations of ∼1 au. We find that the number of massive WDs relative to the total number of WDs in these systems is smaller by an order of magnitude compared to their occurrence among single WDs in the field. One possible reason can be an implicit selection of the WD mass range if these are indeed post-stable-mass-transfer systems. Another reason can be the lack of merger products in our sample compared to the field, due to the relatively tight orbital separations of these systems. In addition, we find that about 14% of these systems have distant tertiary companions within 1 pc.