arXiv subjects

Sara Filali

Publications and source records attributed to Sara Filali.

14 recordsLinked to original sources

arXiv2026

Photometric calibration of the 4m ILMT data

The International Liquid Mirror Telescope (ILMT) is a 4-meter survey telescope continuously observing towards the zenith in the SDSS g', r', and i' bands. This survey telescope is designed to detect various astrophysical transients (for example, supernovae) and very faint objects like multiply-imaged quasars and low surface brightness galaxies. Ensuring accurate and stable calibration of supernova (SN) and quasar measurements is essential for studying their intrinsic variability and long-term brightness evolution. In this work, we present a dedicated calibration approach tailored for ILMT point-like object observations obtained in Time Delayed Integration (TDI) mode. Using ZTF reference sources cross-matched with the Pan-STARRS1 catalog, we derive robust photometric zero-points along the ILMT scan. This calibration procedure enables the production of consistent, high-precision SN and quasar light curves, suitable for variability studies, structure-function analysis, and future reverberation-mapping applications.

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arXiv2026

Efficient Identification and Analysis of Space debris Trails in Time-Domain Astronomy

This paper presents an automated end-to-end pipeline to detect and characterize artificial satellites and space debris streaks. As the exponential rise of orbital debris poses an escalating risk to global space assets, providing scalable, real-time tools is a critical contribution to the global Space Situational Awareness (SSA) network. The architecture utilizes a multi-stage geometric approach for raw image segmentation and high-precision morphological characterization. Experimental results demonstrate robust performance in real-world survey environments, successfully recovering streak profiles with an operational detection limit for features with integrated $SNR \ge 86$ and lengths $\ge 98$ arcseconds. This unsupervised framework strengthens global space sustainability efforts by monitoring RSO populations.

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arXiv2026

Characterisation of the ILMT 4K x 4K TDI CCD imager

In this work, we determine the gain and read-out noise of the 4Kx4K CCD imager used with the 4-m International Liquid Mirror Telescope (ILMT). Due to the zenith-pointing characteristic of the telescope, the CCD operates in the Time Delay Integration (TDI) mode for image acquisition. The CCD frames obtained during rising twilight have non-uniform illumination and can potentially be used to calculate important CCD parameters using two approaches. The first approach directly measures variance in flux along rows with minimum horizontal gradient, despite the strong vertical illumination gradient. The second method divides the CCD frame into regions exposed to similar background illumination and analyses the flux ratio variation between the two regions. Mathematically, we establish a linear relationship between the signal variance and average flux, from which gain is determined, while the read-out noise (RON) is measured independently from the bias frames. Both methods provide a gain value of approximately 3.9 e/ADU, while the RON measured from bias frames is 1.55 ADU (~ 6 e). Estimation of these parameters provides a better understanding of the CCD performance and allows for an accurate calibration.

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arXiv2026

Overview And Present Status Of The 4-m International Liquid Mirror Telescope (ILMT)

The International Liquid Mirror Telescope (ILMT) is an inexpensive optical instrument with a diameter of 4 metres, entirely dedicated to astronomical surveys. Its scientific programmes include the detection and study of transients, variable objects, asteroids, comets, low surface brightness galaxies, space debris, etc. The telescope achieved its first light on 29 April 2022 and was officially inaugurated on 21 March 2023. It has been operating regularly since October 2023 and continuously observes the sky passing through the zenith using the SDSS $g^{\prime}$, $r^{\prime}$, and $i^{\prime}$ filters in a narrow band of the sky ($22.3^{\prime}$) centred around the equatorial declination of $+29°21^{\prime}41^{\prime \prime}$. Here we report on the astrometric and photometric performance of the ILMT and summarise the main results obtained since the BINA3 workshop. A great way to raise public awareness is to build a scale model of a liquid mirror telescope. General guidelines for doing so are provided.

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arXiv2026

Optical observations of supernova candidates with the ILMT

The International Liquid Mirror Telescope (ILMT) is a 4-m optical ({\em gri} spectral bands) zenithal survey telescope with a field of view of 22.3$\times$22.3 arcmin, suitable for discovery and follow-up of faint transients. With the \texttt{PyLMT} pipeline, several new transients, including supernova candidates, are being discovered and detected routinely. We report the discovery of six supernova candidates, AT~2023yjc, AT~2024agkc, AT~2024aifv, AT~2024aiha, AT~2024fxn, and AT~2024zsm, using the ILMT. Ten additional supernova candidates were discovered in other surveys but were also detected with the ILMT. Here, we present the light curves of all supernova candidates observed with the ILMT in the {\em gri} bands. The basic light curve parameters, such as the rise time, decline rate, and peak magnitude, are estimated and reported.

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arXiv2026

Unveiling photometric transients with the ILMT and their follow-up using DOT

The two 4-m class telescopes at the Devasthal site in India are complementary: the 4-m International Liquid Mirror Telescope (ILMT) is dedicated to the discovery of new transient phenomena, which can be followed up using the 3.6-m Devasthal Optical Telescope (DOT) for their optical characterisation. The ILMT continuously surveys a $22.3^\prime \times 22.3^\prime$ wide strip of the zenith sky. A dedicated transient detection and classification pipeline, PyLMT, based on optimised image subtraction and convolutional neural network (CNN) techniques, enables the discovery of numerous photometric transients with the ILMT. However, these transient discoveries still require spectroscopic confirmation to determine their types, for which spectra obtained with the DOT are essential. Here, we present a selection of photometric transients, specifically supernova candidates. We investigate their photometric and spectroscopic properties using ILMT light curves, supplemented with ZTF data and DOT spectroscopic observations.

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arXiv2026

Modelling the Point Spread Function of ILMT images

We have devised an approach to model the empirical Point Spread Function (PSF) of the images taken with the 4-meter International Liquid Mirror Telescope (ILMT). The characteristics of the ILMT PSF are influenced by the nature of the Liquid Mirror, as well as by instrument specific and atmospheric factors. As the telescope uses Time Delay Integration (TDI) mode for tracking the passing objects, the PSF varies along the declination axis due to differential motion and star-trail curvature. The 5-element optical corrector homogenizes the PSF to some degree, but does not completely eliminate the disturbing effects mentioned above. The PSF in the RA direction is changing due to the sky conditions. We show that we can successfully model the PSF of 4k$\times$4k chunks using PSFEx, accounting for second-order X-Y variability. Our software robustly reproduces the PSF and its variation across the field.

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arXiv2026

From Sky to Science: Time Domain Astronomy with the ILMT Survey

The PyLMT transient detection pipeline has been operational since November 2023 and has detected $\sim$10,000 transients and variable objects in ILMT images in near real time. The survey has enabled the discovery and characterisation of a range of bright and faint optical transients. The detected population spans a broad diversity of astrophysical phenomena, including supernovae, variable stars, active galactic nuclei (AGN) variability, and Solar System objects such as asteroids and comets. We provide a concise overview of the detection of a variety of transient and variable classes identified with the ILMT, emphasising the survey's potential contributions to a broad class of astrophysical sources. We highlight representative science cases, including supernovae, AGNs, variable stars, and asteroids, to demonstrate the scientific potential of the ILMT survey.

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arXiv2026

Demonstrating the Time-Domain Capabilities of the 4-m International Liquid Mirror Telescope: An Early Census of Transient and Variable Detections

The PyLMT transient detection pipeline has been operational since November 2023, detecting transient and variable objects in the ILMT images in almost real time. Using the image subtraction technique, nearly 3700 CCD frames have been analyzed by the automated pipeline, generating nearly 23,000 alerts for the detection of verified transient candidates and cataloged variable sources. Around 21,000 of the alerts correspond to known MPC asteroids, nearly 2000 correspond to variable stars (including eclipsing binaries, RR Lyrae, Delta Scuti, T-Tauri, etc.), 509 correspond to variable AGNs (including QSOs, Seyfert galaxies, and blazars), 21 supernova candidates, and several other interesting candidates. We provide a concise overview of the detections and their significance, emphasizing the surveys potential contributions to a broad class of astrophysical and scientific cases. A transient detection dashboard called DART was developed using Streamlit to visualize and categorize candidates based on PyLMT and SIMBAD classifications. It includes cone-search functionality and displays key metadata, offering an intuitive interface that is publicly accessible. Our results demonstrate the viability of liquid mirror telescopes such as the ILMT for time-domain astronomy, emphasizing the important role that small-field survey facilities can play in systematic transient science programs.

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arXiv2026

Astrometric Calibration of the 4-m International Liquid Mirror Telescope Observations

The 4-m International Liquid Mirror Telescope (ILMT) is a dedicated time domain survey telescope that continuously scans the zenithal sky over the Indian Himalayas in the g', r' and i' optical bands. Its unique capability to repeatedly image the same strip of sky every night makes it a highly useful instrument for the photometric and astrometric studies of Solar System, Galactic and extragalactic objects. We present a robust astrometric calibration pipeline developed for the ILMT data obtained in the time delay integration (TDI) mode. The pipeline uses a linear transformation model from pixel to world coordinates, with a second order correction for the asymmetric optical distortions introduced by the telescope's optical corrector, and ties the astrometric solution to the Gaia DR3 reference frame. The pipeline is integrated to the routine ILMT data processing workflow. Using data from the first four observing cycles (2022-2025), we present the first assessment of the astrometric performance of the pipeline based on positional residuals of sources cross-matched with Gaia DR3. The pipeline achieves a typical astrometric precision of ~100 milliarcseconds (mas), reaching ~70-80 mas for moderately bright sources (G~16.5-18.5). These results, based on 347 nights of data, demonstrate the stability and reliability of ILMT astrometry over multi-year timescales. The astrometrically calibrated data from these four observing cycles have been made publicly available to the astronomical community. This work establishes a validated framework for precision astrometry with zenith-pointing TDI surveys and provides a foundation for future time-domain studies with ILMT, including variability characterization, transient localization, and long-term positional monitoring.

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arXiv2026

Detection and identification of asteroids with the 4-m ILMT

The International Liquid Mirror Telescope (ILMT) covers a 22.3' wide strip of sky in declination (δ), centred at δ = +29° 21' 41.4'' and right ascension (α) in the range 0 h <= α < 24 h. Having a short focal length (f /D ~ 2.4) and a large diameter (4 m), makes the ILMT an excellent asteroid hunter. The ILMT began its 4th cycle in October 2024, running through May 2025. The astrometric accuracy has been improved to 0.1'' , and the PyLMT - a detection and classification pipeline -, has been fine-tuned using data from previous cycles. The current detection rate is tens of transients detected each night with high accuracy in classification and identification. We present statistical results for the asteroids detected during ILMT's Cycles 1-4. We first evaluate the astrometric performance of the detections across different ecliptic latitude ranges. We then describe the positions, apparent motions, and V magnitudes predicted by the Minor Planet Center (MPC) for the asteroids observed in the SDSS g', r', and i' bands. Finally, we assess the ILMT's potential for detecting near-Earth objects (NEOs), potentially hazardous asteroids (PHAs), and comets.

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arXiv2026

Performance of the 4-m International Liquid Mirror Telescope tested in two fields at high and low ecliptic and galactic latitudes

The 4-m International Liquid Mirror Telescope (ILMT) offers a unique opportunity to detect transients in a narrow strip of sky. We explore ILMT's potential to detect astrometric and photometric transients at various ecliptic and galactic latitudes. We inspected CCD frames observed at both low and high ecliptic and galactic latitudes during the commissioning phase and the November 2023 - May 2024 observation cycle, respectively. We analysed these images using both visual inspection and the ILMT's transient detection and candidate classification pipeline. In the low ecliptic and galactic latitude field, we detected more than 500 transient candidates. We cross-matched these with the Minor Planet Checker (MPC) database, identifying 504 catalogued asteroids, all with predicted V-magnitudes brighter than 24 mag, representing a total of 152 distinct asteroids. We performed the same steps on the high ecliptic and galactic latitude field, detecting 30 MPC-catalogued asteroids, and one newly discovered photometric transient, named AT 2024fxn. We present the positions, trajectories, and magnitudes of the detected asteroids observed in the SDSS g', r', and i' spectral bands and compare results from both fields. We explore the lightcurve of AT 2024fxn, which shows partial compatibility with a supernova (SN) hypothesis, while the data invites further insights.

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arXiv2025

PyLMT: A transient detection pipeline for the 4-m International Liquid Mirror Telescope

The International Liquid Mirror Telescope (ILMT) is a 4-m aperture, zenith-pointing telescope with a field-of-view of 22', situated in the foothills of the Himalayas. The telescope operates in continuous survey mode, making it a useful instrument for time-domain astronomy, particularly for detecting transients, variable stars, active galactic nuclei variability, and asteroids. This paper presents the PyLMT transient detection pipeline to detect such transient/varying sources in the ILMT images. The pipeline utilises the image subtraction technique to compare a pair of images from the same field, identifying such sources in subtracted images with the help of convolutional neural networks (CNN) based real/bogus classifiers. The test accuracies determined for the real/bogus classifiers ranged from 94% to 98%. The resulting precision of the pipeline calculated over candidate alerts in the ILMT frames is 0.91. It also houses a CNN-aided transient candidate classifier that classifies the transient/variable candidates based on host morphology. The test accuracy of the candidate classifier is 98.6%. It has the provision to identify catalogued asteroids and other solar system objects using public databases. The median execution time of the pipeline is approximately 29 minutes per image of 17 minutes exposure. Relevant CNNs have been trained on data acquired with the ILMT during the cycle of October-November 2022. Subsequent tests on those images have confirmed the detection of numerous catalogued asteroids, variable stars, and other uncatalogued sources. The pipeline has been operational and has detected 12 extragalactic transients, including 2 new discoveries in the November 2023-May 2024 observation cycle.

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arXiv2025

The 4m International Liquid Mirror Telescope: Construction, operation, and science

The International Liquid Mirror Telescope (ILMT) project was motivated by the need for an inexpensive 4 metre diameter optical telescope that could be devoted entirely to astronomical surveys. Its scientific programmes include the detection and study of transients, variable objects, asteroids, comets, space debris and low surface brightness galaxies. To this end, a collaboration was formed between the Institute of Astrophysics and Geophysics (Liège University, Belgium), several Canadian universities (University of British Columbia, Laval University, University of Montreal, University of Toronto, York University, University of Victoria) and the Aryabhatta Research Institute of Observational Sciences (ARIES, India). After several years of design work in Belgium and construction in India on the ARIES Devasthal site, the telescope saw its first light on 29 April 2022. Its commissioning phase lasted from May 2022 until June 2023 (beginning of the monsoon). The ILMT was inaugurated on 21 March 2023 and has been in regular operation since October 2023. The telescope continuously observes the sky passing at the zenith using the SDSS g', r', and i' filters. This paper describes the ILMT, its operation, performance and shows some initial results.

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