Daksha: Finding High Energy Emissions from Gravitational Wave sources | A Talk by Prof Varun Bhalerao
Daksha is a proposed space mission being developed by IIT Bombay for detecting high energy counterparts to gravitational wave sources like GW170817.
The first Gravitational Waves were discovered in September 2015, but it was in August 2017 that a merger of two neutron stars rocked the astronomy world. Advanced gravitational wave detectors - LIGO and Virgo - discovered the coalescence 130 million light-years away from Earth. At nearly the same time, satellites saw an Electromagnetic counterpart. Telescopes around the world got alerts resulting in arguably the most frantic period of activity in modern astronomy. But despite several neutron star mergers being discovered since we never caught any more photons from them - the current satellite network is just not sensitive enough. Enter Daksha Space Mission: a highly ambitious Indian mission to create two satellites that are several times more sensitive to these bursts than anything ever flown.
Professor Varun Bhalerao, the principal investigator of Daksha, gave a recent talk on the topic, ’Daksha: Finding High Energy Emissions from Gravitational Wave sources’. Attending the lecture, I realised that Daksha and its science should be shared. So, here is an article based on the talk given by Professor Varun.
Gravitational Waves Explained
What are Gravitational Waves?
We can understand the gravitational force of attraction. There is a whole different way of looking Gravitation: a bend in the fabric of space-time. I would also make use of the PhD comics to illuminate Gravitational waves, as done by Professor Bhalerao.
The bend in the space-time results in orbits and objects being pushed towards each other. Gravitational Waves are nothing but ripples in this fabric of space-time. When masses accelerate, they change the distortions of space which results in the ripples.
Observing Gravitational Waves
As gravitational waves distort the space between two objects, we need to measure the change in distances. The catch is that our regular length measuring devices will also elongate/shorten. Therefore, our scales are rendered useless. In comes the advanced gravitational wave detectors, LIGO and Virgo.
LIGO and Virgo use laser beams to calculate the time light takes to transfer between two points. Speed of light is fixed for a medium. Therefore, as a gravitational wave travels the distance between the two points change, hence changing the time of travel for the Laser.
EM counterpart
Gravitational Waves are vital as they provide valuable data regarding the masses, spins and other geometric properties of the objects involved. But the information is incomplete without the Electromagnetic counterpart. Because EM waves can be detected through telescopes, we can arrive at precise locations of the mergers. Not only that, but the wide range of EM spectrum also provides much more in-depth insights into the Astrophysics of the source. Therefore, it is essential to be able to observe both GW and EM for a source to have a complete astrophysical picture.
GW170817
During the second run, LIGO made the first direct detection of gravitational waves from merging neutron stars. Almost 1.3 seconds later, a burst of gamma rays was detected by FERMI and INTEGRAL satellite.
Observing Frenzy
All around the world, even in Antarctica, terrestrial telescopes observed the source for many days. Therefore, this neutron star merger was seen by 70 observatories in all the 7 continents and space. Observations were made across all EM spectrums, and a joint paper with 3500 authors from about 1000 different institutes was published. And it was not easy to make these observations. The source was quite near to the sun, so the optical telescopes could only observe at sunset.
The Observations in EM Spectrum
Terrestrial telescopes observed GW170817 in the whole of the Electro-magnetic Spectrum including Radio, UV, Optimal and IR waves. While the space telescopes detected Gamma and X-Ray bursts. Not just optimal photometry, spectroscopy of the source was also done. The observations made are as follows:
The source became brighter with time (100 days after the merger) in the radio frequencies.
For UV waves, it decayed very swiftly (within 1-2 days). While for optical and infrared, the rise could be observed, followed by slow decay.
According to spectroscopy, the object cooled very fast. The peak temperature decayed to longer wavelengths.
The phenomenon was explained using a consistent simple physics-based model: Synchroton Radiation.
Lessons Learnt
Since the GW170817 observation, we have not been able to observer another neutron star merger along with the EM counterpart. The reasons generally point to the larger distances and more 90% probability area for the third observation run of LIGO. Therefore, we have learnt the following lessons based on GW170817 and O3.
Look at the sky at all times. (AstroSat missed the GW170817 by few minutes)
Need 10x higher sensitivity as compared to current missions. (Fainter and farther sources)
Wide Spectral Band (GW170817 Gamma-Ray Burst had very high error bars due to less spectral range)
Daksha Space Mission
Daksha is a proposed space mission for detecting high energy counterparts to gravitational wave sources. It will be an order of magnitude more sensitive than any existing mission.
Specifications
Energy range: 1 keV to > 1 MeV
Effective area (2 satellites): ~1700 cm2, 100 keV
Source localisation: better than 10°
Sensitivity: Better than 3 x 10^-8 ergs/cm2/sec (20-200 keV)
Detectors used:
Silicon Drift Detectors (SDD) {Used in Chandrayaan 2 Orbiter}
Cadmium Zinc Telluride (CZT) {Used in AstroSat}
Scintillators (NaI + SiPM)
With the above specifications Daksha will be an order of magnitude higher in terms of energy range and total active area than current space missions such as FERMI.
As compared to future missions, there is one Chinese mission named GECAM. Daksha will be having a factor of 3 higher sensitivity than GECAM. And there is one American mission, still in very early phases stated for 2030, while Daksha is planned for a faster timeline.
Current Status
Daksha is a joint mission with institutes across India coming together under the umbrella of IIT Bombay. Scientists and teams from PRL, TIFR, IUCAA, RRI, ISRO and IIT Bombay are currently working on the building a proof-of-concept (POC) with the provided seed funding. They are working on the simulations to prove future science along with building the detectors and thermal design of the satellite.
Hopefully, the POC will be complete soon, and Daksha will become an approved ISRO mission.
You can attend the full talk by Professor Varun Bhalerao at the YouTube link below.