1. 
 Coughlin, Michael W., et al.
(författare)

GROWTH on S190425z : Searching Thousands of Square Degrees to Identify an Optical or Infrared Counterpart to a Binary Neutron Star Merger with the Zwicky Transient Facility and Palomar GattiniIR
 2019

Ingår i: Astrophysical Journal Letters.  : American Astronomical Society.  20418205 . 20418213. ; 885:1

Tidskriftsartikel (refereegranskat)abstract
 The third observing run by LVC has brought the discovery of many compact binary coalescences. Following the detection of the first binary neutron star merger in this run (LIGO/Virgo S190425z), we performed a dedicated followup campaign with the Zwicky Transient Facility (ZTF) and Palomar GattiniIR telescopes. The initial skymap of this singledetector gravitational wave (GW) trigger spanned most of the sky observable from Palomar Observatory. Covering 8000 deg(2) of the initial skymap over the next two nights, corresponding to 46% integrated probability, ZTF system achieved a depth of 21 m(AB) in g and rbands. Palomar GattiniIR covered 2200 square degrees in Jband to a depth of 15.5 mag, including 32% integrated probability based on the initial skymap. The revised skymap issued the following day reduced these numbers to 21% for the ZTF and 19% for Palomar GattiniIR. We narrowed 338,646 ZTF transient ?alerts? over the first two nights of observations to 15 candidate counterparts. Two candidates, ZTF19aarykkb and ZTF19aarzaod, were particularly compelling given that their location, distance, and age were consistent with the GW event, and their early optical light curves were photometrically consistent with that of kilonovae. These two candidates were spectroscopically classified as young corecollapse supernovae. The remaining candidates were ruled out as supernovae. Palomar GattiniIR did not identify any viable candidates with multiple detections only after merger time. We demonstrate that even with singledetector GW events localized to thousands of square degrees, systematic kilonova discovery is feasible.


2. 
 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%.

