The Panchromatic Afterglow of GW170817: The Full Uniform Data Set, Modeling, Comparison with Previous Results, and Implications

Creators: Makhathini, S.; Mooley, K. P.; Brightman, M.; Hotokezaka, K.; Nayana, A. J.; Intema, H. T.; Dobie, D.; Lenc, E.; Perley, D. A.; Fremling, C.; Moldòn, J.; Lazzati, D.; Kaplan, D. L.; Balasubramanian, A.; Brown, I. S.; Carbone, D.; Chandra, P.; Corsi, A.; Camilo, F.; Deller, A.; Frail, D. A.; Murphy, T.; Murphy, E. J.; Nakar, E.; Smirnov, O.; Beswick, R. J.; Fender, R.; Hallinan, G.; Heywood, I.; Kasliwal, M.; Lee, B.; Lu, W.; Rana, J.; Perkins, S.; White, S. V.; Józsa, G. I. G.; Hugo, B.; Kamphuis, P.

Abstract

We present the full panchromatic afterglow light-curve data of GW170817, including new radio data as well as archival optical and X-ray data, between 0.5 and 940 days post-merger. By compiling all archival data and reprocessing a subset of it, we have evaluated the impact of differences in data processing or flux determination methods used by different groups and attempted to mitigate these differences to provide a more uniform data set. Simple power-law fits to the uniform afterglow light curve indicate a t^(0.86±0.04) rise, a t^(−1.92±0.12) decline, and a peak occurring at 155 ± 4 days. The afterglow is optically thin throughout its evolution, consistent with a single spectral index (−0.584 ± 0.002) across all epochs. This gives a precise and updated estimate of the electron power-law index, p = 2.168 ± 0.004. By studying the diffuse X-ray emission from the host galaxy, we place a conservative upper limit on the hot ionized interstellar medium density, <0.01 cm⁻³, consistent with previous afterglow studies. Using the late-time afterglow data we rule out any long-lived neutron star remnant having a magnetic field strength between 10^(10.4) and 10¹⁶ G. Our fits to the afterglow data using an analytical model that includes Very Long Baseline Interferometry proper motion from Mooley et al., and a structured jet model that ignores the proper motion, indicates that the proper-motion measurement needs to be considered when seeking an accurate estimate of the viewing angle.

Additional Information

© 2021. The American Astronomical Society. Received 2020 June 3; revised 2021 August 13; accepted 2021 August 16; published 2021 November 26. The authors thank Eleonora Troja and Brendan Connor for advice on the X-ray data analysis and are grateful to Schuyler van Dyk for helpful discussions on HST data. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The authors thank the NRAO staff, especially Mark Claussen and Amy Mioduszewski, for scheduling the VLA observations. The Australia Telescope is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. We acknowledge the Gomeroi people as the traditional owners of the Observatory site. We thank the staff of the GMRT that made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, (SARAO), which is a facility of the National Research Foundation (NRF), an agency of the Department of Science and Technology. K.P.M. is a Jansky Fellow of the National Radio Astronomy Observatory. K.P.M. and G.H. acknowledge support from the National Science Foundation Grant AST-1911199. D.D. is supported by an Australian Government Research Training Program Scholarship. T.M. acknowledges the support of the Australian Research Council through grant DP190100561. Parts of this research were conducted by the Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav), project number CE170100004. We acknowledge support by the GROWTH (Global Relay of Observatories Watching Transients Happen) project funded by the National Science Foundation PIRE (Partnership in International Research and Education) program under grant No. 1545949. D.L. acknowledges support from NASA grants 80NSSC18K1729 (Fermi) and NNX17AK42G (ATP), Chandra grant TM9-20002X, and NSF grant AST-1907955. This research has made use of NASA's Astrophysics Data System Bibliographic Services. C.F. gratefully acknowledges support of his research by the Heising-Simons Foundation. JM acknowledges financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709) and from the grant RTI2018-096228-B-C31 (MICIU/FEDER, EU). D.L.K. was supported by NSF grant AST-1816492. P.K. is partially supported by the BMBF project 05A17PC2for D-MeerKAT. A.B. and A.C. acknowledge support from the National Science Foundation via grant #1907975. Facilities: VLA - Very Large Array, ATCA - , uGMRT - , MeerKAT - , eMERLIN - , HST - , Chandra - , XMM-Newton. - Software: MIRIAD (Sault et al. 1995), CASA (McMullin et al. 2007, release 5.6.2-3.el7), emcee (Foreman-Mackey et al. 2013), corner (Foreman-Mackey 2016), CARACal (Ramatsoku et al. 2020, beta version), AOFlagger (Offringa et al. 2010, version 2.14.0), WSClean (Offringa et al. 2014, version 2.8.0), CubiCal (Kenyon et al. 2018, version 1.2.2), MeqTrees (Noordam & Smirnov 2010, version 1.6.0).

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