Pune: LIGO and Virgo detectors detected merger of black hole and a compact object

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Manasi Saraf Joshi

Pune, June 24, 2020: In yet another important detection the network of two advanced-LIGO detectors (at Hanford, Washington and Livingston, Louisiana, USA) and the advanced-Virgo detector (in Cascina, Italy), detected the gravitational waves from the inspiral and merger of a stellar-mass black hole and another compact object of undetermined nature. A paper about the detection has been published in The Astrophysical Journal Letters.

 

Researchers have named the event GW190814. The gravitational signal from this merger event travelled for nearly 800 million years through the expanding universe and arrived at the LIGO detectors in the wee hours of 15th August 2019, just past 02:40 am Indian Standard Time”, said the press release by Inter-University Centre for Astronomy and Astrophysics (IUCAA).

 

The Indian Contribution: Scientists from several Indian research institutes participated in the analysis and made critical contributions to this discovery. Gravitational radiation from compact binaries is predominantly emitted at twice the orbital frequency. This is analogous to the main sound that you hear when plucking a guitar string. However, just like musical instruments, gravitational radiation from such systems is also predicted to ring at higher harmonics of this fundamental frequency.

The asymmetric masses of the GW190814 system allowed for these subtle (faint) components of the signal to be better “heard”, added the researchers.

 

Scientists from IIT Gandhinagar and Chennai Mathematical Institute collaborated with LIGO-Virgo researchers to analyze data which resulted in the convincing discovery of the faint, sub-dominant components of the signal, as predicted in Einstein’s theory of gravitation.

 

While, scientists from Chennai Mathematical Institute also contributed an analysis which confirmed that the binary is consistent with a binary black hole, though the measurements do give room for one or more of the binary constituents not being a black hole.

 

The Indian team in LIGO includes scientists from CMI Chennai, DCSEM Mumbai, ICTS-TIFR Bangalore, IISER Kolkata, IISER Pune, IIT Bombay, IIT Gandhinagar, IIT Hyderabad, IIT Madras, IPR Gandhinagar, IUCAA Pune, RRCAT Indore and TIFR Mumbai.

 

GW190814 has two outstanding features that make it unique: Before the two objects merged, their masses differed by a factor of 9, making this the most extreme mass ratio known for a gravitational-wave event. In comparison, the recent LIGO-Virgo event GW190412 had a mass ratio of about 4:1.

From the measured mass of the lighter compact object, we can infer it to be either the lightest black hole or ‘the most massive neutron star’ ever discovered in a compact binary system. But at this point, we can’t be sure which one it is. Together, these features challenge our understanding of the masses of compact astrophysical objects and the way they end up in merging systems.

 

How the event unfolded?

The LIGO-Virgo detectors were in the middle of their third observing run when they observed this extremely loud event. GW190814 is the third loudest event that has been seen to date (after the binary neutron star system GW170817 and the first binary black hole system GW150914 observed by the LIGO detectors).

 

Time-frequency spectrograms of data containing GW190814, observed by LIGO Hanford (top), LIGO Livingston (middle), and Virgo (bottom). Times are shown from around 10 seconds before the event. The energy in a certain time-frequency bin is represented by the colour palette. A “chirping” signal can be seen in the middle panel (LIGO Livingston data), where the signal was the loudest.

 

Is the lighter compact object a Neutron Star or A Black Hole?

The lighter compact object’s mass makes it hard to determine if it is either an exceptionally heavy neutron star or an unusually light black hole.

In a merging binary system involving a neutron star, the gravitational force exerted by its companion raises a tide on the neutron star, similar to the ocean tides raised on Earth by the gravity of the Moon. These tides leave an imprint on the gravitational wave signal which are measurable. For a system as massive and asymmetric as GW190814, however, the tidal imprint was too small to measure. Thus, we still do not know for sure if the signal originated from the merger of a black hole and a neutron star, or two black holes”, said the researchers.