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GRAvitational waves from Neutron stars: Investigating Transient Emission

Periodic Reporting for period 1 - GRANITE (GRAvitational waves from Neutron stars: Investigating Transient Emission)

Reporting period: 2016-10-01 to 2018-09-30

The GRANITE project aimed at detecting transient gravitational waves (GWs) from spinning neutron stars. Such signals would provide novel insights about nuclear matter at extreme densities, the mechanisms of pulsar glitches, the formation and population properties of neutron stars in our Galaxy and beyond, and gravity itself.
When the proposal was submitted, humanity had not yet ever detected GWs. But in 2015, a hundred years after Einstein predicted these new cosmic messengers, the LIGO and Virgo collaborations detected the first GWs from merging black holes. Several more GW observations, including one collision of neutron stars, have since been made, and the fellow has contributed to all these discoveries as a member of the LIGO Collaboration. A completely new observational window on the Universe has indeed been opened, though on a different class of sources than initially targeted in this project.
Still, I have dedicated most of my research during this project to searching for GWs from individual neutron stars. The prime targets originally were glitching pulsars in our Galactic neighbourhood. However, with the fortunate detection of the binary neutron star merger GW170817, priorities were shifted towards this fascinating new source. Hence, this project has indeed delivered the promised first model-based search for medium-duration GW transients from neutron stars, but instead of targeting nearby glitching pulsars, I have led an international team of LIGO and Virgo scientists in a search for transient post-merger emission from a putative neutron star remnant of GW170817.
As a third aspect of this MSCA fellowship, method development has continued towards the initial goal of searching for transient GWs from glitching pulsars, improving the computational efficiency of the available analysis methods and preparing post-glitch searches targeted at the nearby Crab and Vela pulsars.
"1) LIGO-Virgo discoveries of compact binary mergers
From the first two observing runs of the advanced detector era, LIGO and Virgo have so far published GW observations of 6 black hole mergers and a binary neutron star event [papers by Abbott et al. in Phys. Rev. Letters, 2016-2017]. My main contribution was in numerical-relativity-based parameter estimation of the energetics and remnant properties of binary black holes [Jimenez-Forteza, Keitel, et al., Phys. Rev. D 95, 064024 (2017); Keitel et al., Phys. Rev. D 96, 024006 (2017)] applied for each detection through 2016-2018.

2) Search for post-merger GWs from GW170817
The binary neutron star event GW170817 led to bountiful multi-messenger observations from gamma-rays to visible light and radio. None of these have conclusively identified the merger remnant: did it collapse to a black hole immediately, did a massive neutron star form but collapse quickly, or was a long-lived neutron star left behind?
Post-merger GW emission can be the key observable to distinguish these outcomes through distinct signatures on different timescales, but is unfortunately expected to be very weak. Still, it was of high importance to analyse LIGO data after the initial GW170817 detection in an attempt to answer this open question. Since the required analysis methods have a significant overlap with other cases of medium-duration transients, I refocused much of this project towards the GW170817 post-merger analysis. I was one of the editors for the 2017 paper ""Search for Post-merger Gravitational Waves from the Remnant of the Binary Neutron Star Merger GW170817"" [Abbott et al., Astrophys. J. Letters 851, L16] focused on short-duration signals (up to a few hundreds of seconds). No detection was made, and the limited detector sensitivity did not allow physically constraining limits of remnant models. Still, this paper was a key contribution to the multi-messenger analysis of GW170817.
Next, I coordinated a longer-timescale analysis targeting a putative stable remnant, leading four search teams from the US, Australia, Spain and Italy using several methods to probe this novel regime, resulting in the recent paper ""Search for gravitational waves from a long-lived remnant of the binary neutron star merger GW170817"" [ submitted to Astrophys. J.]. Again no detection was made, but together these 2 papers delivered the first solid exploration of current GW detectors' capabilities to study post-merger signals, and have laid the foundation for repeating such analysis when detector sensitivity is further improved and more neutron star binary events will be detected.

3) Towards post-glitch GW searches on nearby pulsars
The project originally aimed to search for GW emission after glitches (enigmatic sudden increases in rotation rate) in nearby radio pulsars. An analysis of recent LIGO data targeted at such sources has been delayed due to the detection of GW170817, throughout the GRANITE project I have continued the characterization and improvement of the transient-F-statistic method, a matched filter analysis using a model of GW emission tracking the pulsar’s spin frequency for a limited time after a glitch. In “Faster search for long gravitational-wave transients: GPU implementation of the transient F-statistic"" [Keitel&Ashton, Class. Quant. Grav. 35 (2018), 205003], I have demonstrated speed-up of the analysis through adapting it to modern graphics processors, based on code contributed to open-source software packages. This will make it possible to cover significant parameter ranges of a glitching pulsar, thus better constraining GW emission not exactly following the radio observations, and to increase the number of targeted objects.
Furthermore, I have worked towards a practical analysis on LIGO data from the 2nd observing run aimed at recent glitches of the Vela and Crab pulsars, two young and nearby and thus promising sources. Detailed data quality studies and setup optimization are needed before the actual search, so this is still in progress by the end of the MSCA project.

Dissemination and exploitation of these results was primarily done through journal publications. I have also presented the project and the GW170817 post-merger results at several international conferences. In continuation of the huge impact on global attention made by the first announced GW detection in 2016, I have contributed to several LIGO-Virgo public outreach channels: Every collaboration paper is accompanied by a summary written for general audiences [] we have held several ask-me-anything (AMA) online events, including a joint GW170817 session with colleagues from many astronomical observatories, and continue to present our research to the public at a multitude of local, national and international events."
Targeting medium-duration GWs from perturbed rotating neutron stars, this project has taken key steps in establishing practical analysis tools for the advanced detector era and the eventual detection of such signals. Shortly after the GW window onto the Universe was first opened, we have already started looking at post-merger remnants of binary neutron stars, and important milestones have been achieved towards also targeting glitching pulsars.
Future detections can answer crucial questions in both astro- and fundamental physics. The entirely novel way in which GWs probe the Universe has captured the global public's attention since 2016 and will continue to enlighten us in unprecedented ways.