Detection of the gravitational wave memory effect of core-collapse supernovae

Einstein’s theory of gravity, general relativity, has passed all tests with correct predictions. One prediction that remains is “gravitational wave memory”: the prediction that a passing gravitational wave will permanently change the distance between cosmic objects.

Supernovae – stars that collapse and explode outwards – are considered generators of gravitational waves, although none have yet been definitively detected by gravitational wave interferometers on Earth. The memory effect of gravitational waves has also not been observed, during mergers or supernovae, due to the limited sensitivity of interferometers below wave frequencies of 10 hertz.

But now a new study presents an approach to detect this effect using currently existing gravitational wave observatories. The article is published in Physical Examination Letters.

To date, all gravitational waves detected have come from black hole-black hole mergers, neutron star-neutron star mergers, or mergers of one of each. But collapsing supernovae with masses greater than about 10 solar masses should also emit gravitational waves, although of lower wave amplitude and with a different signature in a gravitational wave interferometer.

In such supernovae, called core collapse supernovae (CCSN), the core of a massive star undergoes a sudden collapse when the energy generated by its fusion energy can no longer counteract the star’s own gravity.

This results in a shock wave coming out of the implosion. Some of the external energy will be in the form of gravitational waves due to the changing quadrupole moment of the star, with a total energy of about 10⁴⁰ joules – unless the star’s matter is isotropically projected. (Unlike electromagnetic waves, gravitational waves do not have a dipole moment due to conservation of momentum.)

Visible light and neutrinos are also emitted, opening the possibility of multi-messenger detection when they arrive on Earth.

Gravitational waves from CCSN would be particularly useful because the supernova’s electromagnetic signals originate from its periphery, while gravitational waves are generated deep inside and therefore contain information that would not otherwise be available.

However, gravitational waves from CCSN have a smaller amplitude than those from black hole-black hole mergers, with a deformation one to two orders of magnitude lower (the deformation depends inversely on the distance of the source from Earth). Their frequencies are generally lower, their duration is shorter and the signal is more complex and less distinct than that of massive mergers of two bodies.

However, at the lower frequency gravitational waves of the CCSN, approximately below 10 hertz, the waves have a gravitational “memory” component due to the anisotropic motion of matter and the aspheric emission of neutrinos. If the CCSN neutrino explosion is not isotropic, it will generate additional gravitational radiation compared to that of the collapse.

Originating from previously emitted waves, these “memory burst” waves constitute a different class of gravitational radiation in which the gravitational disturbance, at any point, rises from zero, oscillates for a few cycles, then, instead of falling back to zero, s ‘installs in a non-zero final value.

The memory effect of gravitational waves has never been detected. High-frequency detectors like advanced LIGO are mostly insensitive to the memory effect because the response time of these detectors is generally much shorter than the characteristic time required for the non-oscillatory part of the gravitational wave signal to reach its peak. final value.

Larger interferometers, like the proposed Laser Interferometer Space Antenna (LISA), are better because they have better sensitivity in lower frequency bands where typical memory sources are more powerful. (Lower frequency means higher wavelength, so detection requires longer interferometer arms.)

Colter J Richardson of the University of Tennessee, with CCSN modeling and data analysis colleagues from the USA, Sweden and Poland, studied the memory effect using three state-of-the-art three-dimensional simulations of Non-rotating CCSNs with increased masses. at 25 solar masses, using a model called CHIMERA.

Their lowest mass of 9.6 solar masses is representative of lower mass CCSNs; their models’ gravitational wave signals all showed “a slow ramp-up to a non-zero strain value that is characteristic of memory,” they wrote.

The gravitational wave signals from the CCSN explosions were largely random, but they found that the increase (in wave amplitudes) and memory phases exhibited “a high degree of regularity” that could be approximated well by the logistical functions typical of population studies. growth.

They found that gravitational wave signals from CCSNs persisted for more than a second. (In contrast, the first gravitational wave signal of 2015 lasted only 0.2 seconds.) They applied filters to the signals to remove noise, which reduced the power buildup to the peak signal but didn’t erase it.

After further refinement, they applied matched filtering to the final signal, which is also used in current gravitational wave detectors, by searching through a large number of previously calculated model waveforms to find those that are strongly correlated to the refined signal from the detector. They found that their model results for a 25 solar mass CCSN can be detected at 10 kiloparsecs (about 30,000 light years) with a false alarm probability of less than 0.05% and within the range of wave interferometers. current gravitational forces.

“Current efforts around the world to detect gravitational waves from supernovas caused by core collapse are considerable,” Richardson said. “In addition to proposing an alternative detection strategy, we hope that this letter will motivate further research in the low-frequency region of gravitational-wave astronomy.”

He noted that there are several avenues for future research, “from applying our methodology to the most common merger events, to studying how the next generation of detectors will be sensitive to memory “.

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