Observational challenges

WHY EVERY OBSERVATIONS MATTERS?

INTERESTING GW SOURCES OF THE THIRD OBSERVATIONAL CAMPAIGN

GW190814

Two years to the day since the first ever three-detector observation of a GW signal, the three interferometers observed another GW signal. This event was produced by the inspiral and merger of two compact objects: a black hole and another object with an undetermined nature. This detection is unique; first, the heavier compact object is nine times heavier than its companion and second, the mass measured for the lightest compact object makes it the lightest black hole or the heavier neutron star ever discovered.

The GW signal analysis yields a chance of less than 1 in 10000 years that the signal could be due to random detector noise. GW190814 was announced with a classification of Mass Gap, but further analysis of the signal enabled a more precise estimate of the masses and an update circulated 11 hours later changed the source classification to NSBH.

The GRANDMA collaboration processed the follow-up of the sky area with the TAROT Calern, Chili and Réunion telescopes only 30 min after the GW trigger (the most responsive telescopes for this event) with more than 2200 images of 90.2% of the sky area (161 square degrees) with a magnitude limit of 18, but these did not turn up any counterpart to the gravitational waves. There was, however, very poorly chance that matter would be ejected in this event.

GW190425

At the time of this detection, the LIGO-Hanford detector was temporarily down, and due to the lower sensitivity of Virgo compared to LIGO, the signal was only above the detection threshold in LIGO-Livingston. The total mass of the system which produced this gravitational wave is between 3.3 and 3.7 times the mass of the Sun. Given this mass range, the most plausible explanation is that two neutron stars collided, but the mass of this binary is significantly larger than any other known binary neutron star system.

As only one interferometer detected the signal, GW190425 was not well localized. In fact, the sky area is a region covering about 16% of the entire sky. This is a huge area of sky even for collaboration like GRANDMA. For this event, GRANDMA covered 2.8% of the sky area (135 square degrees) with TAROT (Calern, Chili and Réunion), Zadko, Abastumani T70, Lisnyky AZT-8, GMG and CAHA telescopes with 1200 images 7 hours after the trigger time.

GW190412

Observed with all three detectors, this gravitational wave was produced by the inspiral and merger of two black holes. However, GW190412 is unique in that it is the first black hole merger where the masses of the two black holes are definitively unequal. One black hole is 3 times heavier than the other. This asymmetry in masses modifies the GW signal in such a way that the other parameters, such as the distance, the inclination of the system, or the spin of the heavier black hole can be measured better.

Even if we didn’t expect any electromagnetic counterpart, GRANDMA processed the follow-up of the sky area. 76% of the sky area (125 square degrees) was observed by TAROT Calern and Reunion 10 hours after the trigger time.

S200114f

This event, still considered as a gravitational wave candidate, was very unexpected and unique. Indeed, it is an unmodeled transient trigger detected by both LIGO Livingston and Hanford but also by Virgo. Five GRANDMA telescopes participated in its follow-up. Only 19 minutes after the trigger time, the FRAM-Auger telescopes made their first observation and 77% of the sky localization area (351 square degrees) was observed 90 hours after the trigger time.

S191213g and S200213t

These two events, classified as a BNS event at a distance of 200 Mpc, had a high probability to have an electromagnetic counterpart. These were the first events where amateurs collaborated with GRANDMA in the optical follow-up of potential counterparts. They observed 39.8 and 22 hours after the trigger time when GRANDMA collaboration observed 0.9 and 0.4 hours after the trigger time and covered 0.6% and 32.8% of the localization area respectively.

THE CURIOUS CASE OF GW190814: THE COALESCENCE OF A STELLAR MASS BLACK HOLE AND A MYSTERY COMPACT OBJECT
https://www.ligo.org/science/Publication-GW190814/flyer.pdf

GW190412: The First Observation of an Unequal-Mass Black Hole Merger
https://www.ligo.org/science/Publication-GW190412/flyer.pdf

GW190425: The heaviest binary neutron star system ever seen?
https://www.ligo.org/science/Publication-GW190425/flyer.pdf