LOFAR 144-MHz follow-up observations of GW170817

Abstract

We present low-radio-frequency follow-up observations of AT 2017gfo, the electromagnetic counterpart of GW170817, which was the first binary neutron star merger to be detected by Advanced LIGO-Virgo. These data, with a central frequency of 144 MHz, were obtained with LOFAR, the Low-Frequency Array. The maximum elevation of the target is just 13. • 7 when observed with LOFAR, making our observations particularly challenging to calibrate and significantly limiting the achievable sensitivity. On timescales of 130-138 and 371-374 days after the merger event, we obtain 3σ upper limits for the afterglow component of 6.6 and 19.5 mJy beam −1 , respectively. Using our best upper limit and previously published, contemporaneous higher-frequency radio data, we place a limit on any potential steepening of the radio spectrum between 610 and 144 MHz: the two-point spectral index α 610 144 −2.5. We also show that LOFAR can detect the afterglows of future binary neutron star merger events occurring at more favourable elevations.

Observations of Gravitational-wave Afterglows

“…After the discovery of GW170817 in GWs, each band of the EM spectrum contributed key follow-up information that has helped the community form an amazingly detailed picture of the complex physics at play in this BNS merger (a non-exhaustive list of important works on this topic includes Alexander et al 2017;Arcavi et al 2017;Balasubramanian et al 2021;Broderick et al 2020;Chornock et al 2017;Corsi et al 2018;Coulter et al 2017;Cowperthwaite et al 2017;Dobie et al 2018;Drout et al 2017;Fong et al 2019;Ghirlanda et al 2019;Goldstein et al 2017;Haggard et al 2017;Hajela et al 2019;Hallinan et al 2017;Kasen et al 2017;Kasliwal et al 2017;Kilpatrick et al 2017;LIGO Scientific Collaboration et al 2017a,b;Lyman et al 2018;Makhathini et al 2021;Margutti et al 2017;Mooley et al 2018a,b,c;Nicholl et al 2017;Pian et al 2017;Sachenko et al 2017;Shappee et al 2017;Smartt et al 2017;Soares-Santos et al 2017;Tanvir et al 2017;Troja et al 2017Troja et al , 2019Troja et al , 2022Valenti et al 2017). The incredible wealth of varied probes that GW170817 enjoyed has provided a unique opportunity to learn about NS structure and astrophysics (e.g., A.…”

Electromagnetic Precursors to Black Hole–Neutron Star Gravitational Wave Events: Flares and Reconnection-powered Fast Radio Transients from the Late Inspiral

“…In fact, prior to the merger, the orbiting NS can feature a strong exterior magnetic field, whose dynamics could be relevant in sourcing additional EM transients (Hansen & Lyutikov 2001;Lyutikov 2019). Indeed, this possibility was investigated for previous GW events (Callister et al 2019;Broderick et al 2020; see also Stachie et al 2021Stachie et al , 2022, with further efforts being proposed for future searches (James et al 2019;Gourdji et al 2020;Sachdev et al 2020;Wang et al 2020;Yu et al 2021;Cooper et al 2023). In the context of BH-NS GW events (Abbott et al 2021), the nondetection of such precursor counterparts was used to constrain the magnetic field strength present in the stars before merger (D'Orazio et al 2022).…”

Electromagnetic precursor flares from the late inspiral of neutron star binaries

“…The prospects for creating early warning systems for electromagnetic follow-up observations of precursor emission has recently been investigated [37][38][39], crucial for their potential future detection [40]. For the event GW170817 attempts have even been made to detect precursor emission [30,41], see also [42,43]. Coincident detection of precursor emission could also be used to (further) constrain the sky localization of the merger site [44].…”