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Eran Ofek

Publications and source records attributed to Eran Ofek.

At least 37 records · Page 2Linked to original sources
arXiv2019

The first tidal disruption flare in ZTF: from photometric selection to multi-wavelength characterization

We present Zwicky Transient Facility (ZTF) observations of the tidal disruption flare AT2018zr/PS18kh reported by Holoien et al. and detected during ZTF commissioning. The ZTF light curve of the tidal disruption event (TDE) samples the rise-to-peak exceptionally well, with 50 days of g- and r-band detections before the time of maximum light. We also present our multi-wavelength follow-up observations, including the detection of a thermal (kT~100 eV) X-ray source that is two orders of magnitude fainter than the contemporaneous optical/UV blackbody luminosity, and a stringent upper limit to the radio emission. We use observations of 128 known active galactic nuclei (AGN) to assess the quality of the ZTF astrometry, finding a median host-flare distance of 0.2" for genuine nuclear flares. Using ZTF observations of variability from known AGN and supernovae we show how these sources can be separated from TDEs. A combination of light-curve shape, color, and location in the host galaxy can be used to select a clean TDE sample from multi-band optical surveys such as ZTF or LSST.

astro-ph.HE↗
arXiv2019

The Zwicky Transient Facility: System Overview, Performance, and First Results

The Zwicky Transient Facility (ZTF) is a new optical time-domain survey that uses the Palomar 48-inch Schmidt telescope. A custom-built wide-field camera provides a 47 deg$^2$ field of view and 8 second readout time, yielding more than an order of magnitude improvement in survey speed relative to its predecessor survey, the Palomar Transient Factory (PTF). We describe the design and implementation of the camera and observing system. The ZTF data system at the Infrared Processing and Analysis Center provides near-real-time reduction to identify moving and varying objects. We outline the analysis pipelines, data products, and associated archive. Finally, we present on-sky performance analysis and first scientific results from commissioning and the early survey. ZTF's public alert stream will serve as a useful precursor for that of the Large Synoptic Survey Telescope.

astro-ph.IM↗
arXiv2018

Constraints on the ejecta of the GW170817 neutron-star merger from its electromagnetic emission

We present a simple analytic model, that captures the key features of the emission of radiation from material ejected by the merger of neutron stars (NS), and construct the multi-band and bolometric luminosity light curves of the transient associated with GW170817, AT\,2017gfo, using all available data. The UV to IR emission is shown to be consistent with a single $\approx0.05$\,M$_\odot$ component ejecta, with a power-law velocity distribution between $\approx 0.1\,c$ and $>0.3\,c$, a low opacity, {$κ<1$\,cm$^2$\,g$^{-1}$}, and a radioactive energy release rate consistent with an initial $Y_{\rm e}<0.4$. The late time spectra require an opacity of $κ_ν\approx0.1$\,cm$^2$\,g$^{-1}$ at 1 to $2μ$m. If this opacity is provided entirely by Lanthanides, their implied mass fraction is $X_{\rm Ln}\approx10^{-3}$, approximately 30 times below the value required to account for the solar abundance. The inferred value of $X_{\rm Ln}$ is uncertain due to uncertainties in the estimates of IR opacities of heavy elements, which also do not allow the exclusion of a significant contribution to the opacity by other elements (the existence of a slower ejecta rich in Lanthanides, that does not contribute significantly to the luminosity, can also not be ruled out). The existence of a relatively massive, $\approx 0.05$\,M$_\odot$, ejecta with high velocity and low opacity is in tension with the results of numerical simulations of NS mergers.

astro-ph.HE↗
arXiv2018

The Robo-AO-2 facility for rapid visible/near-infrared AO imaging and the demonstration of hybrid techniques

We are building a next-generation laser adaptive optics system, Robo-AO-2, for the UH 2.2-m telescope that will deliver robotic, diffraction-limited observations at visible and near-infrared wavelengths in unprecedented numbers. The superior Maunakea observing site, expanded spectral range and rapid response to high-priority events represent a significant advance over the prototype. Robo-AO-2 will include a new reconfigurable natural guide star sensor for exquisite wavefront correction on bright targets and the demonstration of potentially transformative hybrid AO techniques that promise to extend the faintness limit on current and future exoplanet adaptive optics systems.

astro-ph.IM↗
arXiv2017

An Optical and Infrared Time-Domain Study of the Supergiant Fast X-ray Transient Candidate IC 10 X-2

We present an optical and infrared (IR) study of IC 10 X-2, a high-mass X-ray binary in the galaxy IC 10. Previous optical and X-ray studies suggest X-2 is a Supergiant Fast X-ray Transient: a large-amplitude (factor of $\sim$ 100), short-duration (hours to weeks) X-ray outburst on 2010 May 21. We analyze R- and g-band light curves of X-2 from the intermediate Palomar Transient Factory taken between 2013 July 15 and 2017 Feb 14 show high-amplitude ($\gtrsim$ 1 mag), short-duration ($\lesssim8$ d) flares and dips ($\gtrsim$ 0.5 mag). Near-IR spectroscopy of X-2 from Palomar/TripleSpec show He I, Paschen-$γ$, and Paschen-$β$ emission lines with similar shapes and amplitudes as those of luminous blue variables (LBVs) and LBV candidates (LBVc). Mid-IR colors and magnitudes from Spitzer/IRAC photometry of X-2 resemble those of known LBV/LBVcs. We suggest that the stellar companion in X-2 is an LBV/LBVc and discuss possible origins of the optical flares. Dips in the optical light curve are indicative of eclipses from optically thick clumps formed in the winds of the stellar counterpart. Given the constraints on the flare duration ($0.02 - 0.8$ d) and the time between flares ($15.1\pm7.8$ d), we estimate the clump volume filling factor in the stellar winds, $f_V$, to be $0.01 < f_V < 0.71$, which overlaps with values measured from massive star winds. In X-2, we interpret the origin of the optical flares as the accretion of clumps formed in the winds of an LBV/LBVc onto the compact object.

astro-ph.SR↗
arXiv2017

Type Ibn Supernovae Show Photometric Homogeneity and Spectral Diversity at Maximum Light

Type Ibn supernovae (SNe) are a small yet intriguing class of explosions whose spectra are characterized by low-velocity helium emission lines with little to no evidence for hydrogen. The prevailing theory has been that these are the core-collapse explosions of very massive stars embedded in helium-rich circumstellar material (CSM). We report optical observations of six new SNe Ibn: PTF11rfh, PTF12ldy, iPTF14aki, iPTF15ul, SN 2015G, and iPTF15akq. This brings the sample size of such objects in the literature to 22. We also report new data, including a near-infrared spectrum, on the Type Ibn SN 2015U. In order to characterize the class as a whole, we analyze the photometric and spectroscopic properties of the full Type Ibn sample. We find that, despite the expectation that CSM interaction would generate a heterogeneous set of light curves, as seen in SNe IIn, most Type Ibn light curves are quite similar in shape, declining at rates around 0.1 mag/day during the first month after maximum light, with a few significant exceptions. Early spectra of SNe Ibn come in at least two varieties, one that shows narrow P Cygni lines and another dominated by broader emission lines, both around maximum light, which may be an indication of differences in the state of the progenitor system at the time of explosion. Alternatively, the spectral diversity could arise from viewing-angle effects or merely from a lack of early spectroscopic coverage. Together, the relative light curve homogeneity and narrow spectral features suggest that the CSM consists of a spatially confined shell of helium surrounded by a less dense extended wind.

astro-ph.HE↗
arXiv2016

The IPAC Image Subtraction and Discovery Pipeline for the intermediate Palomar Transient Factory

We describe the near real-time transient-source discovery engine for the intermediate Palomar Transient Factory (iPTF), currently in operations at the Infrared Processing and Analysis Center (IPAC), Caltech. We coin this system the IPAC/iPTF Discovery Engine (or IDE). We review the algorithms used for PSF-matching, image subtraction, detection, photometry, and machine-learned (ML) vetting of extracted transient candidates. We also review the performance of our ML classifier. For a limiting signal-to-noise ratio of 4 in relatively unconfused regions, "bogus" candidates from processing artifacts and imperfect image subtractions outnumber real transients by ~ 10:1. This can be considerably higher for image data with inaccurate astrometric and/or PSF-matching solutions. Despite this occasionally high contamination rate, the ML classifier is able to identify real transients with an efficiency (or completeness) of ~ 97% for a maximum tolerable false-positive rate of 1% when classifying raw candidates. All subtraction-image metrics, source features, ML probability-based real-bogus scores, contextual metadata from other surveys, and possible associations with known Solar System objects are stored in a relational database for retrieval by the various science working groups. We review our efforts in mitigating false-positives and our experience in optimizing the overall system in response to the multitude of science projects underway with iPTF.

astro-ph.IM↗
arXiv2016

Unveiling the Dynamic Infrared Sky with Gattini-IR

While optical and radio transient surveys have enjoyed a renaissance over the past decade, the dynamic infrared sky remains virtually unexplored. The infrared is a powerful tool for probing transient events in dusty regions that have high optical extinction, and for detecting the coolest of stars that are bright only at these wavelengths. The fundamental roadblocks in studying the infrared time-domain have been the overwhelmingly bright sky background (250 times brighter than optical) and the narrow field-of-view of infrared cameras (largest is 0.6 sq deg). To begin to address these challenges and open a new observational window in the infrared, we present Palomar Gattini-IR: a 25 sq degree, 300mm aperture, infrared telescope at Palomar Observatory that surveys the entire accessible sky (20,000 sq deg) to a depth of 16.4 AB mag (J band, 1.25um) every night. Palomar Gattini-IR is wider in area than every existing infrared camera by more than a factor of 40 and is able to survey large areas of sky multiple times. We anticipate the potential for otherwise infeasible discoveries, including, for example, the elusive electromagnetic counterparts to gravitational wave detections. With dedicated hardware in hand, and a F/1.44 telescope available commercially and cost-effectively, Palomar Gattini-IR will be on-sky in early 2017 and will survey the entire accessible sky every night for two years. Palomar Gattini-IR will pave the way for a dual hemisphere, infrared-optimized, ultra-wide field high cadence machine called Turbo Gattini-IR. To take advantage of the low sky background at 2.5 um, two identical systems will be located at the polar sites of the South Pole, Antarctica and near Eureka on Ellesmere Island, Canada. Turbo Gattini-IR will survey 15,000 sq. degrees to a depth of 20AB, the same depth of the VISTA VHS survey, every 2 hours with a survey efficiency of 97%.

astro-ph.IM↗
arXiv2015

The SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Quasar Target Selection

As part of the Sloan Digital Sky Survey IV the extended Baryon Oscillation Spectroscopic Survey (eBOSS) will improve measurements of the cosmological distance scale by applying the Baryon Acoustic Oscillation (BAO) method to quasar samples. eBOSS will adopt two approaches to target quasars over 7500 sq. deg. First, a "CORE" quasar sample will combine optical selection in ugriz using a likelihood-based routine called XDQSOz, with a mid-IR-optical color-cut. eBOSS CORE selection (to g < 22 OR r < 22) should return ~ 70 quasars per sq. deg. at redshifts 0.9 < z < 2.2 and ~7 z > 2.1 quasars per sq. deg. Second, a selection based on variability in multi-epoch imaging from the Palomar Transient Factory should recover an additional ~3-4 z > 2.1 quasars per sq. deg. to g < 22.5. A linear model of how imaging systematics affect target density recovers the angular distribution of eBOSS CORE quasars over 96.7% (76.7%) of the SDSS North (South) Galactic Cap area. The eBOSS CORE quasar sample should thus be sufficiently dense and homogeneous over 0.9 < z < 2.2 to yield the first few-percent-level BAO constraint near z~1.5. eBOSS quasars at z > 2.1 will be used to improve BAO measurements in the Lyman-alpha Forest. Beyond its key cosmological goals, eBOSS should be the next-generation quasar survey, comprising > 500,000 new quasars and > 500,000 uniformly selected spectroscopically confirmed 0.9 < z < 2.2 quasars. At the conclusion of eBOSS, the SDSS will have provided unique spectra of over 800,000 quasars.

astro-ph.CO↗
arXiv2013

Disk-Related Bursts and Fades in Young Stars

We present first results from a new, multiyear, time domain survey of young stars in the North America Nebula complex using the Palomar Transient Factory. Our survey is providing an unprecedented view of aperiodic variability in young stars on timescales of days to years. The analyzed sample covers R_PTF = 13.5-18 and spans a range of mid-infrared color, with larger-amplitude optical variables (exceeding 0.4 mag root-mean-squared) more likely to have mid-infrared evidence for circumstellar material. This paper characterizes infrared excess stars with distinct bursts above or fades below a baseline of lower-level variability, identifying 41 examples. The light curves exhibit a remarkable diversity of amplitudes, timescales, and morphologies, with a continuum of behaviors that can not be classified into distinct groups. Among the bursters, we identify three particularly promising sources that may represent theoretically predicted short-timescale accretion instabilities. Finally, we find that fading behavior is approximately twice as common as bursting behavior on timescales of days to years, although the bursting and fading duty cycle for individual objects often varies from year to year.

astro-ph.SR↗
arXiv2011

The Factory and The Beehive I. Rotation Periods For Low-Mass Stars in Praesepe

Stellar rotation periods measured from single-age populations are critical for investigating how stellar angular momentum content evolves over time, how that evolution depends on mass, and how rotation influences the stellar dynamo and the magnetically heated chromosphere and corona. We report rotation periods for 40 late-K to mid-M stars members of the nearby, rich, intermediate-age (~600 Myr) open cluster Praesepe. These rotation periods were derived from ~200 observations taken by the Palomar Transient Factory of four cluster fields from 2010 February to May. Our measurements indicate that Praesepe's mass-period relation transitions from a well-defined singular relation to a more scattered distribution of both fast and slow rotators at ~0.6 Msun. The location of this transition is broadly consistent with expectations based on observations of younger clusters and the assumption that stellar-spin down is the dominant mechanism influencing angular momentum evolution at 600 Myr. However, a comparison to data recently published for the Hyades, assumed to be coeval to Praesepe, indicates that the divergence from a singular mass-period relation occurs at different characteristic masses, strengthening the finding that Praesepe is the younger of the two clusters. We also use previously published relations describing the evolution of rotation periods as a function of color and mass to evolve the sample of Praesepe periods in time. Comparing the resulting predictions to periods measured in M35 and NGC 2516 (~150 Myr) and for kinematically selected young and old field star populations suggests that stellar spin-down may progress more slowly than described by these relations.

astro-ph.SR↗
arXiv2010

The Extreme Hosts of Extreme Supernovae

We use GALEX ultraviolet (UV) and optical integrated photometry of the hosts of seventeen luminous supernovae (LSNe, having peak M_V < -21) and compare them to a sample of 26,000 galaxies from a cross-match between the SDSS DR4 spectral catalog and GALEX interim release 1.1. We place the LSNe hosts on the galaxy NUV-r versus M_r color magnitude diagram (CMD) with the larger sample to illustrate how extreme they are. The LSN hosts appear to favor low-density regions of the galaxy CMD falling on the blue edge of the blue cloud toward the low luminosity end. From the UV-optical photometry, we estimate the star formation history of the LSN hosts. The hosts have moderately low star formation rates (SFRs) and low stellar masses (M_*) resulting in high specific star formation rates (sSFR). Compared with the larger sample, the LSN hosts occupy low-density regions of a diagram plotting sSFR versus M_* in the area having higher sSFR and lower M_*. This preference for low M_*, high sSFR hosts implies the LSNe are produced by an effect having to do with their local environment. The correlation of mass with metallicity suggests that perhaps wind-driven mass loss is the factor that prevents LSNe from arising in higher-mass, higher-metallicity hosts. The massive progenitors of the LSNe (>100 M_sun), by appearing in low-SFR hosts, are potential tests for theories of the initial mass function that limit the maximum mass of a star based on the SFR.

astro-ph.CO↗
arXiv2008

A comprehensive study of GRB 070125, a most energetic gamma ray burst

We present a comprehensive multiwavelength analysis of the bright, long duration gamma-ray burst GRB 070125, comprised of observations in $γ$-ray, X-ray, optical, millimeter and centimeter wavebands. Simultaneous fits to the optical and X-ray light curves favor a break on day 3.78, which we interpret as the jet break from a collimated outflow. Independent fits to optical and X-ray bands give similar results in the optical bands but shift the jet break to around day 10 in the X-ray light curve. We show that for the physical parameters derived for GRB 070125, inverse Compton scattering effects are important throughout the afterglow evolution. While inverse Compton scattering does not affect radio and optical bands, it may be a promising candidate to delay the jet break in the X-ray band. Radio light curves show rapid flux variations, which are interpreted as due to interstellar scintillation, and are used to derive an upper limit of $2.4 \times 10^{17}$ cm on the radius of the fireball in the lateral expansion phase of the jet. Radio light curves and spectra suggest a high synchrotron self absorption frequency indicative of the afterglow shock wave moving in a dense medium. Our broadband modeling favors a constant density profile for the circumburst medium over a wind-like profile ($R^{-2}$). However, keeping in mind the uncertainty of the parameters, it is difficult to unambiguously distinguish between the two density profiles. Our broadband fits suggest that \event is a burst with high radiative efficiency ($> 60 %$).

astro-ph↗
arXiv2008

GRBs 070429B and 070714B: The High End of the Short-Duration Gamma-Ray Burst Redshift Distribution

We present optical spectra of the host galaxies of the short-duration gamma-ray burst GRB 070429B and the likely short-duration with extended emission GRB 070714B. In both cases, we find a single emission line that we identify as O II lambda 3727 at z ~ 0.9. Both events are more distant than any previous short-duration GRB with a secure host association from the sub-arcsecond position of an optical afterglow. GRBs 070429B and 070714B provide strong evidence in support of our previous claims in Berger et al. that a significant fraction of short-duration hosts (>~ 33%) reside at z > 0.7. We discuss the implications of the existence this population on the energetics of short-duration GRBs, as well as on progenitor models. In the context of the degenerate binary merger scenario, such events require progenitor systems with a range of lifetimes and disfavor progenitor models with a long, narrow lifetime distribution.

astro-ph↗
arXiv2006

Random 3CNF formulas elude the Lovasz theta function

Let $ϕ$ be a 3CNF formula with n variables and m clauses. A simple nonconstructive argument shows that when m is sufficiently large compared to n, most 3CNF formulas are not satisfiable. It is an open question whether there is an efficient refutation algorithm that for most such formulas proves that they are not satisfiable. A possible approach to refute a formula $ϕ$ is: first, translate it into a graph $G_ϕ$ using a generic reduction from 3-SAT to max-IS, then bound the maximum independent set of $G_ϕ$ using the Lovasz $\vartheta$ function. If the $\vartheta$ function returns a value $< m$, this is a certificate for the unsatisfiability of $ϕ$. We show that for random formulas with $m < n^{3/2 -o(1)}$ clauses, the above approach fails, i.e. the $\vartheta$ function is likely to return a value of m.

cs.CC↗
arXiv2005

The Progenitors of Short-Hard Gamma-Ray Bursts from an Extended Sample of Events

The detection of the afterglow emission and host galaxies of short-hard gamma-ray bursts (SHBs) is one of the most exciting recent astronomical discoveries. Indications that SHB progenitors belong to old stellar populations, in contrast to those of the long-soft GRBs, provide a strong clue about their physical nature. Definitive conclusions however are limited by the small number of SHBs with known hosts. Here, we present our investigation of SHBs previously localized by the interplanetary network (IPN) using new and archival optical and X-ray observations. We show that we can likely identify the host galaxies/clusters for additional two bursts, significantly increasing the sample of SHBs with known hosts and/or distances. In particular, we determine that the bright SHB 790613 occurred within the rich galaxy cluster Abell 1892, making it probably the nearest SHB currently known. We show that the brightest galaxy within the error box of SHB 000607, at z=0.14, is most likely the host galaxy of this event. Additionally, we rule out the existence of galaxy overdensities (down to ~21mag) near the locations of two other SHBs, and set a lower limit on their probable redshift. We combine our SHB sample with events discovered recently by the Swift and HETE-2 missions, and investigate the properties of the extended sample. Comparison to SNe Ia shows that the progenitors of SHBs are typically older, implying a typical life time of several Gy. We also show that it is unlikely that there is a significant population of progenitors with life time shorter than 1Gy. This result disfavors the popular model of NS-NS mergers as the progenitors of SHBs.

astro-ph↗
arXiv2005

On the expansion of the giant component in percolated (n,d,λ) graphs

Let d \geq d_0 be a sufficiently large constant. A (n,d,c \sqrt{d}) graph G is a d-regular graph over n vertices whose second largest (in absolute value) eigenvalue is at most c \sqrt{d}. For any 0 < p < 1, G_p is the graph induced by retaining each edge of G with probability p. It is known that for p > \frac{1}{d} the graph G_p almost surely contains a unique giant component (a connected component with linear number vertices). We show that for p \geq frac{5c}{\sqrt{d}} the giant component of G_p almost surely has an edge expansion of at least \frac{1}{\log_2 n}.

math.PR↗