1. 
 Andreoni, Igor, et al.
(författare)

Constraining the Kilonova Rate with Zwicky Transient Facility Searches Independent of Gravitational Wave and Short GammaRay Burst Triggers
 2020

Ingår i: Astrophysical Journal.  : American Astronomical Society.  0004637X . 15384357. ; 904:2

Tidskriftsartikel (refereegranskat)abstract
 The first binary neutron star merger, GW170817, was accompanied by a radioactivitypowered optical/infrared transient called a kilonova. To date, no compelling kilonova has been found in allsky optical surveys, independently of short gammaray burst and gravitationalwave triggers. In this work, we searched the first 23 months of the Zwicky Transient Facility (ZTF) data stream for candidate kilonovae in the form of rapidly evolving transients. We combined ZTF alert queries with forced pointspreadfunction photometry and nightly flux stacking to increase our sensitivity to faint and fast transients. Automatic queries yielded >11,200 candidates, 24 of which passed quality checks and selection criteria based on a grid of kilonova models tailored for both binary neutron star and neutron starblack hole mergers. None of the candidates in our sample was deemed a possible kilonova after thorough vetting. The sources that passed our selection criteria are dominated by Galactic cataclysmic variables. We identified two fast transients at high Galactic latitude, one of which is the confirmed afterglow of longduration GRB.190106A, the other is a possible cosmological afterglow. Using a survey simulation code, we constrained the kilonova rate for a range of models including tophat, linearly decaying light curves, and synthetic light curves obtained with radiative transfer simulations. For prototypical GW170817like kilonovae, we constrain the rate to be R < 1775 Gpc(3) yr(1) (95% confidence). By assuming a population of kilonovae with the same geometry and composition of GW170817 observed under a uniform viewing angle distribution, we obtained a constraint on the rate of R.<.4029 Gpc(3) yr(1).


2. 
 Andreoni, Igor, et al.
(författare)

GROWTH on S190814bv : Deep Synoptic Limits on the Optical/Nearinfrared Counterpart to a Neutron StarBlack Hole Merger
 2020

Ingår i: Astrophysical Journal.  : American Astronomical Society.  0004637X . 15384357. ; 890:2

Tidskriftsartikel (refereegranskat)abstract
 On 2019 August 14, the Advanced LIGO and Virgo interferometers detected the highsignificance gravitational wave (GW) signal S190814bv. The GW data indicated that the event resulted from a neutron starblack hole (NSBH) merger, or potentially a lowmass binary BH merger. Due to the low falsealarm rate and the precise localization (23 deg(2) at 90%), S190814bv presented the community with the best opportunity yet to directly observe an optical/nearinfrared counterpart to an NSBH merger. To search for potential counterparts, the GROWTH Collaboration performed realtime image subtraction on six nights of public Dark Energy Camera images acquired in the 3 weeks following the merger, covering >98% of the localization probability. Using a worldwide network of followup facilities, we systematically undertook spectroscopy and imaging of optical counterpart candidates. Combining these data with a photometric redshift catalog, we ruled out each candidate as the counterpart to S190814bv and placed deep, uniform limits on the optical emission associated with S190814bv. For the nearest consistent GW distance, radiative transfer simulations of NSBH mergers constrain the ejecta mass of S190814bv to be Mej < 0.04 Mcircle dot at polar viewing angles, or Mej < 0.03 Mcircle dot if the opacity is kappa < 2 cm(2)g(1). Assuming a tidal deformability for the NS at the high end of the range compatible with GW170817 results, our limits would constrain the BH spin component aligned with the orbital momentum to be chi < 0.7 for mass ratios Q < 6, with weaker constraints for more compact NSs.


3. 
 Kasliwal, Mansi M., et al.
(författare)

Kilonova Luminosity Function Constraints Based on Zwicky Transient Facility Searches for 13 Neutron Star Merger Triggers during O3
 2020

Ingår i: Astrophysical Journal.  : American Astronomical Society.  0004637X . 15384357. ; 905:2

Tidskriftsartikel (refereegranskat)abstract
 We present a systematic search for optical counterparts to 13 gravitational wave (GW) triggers involving at least one neutron star during LIGO/Virgo's third observing run (O3). We searched binary neutron star (BNS) and neutron star black hole (NSBH) merger localizations with the Zwicky Transient Facility (ZTF) and undertook followup with the Global Relay of Observatories Watching Transients Happen (GROWTH) collaboration. The GW triggers had a median localization area of 4480 deg(2), a median distance of 267 Mpc, and falsealarm rates ranging from 1.5 to 10(25) yr(1). The ZTF coverage in the g and r bands had a median enclosed probability of 39%, median depth of 20.8 mag, and median time lag between merger and the start of observations of 1.5 hr. The O3 followup by the GROWTH team comprised 340 UltraViolet/Optical/InfraRed (UVOIR) photometric points, 64 OIR spectra, and three radio images using 17 different telescopes. We find no promising kilonovae (radioactivitypowered counterparts), and we show how to convert the upper limits to constrain the underlying kilonova luminosity function. Initially, we assume that all GW triggers are bona fide astrophysical events regardless of falsealarm rate and that kilonovae accompanying BNS and NSBH mergers are drawn from a common population; later, we relax these assumptions. Assuming that all kilonovae are at least as luminous as the discovery magnitude of GW170817 (16.1 mag), we calculate that our joint probability of detecting zero kilonovae is only 4.2%. If we assume that all kilonovae are brighter than 16.6 mag (the extrapolated peak magnitude of GW170817) and fade at a rate of 1 mag day(1) (similar to GW170817), the joint probability of zero detections is 7%. If we separate the NSBH and BNS populations based on the online classifications, the joint probability of zero detections, assuming all kilonovae are brighter than 16.6 mag, is 9.7% for NSBH and 7.9% for BNS mergers. Moreover, no more than <57% (<89%) of putative kilonovae could be brighter than 16.6 mag assuming flat evolution (fading by 1 mag day(1)) at the 90% confidence level. If we further take into account the online terrestrial probability for each GW trigger, we find that no more than <68% of putative kilonovae could be brighter than 16.6 mag. Comparing to model grids, we find that some kilonovae must have Mej M, Xlan > 10(4), or > 30 degrees to be consistent with our limits. We look forward to searches in the fourth GW observing run; even 17 neutron star mergers with only 50% coverage to a depth of 16 mag would constrain the maximum fraction of bright kilonovae to <25%.

