Towards low-latency real-time detection of gravitational waves from compact binary coalescences in the era of advanced detectors

Electromagnetic (EM) follow-up observations of gravitational wave events will help shed light on the nature of the sources, and more can be learned if the EM follow-ups can start as soon as the gravitational wave event becomes observable. In this paper, we propose a computationally efficient time-domain algorithm capable of detecting inspiral gravitational waves from coalescing binaries of compact objects with nearly no further delay in addition to the time required to condition the data into a time series of calibrated gravitational-wave strain. Our algorithm, if can be expanded to include sky localization, will serve as the first step towards triggering EM observation before the merger. The key to the efficiency of our algorithm arises from the use of chains of so-called infinite impulse response filters, which filter time-series data recursively. Computational cost is further reduced by a template interpolation technique that requires filtering only done for a ``coarse bank'', much sparser than the ``fine bank'' normally required to sufficiently recover the optimal signal-to-noise ratio: the filter chain of each coarse-bank template is divided into several sections, filtering output from these sections are combined appropriately to reconstruct the output of each of the nearby fine-bank templates. The filter construction and interpolation techniques are illustrated in this paper using Newtonian-chirp waveforms, although these will be generalizable to more accurate post-Newtonian waveforms. Towards future detectors with sensitivity extending to lower frequencies, our algorithm's computational cost is shown to increase rather insignificantly compared to the conventional time-domain correlation method using finite impulse response filters.