Detecting continuous gravitational waves (GWs) from rapidly spinning, asymmetric neutron stars in our Galaxy is extremely challenging. These faint signals must be observed for long periods to distinguish them from the noise of gravitational-wave detectors, and despite ongoing searches, no confirmed detection has yet been made.
However, if a massive object lies along the line of sight, it can temporarily amplify the GW signal amplitude, a phenomenon known as lensing. In the context of signals and sources within our Galaxy, a characteristic, time-varying amplification similar to the well-known Paczyński curve in electromagnetic microlensing is produced. This effect could amplify the currently too weak GW signals emitted by rotating, non-axisymmetric neutron-stars making them possible to be detected, while also providing clues about both the lensing object and the source.
A research team from CAMK and NCBJ, led by CAMK’s PhD student Sudhagar Suyamprakasam investigated whether this effect could be detectable for the neutron-star signals using current second-generation detectors: LIGO, Virgo and KAGRA. They considered several possible lensing objects within our Galaxy, such as stellar-mass black holes, intermediate-mass black holes, primordial black holes, and stellar objects that could act as microlenses. Treating these objects as point masses, the team first modeled the expected lensing patterns theoretically and then tested them using simulated GW signals.
To search for microlensed signatures from the simulated signal, the researchers utilized a GW search method based on the time-domain F-statistic formulation, developed by researchers at CAMK. This technique is sensitive to variations in signal strength across short time segments. Since the signal-to-noise ratio in each segment is directly influenced by microlensing amplification, this approach is particularly effective for identifying the microlensed pattern from the signals.
Their results highlight a promising new pathway for discovering continuous GWs, as well as for probing the hidden population of compact and dark objects within our Galaxy.
Figure caption: The top panel shows a simulated 1000 Hz gravitational-wave signal from a neutron star that is temporarily amplified (microlensed) by a 100-solar-mass object using a time-domain F-statistic formulation. The bottom panel shows the same type of signal: it has 100 Hz frequency, but magnified by a 10,000-solar-mass object. In both panels, the dark line represents the theoretical prediction of how the microlensing amplification should evolve over time.
Microlensing of long-duration gravitational wave signals originating from Galactic sources
Sudhagar Suyamprakasam, Sreekanth Harikumar, Paweł Ciecieląg Przemysław Figura, Michał Bejger, and Marek Biesiada
https://journals.aps.org/prd/abstract/10.1103/d1ys-8fw3
