We have studied the performance of the Ultraviolet Imaging Telescope payload on AstroSat and derived a calibration of the far-ultraviolet (FUV) and near-ultraviolet (NUV) instruments on board. We find that the sensitivity of both the FUV and NUV channels is as expected from ground calibrations, with the FUV effective area about 35 per cent and the NUV effective area about the same as that of GALEX. The point spread function of the instrument is on the order of 1.2-1.6''. We have found that pixel-to-pixel variations in the sensitivity are less than 10 per cent with spacecraft motion compensating for most of the flat-field variations. We derived a distortion correction but recommend that it be applied post-processing as part of an astrometric solution.
We present a study of six open clusters (Berkeley 67, King 2, NGC 2420, NGC 2477, NGC 2682 and NGC 6940) using the Ultra Violet Imaging Telescope (UVIT) aboard ASTROSAT and Gaia EDR3. We present their Gaia EDR3 membership catalogues and UV photometric catalogues along with membership probability. We used a supervised machine learning algorithm along with a Gaussian mixture model and used combinations of astrometric, photometric and systematic parameters to train the algorithm. We classified the stars as members, candidates and field using two such combinations. This generic technique is robust, reproducible, versatile in various cluster environments and is applicable to other data-sets. We could detect 200-2500 additional members using this method with respect to Gaia DR2 studies. We estimated cluster properties such as mean space velocities, distances, number of members and core radii. We detected 3 to 700 member stars, which include blue stragglers, main-sequence and red giants in various UVIT images of six clusters. We created UV-Optical colour-magnitude diagrams to find that majority of the sources in NGC 2682 and a few in NGC 2420, NGC 2477 and NGC 6940 showed excess UV flux. NGC 2682 images have 10 white dwarf detections in far-UV. The massive cluster NGC 2477 has 92/576 members detected in the far-UV/near-UV, which will be useful to study the UV properties of stars in the extended turn-off and in various evolutionary stages from main-sequence to red clump. Future studies will carry out panchromatic analysis of noteworthy members detected in this study.
At the distance of NGC 5128 (3.6+/-0.2Mpc) it is possible to resolve globular clusters with high resolution imaging from the ground, thus allowing the globular cluster candidate selection primarily through their morphological properties. I report the discovery of 71 globular clusters in NGC 5128 on VLT UT1+FORS1 images, including the faintest members (M_V_~-5) known to date in this galaxy as well as in 5 previously-known clusters. U- and V-band photometry has been measured for all the candidates and the luminosity function, spanning -10.1<M_V_<-4.9 and -9.3<M_U_<-3.3, constructed. These are the deepest globular cluster luminosity functions in an elliptical galaxy determined so far. The Kolmogorov-Smirnov statistics show that the difference between the globular cluster luminosity functions of NGC 5128 and the Milky Way is not larger than the difference between the ones of M 31 and the Milky Way. The (U-V)_0_ color histogram shows a bimodal distribution. For 23 globular clusters I obtained K-band images with SOFI at the NTT in La Silla. Their positions in the (U-V) vs. (V-K) color-color diagram indicate that they are indeed old globular clusters. Assuming that the globular clusters in NGC 5128 span a similar age range as the ones in the Milky Way and adopting a linear fit between the metallicity and (U-V)0 color, the metal-rich clusters peak at [Fe/H]=-0.6dex and the metal-poor ones peak at [Fe/H]=-1.7dex.
We present a polarimetric and spectroscopic study of the persistent ultra-compact X-ray binary 4U 0614+091 aimed at searching for the emission of a relativistic particle jet and at unveiling the orbital period Porb of the system. We obtained r-band polarimetric observations with the Telescopio Nazionale Galileo (TNG) equipped with the PAOLO polarimeter and with the Nordic Optical Telescope (NOT) equipped with the ALFOSC instrument, covering ~2h and ~0.5h observations, respectively. We carried out low resolution spectroscopy of the system using the ESO Very Large Telescope equipped with FORS1 for ~1.5h (16 spectra covering the range 4300-8000{AA}).
Here we provide the most comprehensive determinations of the rest-frame UV luminosity function (LF) available to date with the Hubble Space Telescope (HST) at z~2-9. Essentially all of the noncluster extragalactic legacy fields are utilized, including the Hubble Ultra Deep Field, the Hubble Frontier Fields parallel fields, and all five CANDELS fields, for a total survey area of 1136arcmin^2^. Our determinations include galaxies at z~2-3 leveraging the deep HDUV, UVUDF, and ERS WFC3/UVIS observations available over an ~150arcmin^2^ area in the GOODS-North and GOODS-South regions. All together, our collective samples include >24000 sources, >2.3x larger than previous selections with HST. We identify 5766, 6332, 7240, 3449, 1066, 601, 246, and 33 sources at z~2, 3, 4, 5, 6, 7, 8, and 9, respectively. Combining our results with an earlier z~10 LF determination by Oesch et al., we quantify the evolution of the UV LF. Our results indicate that there is (1) a smooth flattening of the faint-end slope {alpha} from {alpha}~-2.4 at z~10 to {alpha}~-1.5 at z~2, (2) minimal evolution in the characteristic luminosity M* at z>~2.5, and (3) a monotonic increase in the normalization log_10_\{phi}* from z~10 to 2, which can be well described by a simple second-order polynomial, consistent with an "accelerated" evolution scenario. We find that each of these trends (from z~10 to 2.5 at least) can be readily explained on the basis of the evolution of the halo mass function and a simple constant star formation efficiency model.
We report on the properties of a sample of ultraviolet-luminous galaxies (UVLGs) selected by matching the Galaxy Evolution Explorer (GALEX) All-Sky Imaging and Medium Imaging Surveys with the Sloan Digital Sky Survey third data release. The overlap between these two surveys is roughly 450deg^2^. Of 25362 galaxies (with SDSS spectroscopy) in the range 0.0<z<0.3 detected by GALEX, there are 215 galaxies with L>2x10^10^L{sun} at 1530{AA} (observed wavelength).
We present a robust measurement and analysis of the rest-frame ultraviolet (UV) luminosity functions at z=4-8. We use deep Hubble Space Telescope imaging over the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey/GOODS fields, the Hubble Ultra Deep Field, and the Hubble Frontier Field deep parallel observations near the Abell 2744 and MACS J0416.1-2403 clusters. The combination of these surveys provides an effective volume of 0.6-1.2x10^6^Mpc^3^ over this epoch, allowing us to perform a robust search for faint (M_UV_=-18) and bright (M_UV_<-21) high-redshift galaxies. We select candidate galaxies using a well-tested photometric redshift technique with careful screening of contaminants, finding a sample of 7446 candidate galaxies at 3.5<z<8.5, with >1000 galaxies at z~6-8. We measure both a stepwise luminosity function for candidate galaxies in our redshift samples, and a Schechter function, using a Markov Chain Monte Carlo analysis to measure robust uncertainties. At the faint end, our UV luminosity functions agree with previous studies, yet we find a higher abundance of UV-bright candidate galaxies at z>=6.
We present comprehensive observations and analysis of the energetic H-stripped SN 2016coi (a.k.a. ASASSN-16fp), spanning the {gamma}-ray through optical and radio wavelengths, acquired within the first hours to ~420 days post explosion. Our observational campaign confirms the identification of He in the supernova (SN) ejecta, which we interpret to be caused by a larger mixing of Ni into the outer ejecta layers. By modeling the broad bolometric light curve, we derive a large ejecta-mass-to-kinetic-energy ratio (M_ej_~4-7M_{sun}_, E_k_~(7-8)x10^51^erg). The small [CaII]{lambda}{lambda}7291,7324 to [OI]{lambda}{lambda}6300,6364 ratio (~0.2) observed in our late-time optical spectra is suggestive of a large progenitor core mass at the time of collapse. We find that SN 2016coi is a luminous source of X-rays (L_X_>10^39^erg/s in the first ~100 days post explosion) and radio emission (L_8.5GHz_~7x10^27^erg/s/Hz at peak). These values are in line with those of relativistic SNe (2009bb, 2012ap). However, for SN 2016coi, we infer substantial pre-explosion progenitor mass loss with a rate dM/dt~(1-2)x10^-4^M_{sun}_/yr and a sub-relativistic shock velocity v_sh_~0.15c, which is in stark contrast with relativistic SNe and similar to normal SNe. Finally, we find no evidence for a SN- associated shock breakout {gamma}-ray pulse with energy E_{gamma}_>2x10^46^erg. While we cannot exclude the presence of a companion in a binary system, taken together, our findings are consistent with a massive single-star progenitor that experienced large mass loss in the years leading up to core collapse, but was unable to achieve complete stripping of its outer layers before explosion.
We present a variability study of the lowest-luminosity Seyfert 1 nucleus of the galaxy NGC 4395 based on photometric monitoring campaigns in 2017 and 2018. Using 22 ground-based and space telescopes, we monitored NGC 4395 with a ~5-minute cadence during a period of 10 days and obtained light curves in the ultraviolet (UV), V, J, H, and K/K_s_ bands, as well as narrowband H{alpha}. The rms variability is ~0.13mag in the Swift UVM2 and V filter light curves, decreasing down to ~0.01mag in the K filter. After correcting for the continuum contribution to the H{alpha} narrow band, we measured the time lag of the H{alpha} emission line with respect to the V-band continuum as 55_-31_^+27^-122_-67_^+33^min in 2017 and 49_-14_^+15^-83_-14_^+13^min in 2018, depending on assumptions about the continuum variability amplitude in the H{alpha} narrow band. We obtained no reliable measurements for the continuum-to-continuum lag between UV and V bands and among near-IR bands, owing to the large flux uncertainty of UV observations and the limited time baseline. We determined the active galactic nucleus (AGN) monochromatic luminosity at 5100{AA}, {lambda}L_{lambda}_=(5.75+/-0.40)x10^39^erg/s, after subtracting the contribution of the nuclear star cluster. While the optical luminosity of NGC 4395 is two orders of magnitude lower than that of other reverberation-mapped AGNs, NGC 4395 follows the size-luminosity relation, albeit with an offset of 0.48dex (>=2.5{sigma}) from the previous best-fit relation of Bentz+ 2013ApJ...767..149B
We present the first effort to aggregate, homogenize, and uniformly model the combined ultraviolet, optical, and near-infrared data set for the electromagnetic counterpart of the binary neutron star merger GW170817. By assembling all of the available data from 18 different papers and 46 different instruments, we are able to identify and mitigate systematic offsets between individual data sets and to identify clear outlying measurements, with the resulting pruned and adjusted data set offering an opportunity to expand the study of the kilonova. The unified data set includes 647 individual flux measurements, spanning 0.45-29.4d post-merger, and thus has greater constraining power for physical models than any single data set. We test a number of semi-analytical models and find that the data are well modeled with a three-component kilonova model: a "blue" lanthanide-poor component ({kappa}=0.5cm^2^/g) with M_ej_~0.020M_{sun}_ and v_ej_~0.27c; an intermediate opacity "purple" component ({kappa}=3cm^2^/g) with M_ej_~0.047M_{sun}_ and v_ej_~0.15c; and a "red" lanthanide-rich component ({kappa}=10cm^2^/g) with M_ej_~0.011M_{sun}_ and v_ej_~0.14c. We further explore the possibility of ejecta asymmetry and its impact on the estimated parameters. From the inferred parameters we draw conclusions about the physical mechanisms responsible for the various ejecta components, the properties of the neutron stars, and, combined with an up-to-date merger rate, the implications for r-process enrichment via this channel. To facilitate future studies of this keystone event we make the unified data set and our modeling code public.
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