# Search for gravitational waves from Scorpius X-1 in the second Advanced LIGO observing run with an improved hidden Markov model

## TL;DR

This paper reports a highly sensitive search for continuous gravitational waves from Scorpius X-1 using an advanced hidden Markov model and improved data analysis techniques on LIGO's second observing run, setting new upper limits.

## Contribution

It introduces an improved HMM-based search method with the $$-statistic and applies it to LIGO O2 data, enhancing robustness against spin wandering and achieving the most sensitive limits to date.

## Key findings

- No gravitational waves detected in the searched frequency range.
- Set an upper limit of $h_0^{95\%} = 3.47 \times 10^{-25}$ at 194.6 Hz.
- The search is the most sensitive for Scorpius X-1 to date.

## Abstract

We present results from a semicoherent search for continuous gravitational waves from the low-mass X-ray binary Scorpius X-1, using a hidden Markov model (HMM) to track spin wandering. This search improves on previous HMM-based searches of LIGO data by using an improved frequency domain matched filter, the $\mathcal{J}$-statistic, and by analysing data from Advanced LIGO's second observing run. In the frequency range searched, from $60$ to $650\,\mathrm{Hz}$, we find no evidence of gravitational radiation. At $194.6\,\mathrm{Hz}$, the most sensitive search frequency, we report an upper limit on gravitational wave strain (at 95\% confidence) of $h_0^{95\%} = 3.47 \times 10^{-25}$ when marginalising over source inclination angle. This is the most sensitive search for Scorpius X-1, to date, that is specifically designed to be robust in the presence of spin wandering.