Fast Optical Transients Research Articles

Ultra-stripped supernovae: progenitors and fate

The explosion of ultra-stripped stars in close binaries can lead to ejecta masses < 0.1 M_sun and may explain some of the recent discoveries of weak and fast optical transients. In Tauris et al. (2013), it was demonstrated that helium star companions to neutron stars (NSs) may experience mass transfer and evolve into naked ~1.5 M_sun metal cores, barely above the Chandrasekhar mass limit. Here we present a systematic investigation of the progenitor evolution leading to ultra-stripped supernovae (SNe). In particular, we examine the binary parameter space leading to electron-capture (EC SNe) and iron core-collapse SNe (Fe CCSNe), respectively, and determine the amount of helium ejected with applications to their observational classification as Type Ib or Type Ic. We mainly evolve systems where the SN progenitors are helium star donors of initial mass M_He = 2.5 - 3.5 M_sun in tight binaries with orbital periods of P_orb = 0.06 - 2.0 days, and hosting an accreting NS, but we also discuss the evolution of wider systems and of both more massive and lighter - as well as single - helium stars. In some cases we are able to follow the evolution until the onset of silicon burning, just a few days prior to the SN explosion. We find that ultra-stripped SNe are possible for both EC SNe and Fe CCSNe, and that the amount of helium ejected is correlated with P_orb - the tightest systems even having donors being stripped down to envelopes of less than 0.01 M_sun. We estimate the rise time of ultra-stripped SNe to be in the range 12 hr - 8 days, and light curve decay times between 1 and 50 days. Ultra-stripped SNe may produce NSs in the mass range 1.10 - 1.80 M_sun and are highly relevant for LIGO/VIRGO since most (possibly all) merging double NS systems have evolved through this phase. Finally, we discuss the low-velocity kicks which might be imparted on these resulting NSs at birth. [Abridged]

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.

ULTRA-STRIPPED TYPE Ic SUPERNOVAE FROM CLOSE BINARY EVOLUTION

Recent discoveries of weak and fast optical transients raise the question of their origin. We investigate the minimum ejecta mass associated with core-collapse supernovae (SNe) of Type Ic. We show that mass transfer from a helium star to a compact companion can produce an ultra-stripped core which undergoes iron core collapse and leads to an extremely fast and faint SN Ic. In this Letter, a detailed example is presented in which the pre-SN stellar mass is barely above the Chandrasekhar limit, resulting in the ejection of only ~0.05-0.20 M_sun of material and the formation of a low-mass neutron star. We compute synthetic light curves of this case and demonstrate that SN 2005ek could be explained by our model. We estimate that the fraction of such ultra-stripped to all SNe could be as high as 0.001-0.01. Finally, we argue that the second explosion in some double neutron star systems (for example, the double pulsar PSR J0737-3039B) was likely associated with an ultra-stripped SN Ic.

Searching for Fast Optical Transients using a Veritas Cherenkov Telescope

AbstractAstronomical transients are intrinsically interesting objects to study. However, fast optical transients (μs time-scales) are a largely unexplored field of optical astronomy. Most optical observations use instruments that have integration times of the order of seconds and are thus unable to resolve fast transients. Current-generation atmospheric Cherenkov gamma-ray telescopes such as Veritas, which consists of four 12-m telescopes, have huge collecting areas, much larger than those of any existing optical telescopes. This paper outlines the benefits of using a Cherenkov telescope to detect optical transients, and the implementation of the Veritas Transient Detector (TRenDy), a dedicated multi-channel photometer based on field-programmable gate arrays which can be used on Veritas for such studies without interfering with gamma-ray observations.

Optical transient search strategy via wide-field monitoring

We discuss the search strategy of fast optical transients accompanying gamma-ray bursts in wide-field monitoring. We describe the instrumentation and methods of observational data reduction, allowing to detect optical flashes brighter than 10–11 stellar magnitudes with temporal resolution of 0.13 s. The prospects of both instrument engineering, as well as development of techniques to search and investigate optical transients of various nature in wide fields are also discussed.

Fast Universal Spectrophotopolarimeter for Robotic Telescopes

FUSP is the automatic multimode spectrophotopolarimeter aimed to the study of fast optical transients in the photometric, photopolarimetric (instant measurement of linear polarization), and spectropolarimetric (with or without the slit) modes. The mode change is automatic depending on the transient brightness and the purposes of the observations and may be performed in half a second. The instrument may be equipped with either fast EM‐CCD or a position‐sensitive detector which may provide temporal resolution up to 1 microsecond. FUSP is the robotic instrument for the robotic telescopes.

From TORTORA to MegaTORTORA—Results and Prospects of Search for Fast Optical Transients

To study short stochastic optical flares of different objects (GRBs, SNs, etc.) of unknown localizations as well as NEOs it is necessary to monitor large regions of sky with high‐time resolution. We developed a system consisting of widefield camera with field of view of 400–600 sq.deg. which uses TV‐CCD with 0.13 s temporal resolution to record and classify optical transients, and a fast robotic telescope aimed to perform their spectroscopic and photometric investigation just after detection. Such two‐telescope complex, combining wide‐field camera TORTORA and robotic telescope REM, operated from May 2006 at La Silla ESO observatory. Some results of its operation, including first high time resolution study of optical transient accompanying GRB and discovery of its fine time structure, are presented. Also, prospects for improving the efficiency of such observations are given, and a project of a next generation wide field monitoring system, the MegaTORTORA, is described.

The Zadko Telescope: A Southern Hemisphere Telescope for Optical Transient Searches, Multi-Messenger Astronomy and Education

AbstractThe new 1 m f/4 fast-slew Zadko Telescope was installed in June 2008 about 70 km north of Perth, Western Australia. It is the only metre-class optical facility at this southern latitude between the east coast of Australia and South Africa, and can rapidly image optical transients at a longitude not monitored by other similar facilities. We report on first imaging tests of a pilot program of minor planet searches, and Target of Opportunity observations triggered by the Swift satellite. In 12 months, 6 gamma-ray burst afterglows were detected, with estimated magnitudes; two of them, GRB 090205 (z = 4.65) and GRB 090516 (z = 4.11), are among the most distant optical transients imaged by an Australian telescope. Many asteroids were observed in a systematic 3-month search. In September 2009, an automatic telescope control system was installed, which will be used to link the facility to a global robotic telescope network; future targets will include fast optical transients triggered by high-energy satellites, radio transient detections, and LIGO gravitational wave candidate events. We also outline the importance of the facility as a potential tool for education, training, and public outreach.

Capability of Cherenkov telescopes to observe ultra-fast optical flares

The large optical reflector (∼100 m 2) of a H.E.S.S. Cherenkov telescope was used to search for very fast optical transients of astrophysical origin. Forty-three hours of observations targeting stellar-mass black holes and neutron stars were obtained using a dedicated photometer with microsecond time-resolution. The photometer consists of seven photomultiplier tube pixels: a central one to monitor the target and a surrounding ring of six pixels to veto background events. The light curves of all pixels were recorded continuously and were searched offline with a matched-filtering technique for flares with a duration of 2 μs–100 ms. As expected, many unresolved (<3 μs) and many long (>500 μs) background events originating in the earth’s atmosphere were detected. In the time range 3–500 μs the measurement is essentially background-free, with only eight events detected in 43 h; five from lightning and three presumably from a piece of space debris. The detection of flashes of brightness ∼0.1 Jy and only 20 μs duration from the space debris shows the potential of this setup to find rare optical flares on timescales of tens of microseconds. This timescale corresponds to the light crossing time of stellar-mass black holes and neutron stars.

RAPTOR Observations of Delayed Explosive Activity in the High-Redshift Gamma-Ray Burst GRB 060206

The Rapid Telescopes for Optical Response (RAPTOR) system at Los Alamos National Laboratory observed GRB 060206 starting 48.1 minutes after γ-ray emission triggered the Burst Alert Telescope on board the Swift satellite. The afterglow light curve measured by RAPTOR shows a spectacular rebrightening by ~1 mag about 1 hr after the trigger and peaks at R ~ 16.4 mag. Shortly after the onset of the explosive rebrightening, the optical transient doubled its flux on a timescale of about 4 minutes. The total R-band fluence received from GRB 060206 during this episode is 2.3 × 10-9 ergs cm-2. In the rest frame of the burst (z = 4.045), this yields an isotropic equivalent energy release of Eiso ~ 0.7 × 1050 ergs in just a narrow UV band, λ ≃ 130 ± 22 nm. We discuss the implications of RAPTOR observations for untriggered searches for fast optical transients and studies of GRB environments at high redshift.

Pi of the Sky – all-sky, real-time search for fast optical transients

An apparatus to search for optical flashes in the sky is described. It has been optimized for optical counterparts of gamma ray bursts (GRB). It consists of 2×16 cameras covering all the sky. The sky is monitored continuously and the data are analyzed on-line. The apparatus has self-triggering capability and can react to external triggers with negative delay. The prototype with two cameras has been installed at Las Campanas (Chile) and is operational from July 2004. The paper presents the general idea of the experiment and describes the apparatus in detail. The performance of the prototype is briefly reviewed and perspectives for the future are outlined.

“π of the Sky” – automated search for fast optical transients over the whole sky

Institute of Electronic Systems, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warsaw, Poland Institute of Experimental Physics, Faculty of Physics, Warsaw University, Hoza 69, 00-681 Warsaw, Poland Center for Theoretical Physics PAN, Al. Lotnikow 32/46, 00-668 Warsaw, Poland Astronomical Observatory, Warsaw University, Al. Ujazdowskie 4, 00-478 Warsaw, Poland Soltan Institute for Nuclear Studies, Hoza 69, 00-681 Warsaw, Poland

Determination of the night sky background around the Crab pulsar using its optical pulsation

The poor angular resolution of imaging γ-ray telescopes is offset by the large reflector areas of next generation telescopes such as MAGIC (17 m diameter), which makes the study of optical emission associated with some γ-ray sources feasible. Furthermore, the extremely fast time response of photomultipliers (PMs) makes them ideal detectors for fast (subsecond) optical transients and periodic sources like pulsars. The optical pulse of the Crab pulsar was detected with the HEGRA CT1 central pixel using a modified PM, similar to the future MAGIC camera PMs. The HEGRA CT1 telescope is a 2.1 m radius Čerenkov telescope, which has seen the Crab optical pulsations. The corresponding observation time required for a detection is 200 s, but MAGIC will only require ∼30 s if the same strategy with standard PMs is used. The constancy of the pulsed fraction (with a relatively small error) during the 2 h CT1 measurements shows that the pointing did not change during the observations. The purpose of these periodic observations was to determine the light of the night sky (LONS) for the galactic anticenter Crab region, and to compare it with the LONS for a nearby bright star ( ζ Tauri). These obtained LONS values vary between 4.6 and 4.3 × 10 12 ph m −2 s −1 sr −1 (with a coarse estimated error of 10%), being the first value from the timing signature of the Crab pulsar, and the second one was derived from the technique using the ζ Tauri standard star. Our results are between 2.5 and 3 times larger than the previously measured LONS (outside the galactic plane) of (1.7 ± 0.4) × 10 12 ph m −2 s −1 sr −1, as expected since the Crab pulsar is in the galactic plane, which implies a slightly higher energy threshold for Crab observations, if the higher value of CT1 measured LONS rate for galactic sources is used.

The ROTSE‐III Robotic Telescope System

ABSTRACTThe observation of a prompt optical flash from GRB 990123 convincingly demonstrated the value of autonomous robotic telescope systems. Pursuing a program of rapid follow‐up observations of gamma‐ray bursts, the Robotic Optical Transient Search Experiment (ROTSE) has developed a next‐generation instrument, ROTSE‐III, that will continue the search for fast optical transients. The entire system was designed as an economical robotic facility to be installed at remote sites throughout the world. There are seven major system components: optics, optical tube assembly, CCD camera, telescope mount, enclosure, environmental sensing and protection, and data acquisition. Each is described in turn in the hope that the techniques developed here will be useful in similar contexts elsewhere.