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Register a new userApplying full polarization A-Projection to very wide field of view instruments: An imager for LOFAR
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The aimed high sensitivities and large fields of view of the new generation of interferometers impose to reach high dynamic range of order $sim$1:$10^6$ to 1:$10^8$ in the case of the Square Kilometer Array. The main problem is the calibration and correction of the Direction Dependent Effects (DDE) that can affect the electro-magnetic field (antenna beams, ionosphere, Faraday rotation, etc.). As shown earlier the A-Projection is a fast and accurate algorithm that can potentially correct for any given DDE in the imaging step. With its very wide field of view, low operating frequency ($sim30-250$ MHz), long baselines, and complex station-dependent beam patterns, the Low Frequency Array (LOFAR) is certainly the most complex SKA precursor. In this paper we present a few implementations of A-Projection applied to LOFAR that can deal with non-unitary station beams and non-diagonal Mueller matrices. The algorithm is designed to correct for all the DDE, including individual antenna, projection of the dipoles on the sky, beam forming and ionospheric effects. We describe a few important algorithmic optimizations related to LOFARs architecture allowing us to build a fast imager. Based on simulated datasets we show that A-Projection can give dramatic dynamic range improvement for both phased array beams and ionospheric effects. We will use this algorithm for the construction of the deepest extragalactic surveys, comprising hundreds of days of integration.
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Astronomical widefield imaging of interferometric radio data is computationally expensive, especially for the large data volumes created by modern non-coplanar many-element arrays. We present a new widefield interferometric imager that uses the w-stacking algorithm and can make use of the w-snapshot algorithm. The performance dependencies of CASAs w-projection and our new imager are analysed and analytical functions are derived that describe the required computing cost for both imagers. On data from the Murchison Widefield Array, we find our new method to be an order of magnitude faster than w-projection, as well as being capable of full-sky imaging at full resolution and with correct polarisation correction. We predict the computing costs for several other arrays and estimate that our imager is a factor of 2-12 faster, depending on the array configuration. We estimate the computing cost for imaging the low-frequency Square-Kilometre Array observations to be 60 PetaFLOPS with current techniques. We find that combining w-stacking with the w-snapshot algorithm does not significantly improve computing requirements over pure w-stacking. The source code of our new imager is publicly released.
Contamination due to foregrounds (Galactic and Extra-galactic), calibration errors and ionospheric effects pose major challenges in detection of the cosmic 21 cm signal in various Epoch of Reionization (EoR) experiments. We present the results of a pilot study of a field centered on 3C196 using LOFAR Low Band (56-70 MHz) observations, where we quantify various wide field and calibration effects such as gain errors, polarized foregrounds, and ionospheric effects. We observe a `pitchfork structure in the 2D power spectrum of the polarized intensity in delay-baseline space, which leaks into the modes beyond the instrumental horizon (EoR/CD window). We show that this structure largely arises due to strong instrumental polarization leakage ($sim30%$) towards {Cas,A} ($sim21$ kJy at 81 MHz, brightest source in northern sky), which is far away from primary field of view. We measure an extremely small ionospheric diffractive scale ($r_{text{diff}} approx 430$ m at 60 MHz) towards {Cas,A} resembling pure Kolmogorov turbulence compared to $r_{text{diff}} sim3 - 20$ km towards zenith at 150 MHz for typical ionospheric conditions. This is one of the smallest diffractive scales ever measured at these frequencies. Our work provides insights in understanding the nature of aforementioned effects and mitigating them in future Cosmic Dawn observations (e.g. with SKA-low and HERA) in the same frequency window.
A revolution in radio receiving technology is underway with the development of densely packed phased arrays for radio astronomy. This technology can provide an exceptionally large field of view, while at the same time sampling the sky with high angular resolution. Such an instrument, with a field of view of over 100 square degrees, is ideal for performing fast, all-sky, surveys, such as the intensity mapping experiment to measure the signature of Baryonic Acoustic Oscillations in the HI mass distribution at cosmological redshifts. The SKA, built with this technology, will be able to do a billion galaxy survey. I will present a very brief introduction to radio interferometry, as well as an overview of the Square Kilometre Array project. This will be followed by a description of the EMBRACE prototype and a discussion of results and future plans.
One of the possible approaches to detecting optical counterparts of GRBs requires monitoring large parts of the sky. This idea has gained some instrumental support in recent years, such as with the Pi of the Sky project. The broad sky coverage of the Pi of the Sky apparatus results from using cameras with wide-angle lenses (20x20 deg field of view). Optics of this kind introduce significant deformations of the point spread function (PSF), increasing with the distance from the frame centre. A deformed PSF results in additional uncertainties in data analysis. Our aim was to create a model describing highly deformed PSF in optical astronomy, allowing uncertainties caused by image deformations to be reduced. Detailed laboratory measurements of PSF, pixel sensitivity, and pixel response functions were performed. These data were used to create an effective high quality polynomial model of the PSF. Finally, tuning the model and tests in applications to the real sky data were performed. We have developed a PSF model that accurately describes even very deformed stars in our wide-field experiment. The model is suitable for use in any other experiment with similar image deformation, with a simple tuning of its parameters. Applying this model to astrometric procedures results in a significant improvement over standard methods, while basic photometry precision performed with the model is comparable to the results of an optimised aperture algorithm. Additionally, the model was used to search for a weak signal -- namely a possible gamma ray burst optical precursor -- showing very promising results. Precise modelling of the PSF function significantly improves the astrometric precision and enhances the discovery potential of a wide-field system with lens optics.
Kilonovae are ultraviolet, optical, and infrared transients powered by the radioactive decay of heavy elements following a neutron star merger. Joint observations of kilonovae and gravitational waves can offer key constraints on the source of Galactic $r$-process enrichment, among other astrophysical topics. However, robust constraints on heavy element production requires rapid kilonova detection (within $sim 1$ day of merger) as well as multi-wavelength observations across multiple epochs. In this study, we quantify the ability of 13 wide field-of-view instruments to detect kilonovae, leveraging a large grid of over 900 radiative transfer simulations with 54 viewing angles per simulation. We consider both current and upcoming instruments, collectively spanning the full kilonova spectrum. The Roman Space Telescope has the highest redshift reach of any instrument in the study, observing kilonovae out to $z sim 1$ within the first day post-merger. We demonstrate that BlackGEM, DECam, GOTO, the Vera C. Rubin Observatorys LSST, ULTRASAT, and VISTA can observe some kilonovae out to $z sim 0.1$ ($sim$475 Mpc), while DDOTI, MeerLICHT, PRIME, $Swift$/UVOT, and ZTF are confined to more nearby observations. Furthermore, we provide a framework to infer kilonova ejecta properties following non-detections and explore variation in detectability with these ejecta parameters.
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