Optical Transient Search Research Articles

A search for extragalactic fast optical transients in the Tomo-e Gozen high-cadence survey

ABSTRACT The population of optical transients evolving within a time-scale of a few hours or a day (so-called fast optical transients, FOTs) has recently been debated extensively. In particular, our understanding of extragalactic FOTs and their rates is limited. We present a search for extragalactic FOTs with the Tomo-e Gozen high-cadence survey. Using the data taken from 2019 August to 2022 June, we obtain 113 FOT candidates. Through light curve analysis and cross-matching with other survey data, we find that most of these candidates are in fact supernovae, variable quasars, and Galactic dwarf novae that were partially observed around their peak brightness. We find no promising candidate of extragalactic FOTs. From this non-detection, we obtain upper limits on the event rate of extragalactic FOTs as a function of their time-scale. For a very luminous event (absolute magnitude M < −26 mag), we obtain the upper limits of 4.4 × 10−9 Mpc−3 yr−1 for a time-scale of 4 h, and 7.4 × 10−10 Mpc−3 yr−1 for a time-scale of 1 d. Thanks to our wide (although shallow) surveying strategy, our data are less affected by the cosmological effects, and thus, give one of the more stringent limits to the event rate of intrinsically luminous transients with a time-scale of <1 d.

A Search for Extragalactic Fast Blue Optical Transients in ZTF and the Rate of AT2018cow-like Transients

We present a search for extragalactic fast blue optical transients (FBOTs) during Phase I of the Zwicky Transient Facility (ZTF). We identify 38 candidates with durations above half-maximum light 1 day < t 1/2 < 12 days, of which 28 have blue (g − r ≲ −0.2 mag) colors at peak light. Of the 38 transients (28 FBOTs), 19 (13) can be spectroscopically classified as core-collapse supernovae (SNe): 11 (8) H- or He-rich (Type II/IIb/Ib) SNe, 6 (4) interacting (Type IIn/Ibn) SNe, and 2 (1) H&He-poor (Type Ic/Ic-BL) SNe. Two FBOTs (published previously) had predominantly featureless spectra and luminous radio emission: AT2018lug (The Koala) and AT2020xnd (The Camel). Seven (five) did not have a definitive classification: AT 2020bdh showed tentative broad Hα in emission, and AT 2020bot showed unidentified broad features and was 10 kpc offset from the center of an early-type galaxy. Ten (eight) have no spectroscopic observations or redshift measurements. We present multiwavelength (radio, millimeter, and/or X-ray) observations for five FBOTs (three Type Ibn, one Type IIn/Ibn, one Type IIb). Additionally, we search radio-survey (VLA and ASKAP) data to set limits on the presence of radio emission for 24 of the transients. All X-ray and radio observations resulted in nondetections; we rule out AT2018cow-like X-ray and radio behavior for five FBOTs and more luminous emission (such as that seen in the Camel) for four additional FBOTs. We conclude that exotic transients similar to AT2018cow, the Koala, and the Camel represent a rare subset of FBOTs and use ZTF’s SN classification experiments to measure the rate to be at most 0.1% of the local core-collapse SN rate.

Search for Astrophysical Nanosecond Optical Transients with TAIGA-HiSCORE Array

A wide-angle Cerenkov array TAIGA-HiSCORE (FOV $\sim$0.6 sr), was originally created as a part of TAIGA installation for high-energy gamma-ray astronomy and cosmic ray physics. Array now consist on nearly 100 optical stations on the area of 1 km$^2$. Due to high accuracy and stability ($\sim$1 ns) of time synchronization of the optical stations the accuracy of EAS arrival direction reconstruction is reached 0.1$^\mathrm{o}$. It was proven that the array can also be used to search for nanosecond events of the optical range. The report discusses the method of searching for optical transients using the HiSCORE array and demonstrates its performance on a real example of detecting signals from an artificial Earth satellite. The search for this short flares in the HiSCORE data of the winter season 2018--2019 is carried out. One candidate for double repeater has been detected, but the estimated probability of random simulation of such a transient by background EAS events is not less than 10%, which does not allow us to say that the detected candidate corresponds to a real astrophysical transient. An upper bound on the frequency of optical spikes with flux density of more than $10^{-4} \mathrm{erg/s/cm}^2$ and a duration of more than 5\,ns is established as $\sim 2 \times 10^{-3}$ events/sr/hour.

High-cadence optical transient searches using drift scan imaging III: Development of an inexpensive drive control system and characterisation and correction of drive system periodic errors

Abstract In order to further develop and implement novel drift scan imaging experiments to undertake wide-field, high time resolution surveys for millisecond optical transients, an appropriate telescope drive system is required. This paper describes the development of a simple and inexpensive hardware and software system to monitor, characterise, and correct the primary category of telescope drive errors and periodic errors due to imperfections in the drive and gear chain. A model for the periodic errors is generated from direct measurements of the telescope drive shaft rotation, verified by comparison to astronomical measurements of the periodic errors. The predictive model is generated and applied in real time in the form of corrections to the drive rate. A demonstration of the system shows that inherent periodic errors of peak-to-peak amplitude ${\sim}{100}''$ are reduced to below the seeing limit of ${\sim}3''$ . This demonstration allowed an estimate of the uncertainties on the transient sensitivity timescales of the prototype survey of Tingay $\&amp;$ Joubert (2021), with the nominal timescale sensitivity of 21 ms revised to be in the range of $20\!-\!22$ ms, which does not significantly affect the results of the experiment. The correction system will be adopted into the final version of high-cadence imaging experiment, which is currently under construction. The correction system is inexpensive ( $&lt;\!{\$}$ A100) and composed of readily available hardware and is readily adaptable to other applications. Design details and codes are therefore made publicly available.

High-cadence optical transient searches using drift scan imaging I: Proof of concept with a pre-prototype system

AbstractAn imaging technique with sensitivity to short duration optical transients is described. The technique is based on the use of wide-field cameras operating in a drift scanning mode, whereby persistent objects produce trails on the sensor and short duration transients occupy localised groups of pixels. A benefit of the technique is that sensitivity to short duration signals is not accompanied by massive data rates, because the exposure time is much greater than the transient duration. The technique is demonstrated using a pre-prototype system composed of readily available and inexpensive commercial components, coupled with common coding environments, commercially available software, and free web-based services. The performance of the technique and the pre-prototype system is explored, including aspects of photometric and astrometric calibration, detection sensitivity, characterisation of candidate transients, and the differentiation of astronomical signals from non-astronomical signals (primarily glints from satellites in Earth orbit and cosmic ray hits on sensor pixels). Test observations were made using the pre-prototype system, achieving sensitivity to transients with 21-ms duration, resulting in the detection of five candidate transients. An investigation of these candidates concludes they are most likely due to cosmic ray hits on the sensor and/or satellites. The sensitivity obtained with the pre-prototype system is such that, under some models for the optical emission from fast radio bursts (FRBs), the detection of a typical FRB, such as FRB181228, to a distance of approximately 100 Mpc is plausible. Several options for improving the system/technique in the future are described.

The Palomar Transient Factory Sky2Night programme

Abstract We present results of the Sky2Night project: a systematic, unbiased search for fast optical transients with the Palomar Transient Factory. We have observed 407deg2 in R-band for eight nights at a cadence of 2 h. During the entire duration of the project, the 4.2 m William Herschel Telescope on La Palma was dedicated to obtaining identification spectra for the detected transients. During the search, we found 12 supernovae, 10 outbursting cataclysmic variables, nine flaring M-stars, three flaring active galactic nuclei, and no extragalactic fast optical transients. Using this systematic survey for transients, we have calculated robust observed rates for the detected types of transients, and upper limits of the rate of extragalactic fast optical transients of $\mathcal {R}\lt 37\times 10^{-4}$deg−2 d−1 and $\mathcal {R}\lt 9.3\times 10^{-4}$deg−2 d−1 for time-scales of 4 h and 1 d and a limiting magnitude of R ≈ 19.7. We use the results of this project to determine what kind of and how many astrophysical false positives we can expect when following up gravitational wave detections in search for kilonovae.

A search for optical transients associated with fast radio burst 150418

Abstract We have searched for optical variability in the host galaxy of the radio variable source possibly associated with fast radio burst (FRB) 150418. We compare images of the galaxy taken 1 day after the burst using Subaru/Suprime-Cam with images taken ∼1 yr after the burst using Gemini-South/GMOS. No optical variability is found between the two epochs with a limiting absolute magnitude ≳ −19 (AB). This limit applies to the optical variability of the putative active galactic nucleus in the galaxy on a timescale of ∼1 yr, and also to the luminosity of an optical counterpart of FRB 150418 one day after the burst should it have occurred in this galaxy.

PREPARING FOR ADVANCED LIGO: A STAR–GALAXY SEPARATION CATALOG FOR THE PALOMAR TRANSIENT FACTORY

Abstract The search for fast optical transients, such as the expected electromagnetic counterparts to binary neutron star mergers, is riddled with false positives (FPs) ranging from asteroids to stellar flares. While moving objects are readily rejected via image pairs separated by ∼1 hr, stellar flares represent a challenging foreground, significantly outnumbering rapidly evolving explosions. Identifying stellar sources close to and fainter than the transient detection limit can eliminate these FPs. Here, we present a method to reliably identify stars in deep co-adds of Palomar Transient Factory (PTF) imaging. Our machine-learning methodology utilizes the random forest (RF) algorithm, which is trained using sources with Sloan Digital Sky Survey (SDSS) spectra. When evaluated on an independent test set, the PTF RF model outperforms the SExtractor star classifier by ∼4%. For faint sources ( mag), which dominate the field population, the PTF RF model produces a ∼19% improvement over SExtractor. To avoid false negatives in the PTF transient-candidate stream, we adopt a conservative stellar classification threshold, corresponding to a galaxy misclassification rate of 0.005. Ultimately, ∼ objects are included in our PTF point-source catalog, of which only ∼106 are expected to be galaxies. We demonstrate that the PTF RF catalog reveals transients that otherwise would have been missed. To leverage its superior image quality, we additionally create an SDSS point-source catalog, which is also tuned to have a galaxy misclassification rate of 0.005. These catalogs have been incorporated into the PTF real-time pipelines to automatically reject stellar sources as non-extragalactic transients.

THE OBSERVABLE SIGNATURES OF GRB COCOONS

ABSTRACT As a long gamma-ray burst (GRB) jet propagates within the stellar atmosphere it creates a cocoon composed of an outer Newtonian shocked stellar material and an inner (possibly relativistic) shocked jet. The jet deposits erg into this cocoon. This is comparable to the energies of the GRB and of the accompanying supernova, yet the cocoon’s signature has been largely ignored. The cocoon radiates a fraction of this energy as it expands, following the breakout from the star, and later as it interacts with the surrounding matter. We explore the possible signatures of this emission and outline a framework to calculate them from the conditions of the cocoon at the time of the jet breakout. The cocoon signature depends strongly on the, currently unknown, mixing between the shocked jet and shocked stellar material. With no mixing the γ-ray emission from the cocoon is so bright that it should have been already detected. The lack of such detections indicates that some mixing must take place. For partial and full mixing the expected signals are weaker than regular GRB afterglows. However, the latter are highly beamed while the former are wider. Future optical, UV, and X-ray transient searches, like LSST, ZTF, ULTRASAT, ISS-Lobster, and others, will most likely detect such signals, providing a wealth of information on the progenitors and jets of GRBs. While we focus on long GRBs, analogous (but weaker) cocoons may arise in short GRBs. Their signatures might be the most promising electromagnetic counterparts for gravitational wave signals from compact binary mergers.

Untriggered search for rapid optical transients with Mini-MegaTORTORA wide-field monitoring system

AbstractHere we report ongoing efforts for an untriggered search of rapid optical transients of various astrophysical and non-astrophysical origins on time scales down to fractions of a second with Mini-MegaTORTORA. Mini-MegaTORTORA is a novel 9-channel wide-field optical monitoring system in operation since 2014 at Special Astrophysical Observatory on Russian Caucasus.

THE OPTICAL LUMINOSITY FUNCTION OF GAMMA-RAY BURSTS DEDUCED FROM ROTSE-III OBSERVATIONS

We present the optical luminosity function (LF) of gamma-ray bursts (GRBs) estimated from a uniform sample of 58 GRBs from observations with the Robotic Optical Transient Search Experiment III (ROTSE-III). Our GRB sample is divided into two sub-samples: detected afterglows (18 GRBs), and those with upper limits (40 GRBs). The $R$ band fluxes 100s after the onset of the burst for these two sub-samples are derived. The optical LFs at 100s are fitted by assuming that the co-moving GRB rate traces the star-formation rate. The detection function of ROTSE-III is taken into account during the fitting of the optical LFs by using Monte Carlo simulations. We find that the cumulative distribution of optical emission at 100s is well-described with an exponential rise and power-law decay (ERPLD), broken power-law (BPL), and Schechter LFs. A single power-law (SPL) LF, on the other hand, is ruled out with high confidence.

A SEARCH FOR FAST OPTICAL TRANSIENTS IN THE Pan-STARRS1 MEDIUM-DEEP SURVEY: M-DWARF FLARES, ASTEROIDS, LIMITS ON EXTRAGALACTIC RATES, AND IMPLICATIONS FOR LSST

[Abridged] We present a search for fast optical transients (~0.5 hr-1 day) using repeated observations of the Pan-STARRS1 Medium-Deep Survey (PS1/MDS) fields. Our search takes advantage of the consecutive g/r-band observations (16.5 min in each filter), by requiring detections in both bands, with non-detections on preceding and subsequent nights. We identify 19 transients brighter than 22.5 AB mag (S/N>10). Of these, 11 events exhibit quiescent counterparts in the deep PS1/MDS templates that we identify as M4-M9 dwarfs. The remaining 8 transients exhibit a range of properties indicative of main-belt asteroids near the stationary point of their orbits. With identifications for all 19 transients, we place an upper limit of R_FOT(0.5hr)<0.12 deg^-2 d^-1 (95% confidence level) on the sky-projected rate of extragalactic fast transients at <22.5 mag, a factor of 30-50 times lower than previous limits; the limit for a timescale of ~day is R_FOT<2.4e-3 deg^-2 d^-1. To convert these sky-projected rates to volumetric rates, we explore the expected peak luminosities of fast optical transients powered by various mechanisms, and find that non-relativistic events are limited to M~-10 mag (M~-14 mag) for a timescale of ~0.5 hr (~day), while relativistic sources (e.g., GRBs, magnetar-powered transients) can reach much larger luminosities. The resulting volumetric rates are <13 (M~-10 mag), <0.05 (M~-14 mag) and <1e-6 Mpc^-3 yr^-1 (M~-24 mag), significantly above the nova, supernova, and GRB rates, respectively, indicating that much larger surveys are required to provide meaningful constraints. Motivated by the results of our search we discuss strategies for identifying fast optical transients in the LSST main survey, and reach the optimistic conclusion that the veil of foreground contaminants can be lifted with the survey data, without the need for expensive follow-up observations.

Consequences of strong compression in tidal disruption events

The tidal disruption of a star by a supermassive black hole (SMBH) is a highly energetic event with consequences dependent on the degree to which the star plunges inside the SMBH's tidal sphere. We introduce a new analytic model for tidal disruption events (TDEs) to analyze the dependence of these events on beta, the ratio of the tidal radius to the orbital pericenter. We find, contrary to most previous work, that the spread in debris energy for a TDE is largely constant for all beta. This result has important consequences for optical transient searches targeting TDEs, which we discuss. We quantify leading-order general relativistic corrections to this spread in energy and find that they are small. We also examine the role of stellar spin, and find that a combination of spin-orbit misalignment, rapid rotation, and high beta may increase the spread in debris energy. Finally, we quantify for the first time the gravitational wave emission due to the strong compression of a star in a high-beta TDE. Although this signal is unlikely to be detectable for disruptions of main sequence stars, the tidal disruption of a white dwarf by an intermediate mass black hole can produce a strong signal visible to Advanced LIGO at tens of megaparsecs.

Parallax in “Pi of the Sky” project

The main goal of the “Pi of the Sky” project is search for optical transients (OTs) of astrophysical origin, in particular those related to gamma-ray bursts (GRBs). Since March 2011 the project has two running observatories: one in northern Chile and the other one in southern Spain. This allows for regular observations of a common sky fields, visible from both observatories which are scheduled usually 1–2h per night. In such a case, the on-line flash recognition algorithm, looking for optical transients, can use parallax information to assure that events observed from both sites have parallax angle smaller than the error of astrometry. On the other hand, the remaining OT candidates can be verified against a hypothesis of being near-Earth objects. This paper presents algorithm using parallax information for identification of near-Earth objects, which might be satellites, or space debris elements. Preliminary results of the algorithm are also presented.

Pan-STARRS, ATLAS and optical transient searches

The Pan-STARRS1 survey is collecting multi-epoch, multi-colour observations of the sky north of declination -30°, and has designated 70 deg(2) for nightly observations that are particularly useful for transient detection. A duplicate, Pan-STARRS2, is nearing completion that offers opportunities to improve the quality of transient search and observation, as well as simply increasing the number of detections. A new system, the Asteroid Terrestrial-impact Last Alert System (ATLAS), increases the search area to all-sky in return for diminished sensitivity, and highlights tension among optimization for static sky images, optimization for faint transients and optimization for an unbiased number of transients. ATLAS gives up sub-arcsecond images and full colour information to specialize in the third category, but should detect many more transients than the Pan-STARRS1 Medium Deep fields or the Palomar Transient Factory, with examples of transient classes that are considerably closer and brighter.

Rates of superluminous supernovae at z ∼ 0.2

We calculate the volumetric rate of superluminous supernovae (SLSNe) based on five events discovered with the Robotic Optical Transient Search Experiment-IIIb (ROTSE-IIIb) telescope. We gather light curves of 19 events from the literature and our own unpublished data and employ crude k-corrections to constrain the pseudo-absolute magnitude distributions in the rest-frame ROTSE-IIIb (unfiltered) bandpass for both the hydrogen-poor (SLSN-I) and hydrogen-rich (SLSN-II) populations. We find that the peak magnitudes of the available SLSN-I are narrowly distributed (M = −21.7 ± 0.4) in our unfiltered band pass and may suggest an even tighter intrinsic distribution when the effects of dust are considered, although the sample may be skewed by selection and publication biases. The presence of O ii features near maximum light may uniquely signal a high-luminosity event, and we suggest further observational and theoretical work is warranted to assess the possible utility of such SN 2005ap-like SLSN-I as distance indicators. Using the pseudo-absolute magnitude distributions derived from the light-curve sample, we measure the SLSN-I rate to be about (32|$ {^{+ 77}_{-26}} $|⁠) events Gpc−3 yr−1 h371 at a weighted redshift of |$\overline{z} = 0.17$|⁠, and the SLSN-II rate to be about (151|$ {^{+ 151}_{-82}} $|⁠) events Gpc−3 yr−1 h371 at |$\overline{z} = 0.15$|⁠. Given that the exact nature and limits of these populations are still unknown, we discuss how it may be difficult to distinguish these rare SLSNe from other transient phenomena such as active galactic nucleus activity and tidal disruption events even when multiband photometry, spectroscopy or even high-resolution imaging are available. Including one spectroscopically peculiar event, we determine a total rate for SLSN-like events of (199|$ {^{+ 137}_{-86}} $|⁠) events Gpc−3 yr−1 h371 at |$\overline{z} = 0.16$|⁠.

A Study on W Ursae Majoris-Type Systems Recognised by the ROTSE-IIId Experiment

AbstractWe present a study on characterising the light curves of W UMa-type systems gathered from the archive containing 5 years of data observed with the Robotic Optical Transient Search Experiment Telescope (ROTSE-IIId) located in Turkey. A sample of 45 W UMa-type systems was studied on the basis of Fourier decomposition of light curves and some basic geometrical parameters, namely degree of contact (f), mass ratio (q), and orbital inclination (i), as approximated values for these systems were determined. Moreover, methods based on the Fourier transform technique were applied to the discrete data to determine the orbital periods of those systems. Preliminary estimates for the system parameters were presented and compared with the values available in the literature.

Searching for soft relativistic jets in core-collapse supernovae with the IceCube optical follow-up program

Context. Transient neutrino sources such as Gamma-Ray Bursts (GRBs) and Supernovae (SNe) are hypothesized to emit bursts of high-energy neutrinos on a time-scale of \lesssim 100 s. While GRB neutrinos would be produced in high relativistic jets, core-collapse SNe might host soft-relativistic jets, which become stalled in the outer layers of the progenitor star leading to an efficient production of high-energy neutrinos. Aims. To increase the sensitivity to these neutrinos and identify their sources, a low-threshold optical follow-up program for neutrino multiplets detected with the IceCube observatory has been implemented. Methods. If a neutrino multiplet, i.e. two or more neutrinos from the same direction within 100 s, is found by IceCube a trigger is sent to the Robotic Optical Transient Search Experiment, ROTSE. The 4 ROTSE telescopes immediately start an observation program of the corresponding region of the sky in order to detect an optical counterpart to the neutrino events. Results. No statistically significant excess in the rate of neutrino multiplets has been observed and furthermore no coincidence with an optical counterpart was found. Conclusion. The search allows, for the first time, to set stringent limits on current models predicting a high-energy neutrino flux from soft relativistic hadronic jets in core-collapse SNe. We conclude that a sub-population of SNe with typical Lorentz boost factor and jet energy of 10 and 3\times10^{51} erg, respectively, does not exceed 4.2% at 90% confidence.

Long-term variations and periods of Mira stars from ROTSE–IIId

We have studied the long-term variations of Mira type variables observed with Robotic Optical Transient Search Experiment telescope (ROTSE–IIId) between 2004 and 2009 located at TÜBİTAK National Observatory (TUG) in Antalya, Turkey. The actual pulsation periods, variability amplitudes, epochs of maximums and light curves of selected 70 Mira type variables already defined in the SIMBAD database were investigated. In these variables, 17 periods are identified for the first time.

Systematically Bridging the Gap between Novæ and Supernovæ

AbstractUntil recently, the venerable field of cosmic explosions has been plagued with a glaring six-magnitude luminosity gap between the brightest novæ and the faintest supernovæ. A key science driver of the Palomar Transient Factory was a systematic search for optical transients that are fainter, faster and rarer than supernovæ. Theorists predict a variety of mechanisms to produce transients in that “gap”, and observers have the best chance of finding them in the local universe. The talk presented the discoveries and the unique physics of cosmic explosions which bridge that gap between novæ and supernovæ. As Fig. 1 illustrates, there is now evidence of multiple, distinct populations of rare transients in the “gap”.