G. Duggan

iPTF search for an optical counterpart to gravitational-wave transient GW150914

The intermediate Palomar Transient Factory (iPTF) autonomously responded to and promptly tiled the error region of the first gravitational-wave event GW150914 to search for an optical counterpart. Only a small fraction of the total localized region was immediately visible in the northern night sky, due both to Sun-angle and elevation constraints. Here, we report on the transient candidates identified and rapid follow-up undertaken to determine the nature of each candidate. Even in the small area imaged of 126 deg^2, after extensive filtering, eight candidates were deemed worthy of additional follow-up. Within two hours, all eight were spectroscopically classified by the Keck II telescope. Curiously, even though such events are rare, one of our candidates was a superluminous supernova. We obtained radio data with the Jansky Very Large Array and X-ray follow-up with the Swift satellite for this transient. None of our candidates appear to be associated with the gravitational-wave trigger, which is unsurprising given that GW150914 came from the merger of two stellar-mass black holes. This end-to-end discovery and follow-up campaign bodes well for future searches in this post-detection era of gravitational waves.

PTF13efv - An Outburst 500 Days Prior to the SNHUNT 275 Explosion and Its Radiative Efficiency

The progenitors of some supernovae (SNe) exhibit outbursts with super-Eddington luminosities prior to their final explosions. This behavior is common among SNe IIn, but the driving mechanisms of these precursors are not yet well-understood. SNHunt 275 was announced as a possible new SN during 2015 May. Here we report on pre-explosion observations of the location of this event by the Palomar Transient Factory (PTF) and report the detection of a precursor about 500 days prior to the 2015 May activity (PTF 13efv). The observed velocities in the 2015 transient and its 2013 precursor absorption spectra are low (1000–2000 km s^(−1)), so it is not clear yet if the recent activity indeed marks the final disruption of the progenitor. Regardless of the nature of this event, we use the PTF photometric and spectral observations, as well as Swift-UVOT observations, to constrain the efficiency of the radiated energy relative to the total kinetic energy of the precursor. We find that, using an order-of-magnitude estimate and under the assumption of spherical symmetry, the ratio of the radiated energy to the kinetic energy is in the range of 4 × 10^(−2) to 3.4 × 10^3.

Long-rising Type II supernovae from Palomar Transient Factory and Caltech Core-Collapse Project

Context. Supernova (SN) 1987A was a peculiar hydrogen-rich event wit h a long-rising ( ∼ 84 d) light curve, stemming from the explosion of a compact blue supergiant star. Only a few simil ar events have been presented in the literature in recent dec ades. Aims. We present new data for a sample of six long-rising Type II SNe (SNe II), three of which were discovered and observed by the Palomar Transient Factory (PTF) and three observed by th e Caltech Core-Collapse Project (CCCP). Our aim is to enlarg e this small family of long-rising SNe II, characterizing their di fferences in terms of progenitor and explosion parameters. We also study the metallicity of their environments. Methods. Optical light curves, spectra, and host-galaxy properties of these SNe are presented and analyzed. Detailed compariso ns with known SN 1987A-like events in the literature are shown, ith particular emphasis on the absolute magnitudes, color s, expansion velocities, and host-galaxy metallicities. Bolometric pr o erties are derived from the multiband light curves. By mod eling the earlytime emission with scaling relations derived from the Super Nova Explosion Code (SNEC) models of MESA progenitor stars, we estimate the progenitor radii of these transients. The mode ling of the bolometric light curves also allows us to estimat e other progenitor and explosion parameters, such as the ejected Ni mass, the explosion energy, and the ejecta mass. Results. We present PTF12kso, a long-rising SN II that is estimated to have the largest amount of ejected Ni mass measured for this class. PTF09gpn and PTF12kso are found at the lowest hos t metallicities observed for this SN group. The variety of ea rly lightcurve luminosities depends on the wide range of progenitor r adii of these SNe, from a few tens of R ⊙ (SN 2005ci) up to thousands (SN 2004ek) with some intermediate cases between 100 R ⊙ (PTF09gpn) and 300 R ⊙ (SN 2004em). Conclusions. We confirm that long-rising SNe II with light-curve shapes cl osely resembling that of SN 1987A generally arise from blue supergiant (BSG) stars. However, some of them, such as S N 2004em, likely have progenitors with larger radii ( ∼ 300 R⊙, typical of yellow supergiants) and can thus be regarded as in termediate cases between normal SNe IIP and SN 1987A-like SN e. ome extended red supergiant (RSG) stars such as the progenitor o f SN 2004ek can also produce long-rising SNe II if they synthe sized a large amount ofNi in the explosion. Low host metallicity is confirmed as a cha racteristic of the SNe arising from compact BSG stars.Context. Supernova (SN) 1987A was a peculiar hydrogen-rich event with a long-rising (~ 84 d) light curve, stemming from the explosion of a compact blue supergiant star. Only a few similar events have been presented in the literature in recent decades. Aims. We present new data for a sample of six long-rising Type II SNe (SNe II), three of which were discovered and observed by the Palomar Transient Factory (PTF) and three observed by the Caltech Core-Collapse Project (CCCP). Our aim is to enlarge this small family of long-rising SNe II, characterizing their differences in terms of progenitor and explosion parameters. We also study the metallicity of their environments. Methods. Optical light curves, spectra, and host-galaxy properties of these SNe are presented and analyzed. Detailed comparisons with known SN 1987A-like events in the literature are shown, with particular emphasis on the absolute magnitudes, colors, expansion velocities, and host-galaxy metallicities. Bolometric properties are derived from the multiband light curves. By modeling the early time emission with scaling relations derived from the SuperNova Explosion Code (SNEC) models of MESA progenitor stars, we estimate the progenitor radii of these transients. The modeling of the bolometric light curves also allows us to estimate other progenitor and explosion parameters, such as the ejected ^(56)Ni mass, the explosion energy, and the ejecta mass. Results. We present PTF12kso, a long-rising SN II that is estimated to have the largest amount of ejected ^(56)Ni mass measured for this class. PTF09gpn and PTF12kso are found at the lowest host metallicities observed for this SN group. The variety of early light curve luminosities depends on the wide range of progenitor radii of these SNe, from a few tens of R_⊙ (SN 2005ci) up to thousands (SN 2004ek) with some intermediate cases between 100 R_⊙ (PTF09gpn) and 300 R_⊙ (SN 2004em). Conclusions. We confirm that long-rising SNe II with light-curve shapes closely resembling that of SN 1987A generally arise from blue supergiant (BSG) stars. However, some of them, such as SN 2004em, likely have progenitors with larger radii (~ 300 R_⊙, typical of yellow supergiants) and can thus be regarded as intermediate cases between normal SNe IIP and SN 1987A-like SNe. Some extended red supergiant (RSG) stars such as the progenitor of SN 2004ek can also produce long-rising SNe II if they synthesized a large amount of ^(56 0Ni in the explosion. Low host metallicity is confirmed as a characteristic of the SNe arising from compact BSG stars.

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

This work is based on observations obtained with the 48 inch Samuel Oschin Telescope and the 60 inch telescope at the Palomar Observatory as part of the intermediate Palomar Transient Factory (iPTF) project, a scientific collaboration among the California Institute of Technology, Los Alamos National Laboratory, the University of Wisconsin–Milwaukee, the Oskar Klein Center, the Weizmann Institute of Science, the TANGO Program of the University System of Taiwan, and the Kavli Institute for the Physics and Mathematics of the universe; the New Technology Telescope, operated by the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, as part of PESSTO, ESO program 191.D-0935(C); the Las Cumbres Observatory Global Telescope Network; both the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association, and the Telescopio Nazionale Galileo, operated by the Fundacion Galileo Galilei of the Italian Istituto Nazionale di Astrofisica, at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias; the Lick Observatory owned and operated by the University of California; and the W. M. Keck Observatory, which was made possible by the generous financial support of the W. M. Keck Foundation and is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration (NASA). We thank the staffs at all of these observatories for their assistance with the observations. We thank Lars Bildsten and Matteo Cantiello for useful discussions, and all those whose observations and data reduction contributed to this work. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. The authors made extensive use of the Astropy package Astropy Collaboration et al. (2013) for data analysis. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. LANL participation in iPTF was funded by the US Department of Energy as part of the Laboratory Directed Research and Development program. G.H., D.A.H., and C.M. are supported by the National Science Foundation (NSF) under Grant No. 1313484. A.P., S.B., and N.E.R. are partially supported by PRININAF 2014 with the project “Transient universe: unveiling new types of stellar explosions with PESSTO.” J.S., C.F., E.K., and F.T. gratefully acknowledge support from the Knut and Alice Wallenberg Foundation. The Oskar Klein Centre is funded by the Swedish Research Council. M.F., A.G.-Y., and M.S. acknowledge support from the European Union FP7 programme through ERC grant numbers 320360, 307260, and 615929, respectively. A.G.-Y. is also supported by the Quantum universe I-Core program by the Israeli Committee for Planning and Budgeting and the ISF; by Minerva and ISF grants; by the Weizmann-UK “making connections” program; and by Kimmel and YeS awards. A.C. acknowledges support from NSF CAREER award #1455090. M.M.K. acknowledges support from NSF PIRE program grant 1545949. The supernova research of A.V.F.’s group at UC Berkeley is supported by the Christopher R. Redlich Fund, the TABASGO Foundation, and NSF grant AST–1211916. KAIT and its ongoing operation were made possible by donations from Sun Microsystems, Inc., the Hewlett-Packard Company, AutoScope Corporation, Lick Observatory, the NSF, the University of California, the Sylvia & Jim Katzman Foundation, and the TABASGO Foundation. Research at Lick Observatory is partially supported by a generous gift from Google.

Triangulum II. Not Especially Dense After All

Among the Milky Way satellites discovered in the past three years, Triangulum II has presented the most difficulty in revealing its dynamical status. Kirby et al. identified it as the most dark-matter-dominated galaxy known, with a mass-to-light ratio within the half-light radius of 3600_(-2100)^(+3500)M_☉L_☉^(-1). On the other hand, Martin et al. measured an outer velocity dispersion that is 3.5 ± 2.1 times larger than the central velocity dispersion, suggesting that the system might not be in equilibrium. From new multi-epoch Keck/DEIMOS measurements of 13 member stars in Triangulum II, we constrain the velocity dispersion to be σ_v < 3.4 km s^(−1) (90% C.L.). Our previous measurement of σ_v, based on six stars, was inflated by the presence of a binary star with variable radial velocity. We find no evidence that the velocity dispersion increases with radius. The stars display a wide range of metallicities, indicating that Triangulum II retained supernova ejecta and therefore possesses, or once possessed, a massive dark matter halo. However, the detection of a metallicity dispersion hinges on the membership of the two most metal-rich stars. The stellar mass is lower than galaxies of similar mean stellar metallicity, which might indicate that Triangulum II is either a star cluster or a tidally stripped dwarf galaxy. Detailed abundances of one star show heavily depressed neutron-capture abundances, similar to stars in most other ultra-faint dwarf galaxies but unlike stars in globular clusters.

The Zwicky Transient Facility Camera

The Zwicky Transient Facility Camera (ZTFC) is a key element of the ZTF Observing System, the integrated system of optoelectromechanical instrumentation tasked to acquire the wide-field, high-cadence time-domain astronomical data at the heart of the Zwicky Transient Facility. The ZTFC consists of a compact cryostat with large vacuum window protecting a mosaic of 16 large, wafer-scale science CCDs and 4 smaller guide/focus CCDs, a sophisticated vacuum interface board which carries data as electrical signals out of the cryostat, an electromechanical window frame for securing externally inserted optical filter selections, and associated cryo-thermal/vacuum system support elements. The ZTFC provides an instantaneous 47 deg2 field of view, limited by primary mirror vignetting in its Schmidt telescope prime focus configuration. We report here on the design and performance of the ZTF CCD camera cryostat and report results from extensive Joule-Thompson cryocooler tests that may be of broad interest to the instrumentation community.

A hot and fast ultra-stripped supernova that likely formed a compact neutron star binary

Explosive origin of a binary neutron star Some types of core-collapse supernovae are known to produce a neutron star (NS). A binary NS merger was recently detected from its gravitational wave emission, but it is unclear how such a tight binary system can be formed. De et al. discovered a core-collapse supernova with unusual properties, including the removal of the outer layers of the star before the explosion. They interpret this as the second supernova in an interacting binary system that already contains one NS. Because the explosion probably produced a second NS (rather than a black hole) in a tight orbit, it could be an example of how binary NS systems form. Science, this issue p. 201 An unusual core-collapse supernova appears to have formed a binary neutron star in a tight orbit. Compact neutron star binary systems are produced from binary massive stars through stellar evolution involving up to two supernova explosions. The final stages in the formation of these systems have not been directly observed. We report the discovery of iPTF 14gqr (SN 2014ft), a type Ic supernova with a fast-evolving light curve indicating an extremely low ejecta mass (≈0.2 solar masses) and low kinetic energy (≈2 × 1050 ergs). Early photometry and spectroscopy reveal evidence of shock cooling of an extended helium-rich envelope, likely ejected in an intense pre-explosion mass-loss episode of the progenitor. Taken together, we interpret iPTF 14gqr as evidence for ultra-stripped supernovae that form neutron stars in compact binary systems.

Aligning the ZTF science focal plane using stellar images

The Zwicky Transient Facility (ZTF) is a next-generation, optical, synoptic survey that leverages the success of the Palomar Transient Factory (PTF). ZTF has a large science focal plane (SFP) that needs to be aligned such that all portions of the CCDs are simultaneously placed in focus to optimize the survey’s efficiency. The SFP consists of 16 large, wafer-scale science CCDs, which are mosaicked to achieve 47 deg2 field of view. The SFP is aligned by repositioning each CCD based on the measured height map, which is a map of the camera’s z position at which each portion of the CCD is in focus. This height map is measured using on-sky stellar images in order to recreate the optical path that will be used throughout the survey. We present our technique for placing the SFP in focus, which includes two different methods to measure the height map of the SFP. The first method measures the height at which a star is in focus by fitting a parabola to each star’s photometric width as the star is moved in and out of focus. The second method measures the height by decomposing a defocused star into its image moments. We will discuss the strengths and limitations of each method and their outputs. By repositioning the CCDs, we were able to reduce the standard deviation of the height map from 33 to 14microns, which improved the survey’s speed by ∼ 81%.