arXiv subjects

R. Ansari

Publications and source records attributed to R. Ansari.

At least 19 recordsLinked to original sources

arXiv2026

Photometric redshifts for active galactic nuclei with LePHARE for the Vera C. Rubin Observatory

Active Galactic Nuclei (AGN) play a crucial role in galaxy evolution, but they are a minority of extragalactic sources with diverse Spectral Energy Distributions (SEDs), which depend on their means of selection. Upcoming large-scale surveys such as LSST will identify many AGN, but analysis tools are not optimized for them. The limited number of photometric bands in these surveys impacts the calculation of photometric redshifts for AGN, which are essential for scientific advancement. We use LePHARE to demonstrate the impact that a limited number of bands and erroneous assumptions have on the determination of the photometric redshifts of AGN. We conduct tests on six AGN samples selected using X-ray, radio, infrared, variability, color, and spectroscopic criteria in the COSMOS field, using photometry from HSC-CLAUDS, which is closest in depth and wavelength coverage to LSST. We present the LSST pipeline for LePHARE within the Redshift Assessment Infrastructure Layers (RAIL), facilitating comparison between SED fitting and machine learning algorithms. AGN that appear as point-like sources in optical data will be assigned highly unreliable photometric redshifts if they are processed using galaxy templates. Additionally, shallow all-sky surveys (like eROSITA, WISE, and ZTF) miss many AGN. As a result, these "hidden" AGN are often misidentified as galaxies in public survey data, leading to incorrect photometric redshift. We provide the configurations that are suggested for each type of AGN alongside measures of expected performance as a function of redshift, magnitude, and selection. To facilitate studies with a panchromatic view of AGN, we also release photometric redshifts and posterior distributions for all AGN sources identified in the COSMOS field using the six criteria, based on 28-band photometry.

astro-ph.GA↗

arXiv2025

EROS light curve data base

The EROS project (Exp\'erience de Recherche d'Objets Sombres) carried out photometric surveys of dense stellar fields towards the Magellanic Clouds (LMC and SMC), the Galactic Bulge and Galactic spiral arms, over the period 1990-2003. The main goal of the experiment was to search for the Galactic Dark Matter in the form of massive compact objects (machos), through the gravitational microlensing effect. The historical record of the flux variations of the monitored stars by EROS-2 will be a unique asset for time domain astronomy and to complement current and future searches of transient sources. We describe the set of light curves obtained by EROS-2 program over the years 1996 to 2003, monitoring more than 86 million stars, which is publicly released through the Centre de Donn\'ees de Strasbourg (CDS). The numbers of light curves in this data set are 28.7 and 4.0 million in LMC, respectively SMC, 42.9 million in the Galactic Bulge and 10.4 million towards the Galactic spiral arms, with several hundred measurements for each object. Data from EROS-1 is also being released. The object catalog and light curves and images are accessible through the CDS portal. This will be useful for checking the past behavior of newly discovered variable objects.

astro-ph.GA↗

arXiv2022

New limits from microlensing on Galactic Black Holes in the mass range $10M_{\odot}<M<1000M_{\odot}$

We have searched for long duration microlensing events originating from intermediate mass Black Holes (BH) in the halo of the Milky Way, using archival data from EROS-2 and MACHO photometric surveys towards the Large Magellanic Cloud. We combined data from these two surveys to create a common database of light curves for 14.1 million objects in LMC, covering a total duration of 10.6 years, with flux series measured through four wide passbands. We have carried out a microlensing search on these light curves, complemented by the light curves of 22.7 million objects, observed by EROS-2 only or MACHO only over about 7 years, with flux series measured through only two passbands. A likelihood analysis, taking into account LMC self lensing and Milky Way disk contributions allows us to conclude that compact objects with masses in the range $10 - 100 M_{\odot}$ cannot make up more than $\sim 15\%$ of a standard halo total mass (at $95\%$ confidence level). Our analysis sensitivity weakens for heavier objects, although we still exclude that $\sim 50\%$ of the halo be made of $\sim 1000 M_{\odot}$ BHs. Combined with previous EROS results, an upper limit of $\sim 15\%$ of the total halo mass can be obtained for the contribution of compact halo objects in the mass range $10^{-6} - 10^2 M_{\odot}$.

astro-ph.GA↗

arXiv2020

A DFT study on the mechanical properties of hydrogenated and fluorinated germanene sheets

The density functional theory (DFT) is used to investigate the mechanical properties of pure, fully hydrogenated, semi-hydrogenated, fully fluorinated, and semi-fluorinated germanene sheets, including Young's and bulk moduli, and plastic properties. Also, the electronic properties, namely the planar average electron difference density, are considered to evaluate the bonding characteristics of pure and adsorbed germanenes. The results show that the effect of adsorption decreases the electron accumulation between Ge atoms, i.e. weaker covalent bonds. This weakening in the covalent bonds results in the reduction of Young's and bulk moduli, which are calculated through the second derivation of the total energy versus strain. Furthermore, it is observed that the yield strain of fully fluorinated germanene remains unchanged under uniaxial loading as compared to the pristine structure. Moreover, the yield strain of the germanene is increased under biaxial loading, while it is affected by full hydrogenation adsorption.

cond-mat.mtrl-sci↗

arXiv2020

Design, operation and performance of the PAON4 prototype transit interferometer

PAON4 is an L-band (1250-1500 MHz) small interferometer operating in transit mode deployed at the Nançay observatory in France, designed as a prototype instrument for Intensity Mapping. It features four 5~meter diameter dishes in a compact triangular configuration, with a total geometric collecting area of $\sim75 \mathrm{m^2}$, and equipped with dual polarization receivers. A total of 36 visibilities are computed from the 8 independent RF signals by the software correlator over the full 250~MHz RF band. The array operates in transit mode, with the dishes pointed toward a fixed declination, while the sky drifts across the instrument. Sky maps for each frequency channel are then reconstructed by combining the time-dependent visibilities from the different baselines observed at different declinations. This paper presents an overview of the PAON4 instrument design and goals, as a prototype for dish arrays to map the Large Scale Structure in radio, using intensity mapping of the atomic hydrogen $21~\mathrm{cm}$ line. We operated PAON4 over several years and use data from observations in different periods to assess the array performance. We present preliminary analysis of a large fraction of this data and discuss crucial issues for this type of instrument, such as the calibration strategy, instrument response stability, and noise behaviour.

astro-ph.IM↗

arXiv2018

French SKA White Book - The French Community towards the Square Kilometre Array

The "Square Kilometre Array" (SKA) is a large international radio telescope project characterised, as suggested by its name, by a total collecting area of approximately one square kilometre, and consisting of several interferometric arrays to observe at metric and centimetric wavelengths. The deployment of the SKA will take place in two sites, in South Africa and Australia, and in two successive phases. From its Phase 1, the SKA will be one of the most formidable scientific machines ever deployed by mankind, and by far the most impressive in terms of data throughput and required computing power. With the participation of almost 200 authors from forty research institutes and six private companies, the publication of this French SKA white paper illustrates the strong involvement in the SKA project of the French astronomical community and of a rapidly growing number of major scientific and technological players in the fields of Big Data, high performance computing, energy production and storage, as well as system integration.

astro-ph.IM↗

arXiv2015

On sky characterization of the BAORadio wide band digital backend: Search for HI emission in Abell85, Abell1205 and Abell2440 galaxy clusters

We have observed regions of three galaxy clusters at z$\sim$ [0.06, 0.09] (Abell85, Abell1205, Abell2440), as well as calibration sources with the Nancay radiotelescope (NRT) to search for 21 cm emission and fully characterize the FPGA based BAORadio digital backend. The total observation time of few hours per source have been distributed over few months, from March 2011 to January 2012, due to scheduling constraints of the NRT, which is a transit telescope. Data have been acquired in parallel with the NRT standard correlator (ACRT) back-end, as well as with the BAORadio data acquisition system. The latter enables wide band instantaneous observation of the [1250, 1500]MHz frequency range, as well as the use of powerful RFI mitigation methods thanks to its fine time sampling. A number of questions related to instrument stability, data processing and calibration are discussed. We have obtained the radiometer curves over the integration time range [0.01,10 000] seconds and we show that sensitivities of few mJy over most of the wide frequency band can be reached with the NRT. It is clearly shown that in blind line search, which is the context of HI intensity mapping for Baryon Acoustic Oscillations, the new acquisition system and processing pipeline outperforms the standard one. We report a positive detection of 21 cm emission at 3 sigma-level from galaxies in the outer region of Abell85 at 1352 MHz (14 400 km/s) corresponding to a line strength of 0.8 Jy km/s. We observe also an excess power around 1318 MHz (21 600 km/s), although at lower statistical significance, compatible with emission from Abell1205 galaxies. Detected radio line emissions have been cross matched with optical catalogs and we have derived hydrogen mass estimates.

astro-ph.IM↗

arXiv2013

Spectroscopic Needs for Imaging Dark Energy Experiments: Photometric Redshift Training and Calibration

Large sets of objects with spectroscopic redshift measurements will be needed for imaging dark energy experiments to achieve their full potential, serving two goals:_training_, i.e., the use of objects with known redshift to develop and optimize photometric redshift algorithms; and_calibration_, i.e., the characterization of moments of redshift (or photo-z error) distributions. Better training makes cosmological constraints from a given experiment stronger, while highly-accurate calibration is needed for photo-z systematics not to dominate errors. In this white paper, we investigate the required scope of spectroscopic datasets which can serve both these purposes for ongoing and next-generation dark energy experiments, as well as the time required to obtain such data with instruments available in the next decade. Large time allocations on kilo-object spectrographs will be necessary, ideally augmented by infrared spectroscopy from space. Alternatively, precision calibrations could be obtained by measuring cross-correlation statistics using samples of bright objects from a large baryon acoustic oscillation experiment such as DESI. We also summarize the additional work on photometric redshift methods needed to prepare for ongoing and future dark energy experiments.

astro-ph.CO↗

arXiv2013

Search for hidden turbulent gas through interstellar scintillation

Stars twinkle because their light propagates through the atmosphere. The same phenomenon is expected when the light of remote stars crosses a Galactic - disk or halo - refractive medium such as a molecular cloud. We present the promising results of a test performed with the ESO-NTT and the perspectives of detection.

astro-ph.GA↗

arXiv2012

Search for Turbulent Gas through Interstellar Scintillation

Stars twinkle because their light propagates through the atmosphere. The same phenomenon is expected when the light of remote stars crosses a Galactic - disk or halo - refractive medium such as a molecular cloud.We present the promising results of a test performed with the ESO-NTT and the perspectives.

astro-ph.GA↗

arXiv2011

Planck early results. VI. The High Frequency Instrument data processing

We describe the processing of the 336 billion raw data samples from the High Frequency Instrument (HFI) which we performed to produce six temperature maps from the first 295 days of Planck-HFI survey data. These maps provide an accurate rendition of the sky emission at 100, 143, 217, 353, 545 and 857 GHz with an angular resolution ranging from 9.9 to 4.4^2. The white noise level is around 1.5 μK degree or less in the 3 main CMB channels (100--217GHz). The photometric accuracy is better than 2% at frequencies between 100 and 353 GHz and around 7% at the two highest frequencies. The maps created by the HFI Data Processing Centre reach our goals in terms of sensitivity, resolution, and photometric accuracy. They are already sufficiently accurate and well-characterised to allow scientific analyses which are presented in an accompanying series of early papers. At this stage, HFI data appears to be of high quality and we expect that with further refinements of the data processing we should be able to achieve, or exceed, the science goals of the Planck project.

astro-ph.CO↗

arXiv2011

21 cm observation of LSS at z~1 Instrument sensitivity and foreground subtraction

Large Scale Structures (LSS) in the universe can be traced using the neutral atomic hydrogen HI through its 21cm emission. Such a 3D matter distribution map can be used to test the Cosmological model and to constrain the Dark Energy properties or its equation of state. A novel approach, called intensity mapping can be used to map the HI distribution, using radio interferometers with large instantaneous field of view and waveband. In this paper, we study the sensitivity of different radio interferometer configurations, or multi-beam instruments for the observation of large scale structures and BAO oscillations in 21cm and we discuss the problem of foreground removal. For each configuration, we determine instrument response by computing the (u,v) or Fourier angular frequency plane coverage using visibilities. The (u,v) plane response is the noise power spectrum, hence the instrument sensitivity for LSS P(k) measurement. We describe also a simple foreground subtraction method to separate LSS 21 cm signal from the foreground due to the galactic synchrotron and radio sources emission. We have computed the noise power spectrum for different instrument configuration as well as the extracted LSS power spectrum, after separation of 21cm-LSS signal from the foregrounds. We have also obtained the uncertainties on the Dark Energy parameters for an optimized 21 cm BAO survey. We show that a radio instrument with few hundred simultaneous beams and a collecting area of ~10000 m^2 will be able to detect BAO signal at redshift z ~ 1 and will be competitive with optical surveys.

astro-ph.CO↗

arXiv2011

BAORadio : Cartographie 3D de la distribution de gaz H$_I$ dans l'Univers

3D mapping of matter distribution in the universe through the 21 cm radio emission of atomic hydrogen is a complementary approach to optical surveys for the study of the Large Scale Structures, in particular for measuring the BAO (Baryon Acoustic Oscillation) scale up to redshifts z <~ 3 and constrain dark energy. We propose to carry such a survey through a novel method, called intensity mapping, without detecting individual galaxies radio emission. This method requires a wide band instrument, 100 MHz or larger, and multiple beams, while a rather modest angular resolution of 10 arcmin would be sufficient. The instrument would have a few thousand square meters of collecting area and few hundreds of simultaneous beams. These constraints could be fulfilled with a dense array of receivers in interferometric mode, or a phased array at the focal plane of a large antenna.

astro-ph.IM↗

arXiv2011

Planck early results: first assessment of the High Frequency Instrument in-flight performance

The Planck High Frequency Instrument (HFI) is designed to measure the temperature and polarization anisotropies of the Cosmic Microwave Background and galactic foregrounds in six wide bands centered at 100, 143, 217, 353, 545 and 857 GHz at an angular resolution of 10' (100 GHz), 7' (143 GHz), and 5' (217 GHz and higher). HFI has been operating flawlessly since launch on 14 May 2009. The bolometers cooled to 100 mK as planned. The settings of the readout electronics, such as the bolometer bias current, that optimize HFI's noise performance on orbit are nearly the same as the ones chosen during ground testing. Observations of Mars, Jupiter, and Saturn verified both the optical system and the time response of the detection chains. The optical beams are close to predictions from physical optics modeling. The time response of the detection chains is close to pre-launch measurements. The detectors suffer from an unexpected high flux of cosmic rays related to low solar activity. Due to the redundancy of Planck's observations strategy, the removal of a few percent of data contaminated by glitches does not affect significantly the sensitivity. The cosmic rays heat up significantly the bolometer plate and the modulation on periods of days to months of the heat load creates a common drift of all bolometer signals which do not affect the scientific capabilities. Only the high energy cosmic rays showers induce inhomogeneous heating which is a probable source of low frequency noise.

astro-ph.IM↗

arXiv2010

Searching for Galactic hidden gas through interstellar scintillation: Results from a test with the NTT-SOFI detector

Aims: Stars twinkle because their light propagates through the atmosphere. The same phenomenon is expected at a longer time scale when the light of remote stars crosses an interstellar molecular cloud, but it has never been observed at optical wavelength. In a favorable case, the light of a background star can be subject to stochastic fluctuations on the order of a few percent at a characteristic time scale of a few minutes. Our ultimate aim is to discover or exclude these scintillation effects to estimate the contribution of molecular hydrogen to the Galactic baryonic hidden mass. This feasibility study is a pathfinder toward an observational strategy to search for scintillation, probing the sensitivity of future surveys and estimating the background level. Methods: We searched for scintillation induced by molecular gas in visible dark nebulae as well as by hypothetical halo clumpuscules of cool molecular hydrogen ($\mathrm{H_2-He}$) during two nights. We took long series of 10s infrared exposures with the ESO-NTT telescope toward stellar populations located behind visible nebulae and toward the Small Magellanic Cloud (SMC). We therefore searched for stars exhibiting stochastic flux variations similar to what is expected from the scintillation effect. According to our simulations of the scintillation process, this search should allow one to detect (stochastic) transverse gradients of column density in cool Galactic molecular clouds of order of $\sim 3\times 10^{-5}\,\mathrm{g/cm^2/10\,000\,km}$. Results: We found one light-curve that is compatible with a strong scintillation effect through a turbulent structure characterized by a diffusion radius $R_{diff}<100\, km$ in the B68 nebula. Complementary observations are needed to clarify the status of this candidate, and no firm conclusion can be established from this single observation. We can also infer limits on the existence of turbulent dense cores (of number density $n>10^9\, cm^{-3}$) within the dark nebulae. Because no candidate is found toward the SMC, we are also able to establish upper limits on the contribution of gas clumpuscules to the Galactic halo mass. Conclusions: The limits set by this test do not seriously constrain the known models, but we show that the short time-scale monitoring for a few $10^6 star\times hour$ in the visible band with a $>4$ meter telescope and a fast readout camera should allow one to quantify the contribution of turbulent molecular gas to the Galactic halo. The LSST (Large Synoptic Survey Telescope) is perfectly suited for this search.

astro-ph.GA↗

arXiv2010

Partial CMB maps: bias removal and optimal binning of the angular power spectrum

We present a semi-analytical method to investigate the systematic effects and statistical uncertainties of the calculated angular power spectrum when incomplete spherical maps are used. The computed power spectrum suffers in particular a loss of angular frequency resolution, which can be written as δ_l ~ π/γ_max, where γ_max is the effective maximum extent of the partial spherical maps. We propose a correction algorithm to reduce systematic effects on the estimated C_l, as obtained from the partial map projection on the spherical harmonic Ylm(l,m) basis. We have derived near optimal bands and weighting functions in l-space for power spectrum calculation using small maps, and a correction algorithm for partially masked spherical maps that contain information on the angular correlations on all scales.

astro-ph.IM↗

arXiv2010

Baryonic acoustic oscillations simulations for the Large Synoptic Survey Telescope (LSST)

The baryonic acoustic oscillations are features in the spatial distribution of the galaxies which, if observed at different epochs, probe the nature of the dark energy. In order to be able to measure the parameters of the dark energy equation of state to high precision, a huge sample of galaxies has to be used. The Large Synoptic Survey Telescope will survey the optical sky with 6 filters from 300nm and 1100nm, such that a catalog of galaxies with photometric redshifts will be available for dark energy studies. In this article, we will give a rough estimate of the impact of the photometric redshift uncertainties on the computation of the dark energy parameter through the reconstruction of the BAO scale from a simulated photometric catalog.

astro-ph.CO↗

arXiv2009

New Magellanic Cloud R Coronae Borealis and DY Per type stars from the EROS-2 database: the connection between RCBs, DYPers and ordinary carbon stars

R Coronae Borealis stars (RCB) are a rare type of evolved carbon-rich supergiant stars that are increasingly thought to result from the merger of two white dwarfs, called the Double degenerate scenario. This scenario is also studied as a source, at higher mass, of type Ia Supernovae (SnIa) explosions. Therefore a better understanding of RCBs composition would help to constrain simulations of such events. We searched for and studied RCB stars in the EROS Magellanic Clouds database. We also extended our research to DY Per type stars (DYPers) that are expected to be cooler RCBs (T~3500 K) and much more numerous than their hotter counterparts. The light curves of ~70 millions stars have been analysed to search for the main signature of RCBs and DYPers: a large drop in luminosity. Follow-up optical spectroscopy was used to confirm each photometric candidate found. We have discovered and confirmed 6 new Magellanic Cloud RCB stars and 7 new DYPers, but also listed new candidates: 3 RCBs and 14 DYPers. We estimated a range of Magellanic RCB shell temperatures between 360 and 600 K. We confirm the wide range of absolute luminosity known for RCB stars, M_V~-5.2 to -2.6. Our study further shows that mid-infrared surveys are ideal to search for RCB stars, since they have thinner and cooler circumstellar shells than classical post-AGB stars. In addition, by increasing the number of known DYPers by ~400%, we have been able to shed light on the similarities in the spectral energy distribution between DYPers and ordinary carbon stars. We also observed that DYPer circumstellar shells are fainter and hotter than those of RCBs. This suggests that DYPers may simply be ordinary carbon stars with ejection events, but more abundance analysis is necessary to give a status on a possible evolutionnary connexion between RCBs and DYPers.

astro-ph.SR↗