EARLY SIGNATURES OF A GRAVITATIONALWAVE BACKGROUND SEEN IN PULSARTIMING ARRAY DATASETS!

An international team of Astronomers with the European Pulsar Timing Array and theIndian Pulsar Timing Array see an emerging signal of gravitational waves from merging
black holes, several million to billion times heavier than the sun.
We used pulsars, rapidly rotating remains of dead stars as clocks to detect gravitational
waves which perturb space and time using radio Telescopes for our observations.
These results were reported together with other IPTA members such as NANOGrav and theParkes Pulsar timing Array, which see a similar signal.
More observations are needed to determine the exact nature of the signal, eventually theywill let us probe the some of the oldest supermassive black holes in the universe andperhaps even shed light on some possible exotic physics such as cosmic strings and phasetransitions in the early universe. A lot of new science awaits.

A new window in Graviatianal Wave astronomy has been opened.

Made by Neel Kolhe

WHERE DO THESE GRAVITATIONAL WAVES COME FROM?

Almost all galaxies are expected to havesupermassive black holes, which weighseveral million to many billion times themass of the sun, at their centers.

Merging black holes emit gravitational
waves, which stretch and squeeze space
and time as they propagate!
Galaxies merge with eachother, and the black holes
at their centers are also
expected to merge after along spiral dance.
Neel Kolhe

THE GRAVITATIONAL WAVE BACKGROUND
These waves stretch along for many lightyears, and cannot be detected byexperiments such as LIGO, VIRGO or KAGRA,
which have been detecting gravitational
waves from merging black holes only 10s oftimes the mass of the sun

As black holes merge all across the universe, in every directionin the sky, these waves overlap and form a background hum ofgravitational waves. This thrumming hum is called as thestochastic gravitational wave background, and it has been theongoing goal of PTA experiments to detect it.

Neel Kolhe

Pulsars are rapidly rotatingcompact dead stars, whichemit bright beams of radio
light from their poles

ENTER: PULSARS!

Some pulsars rotate ~ 100 times asecond without missing a beat, hence
called millisecond pulsars, and are
the ones observed by PTAs
Neel Kolhe

Radio telescopes, such as theupgraded Giant Meterwave RadioTelescope (uGMRT) used by InPTA
pick up these radio beams as timely
pulses as they sweep the earth!

Due to the regularity of these pulses,
we can use them as extremely accurate
clocks, correct up to a nanosecond!
This ability to keep time precisely is
whatPTAs
exploittodetectgravitational waves.

WHAT ARE PULSAR TIMING ARRAYS?
We observe an array of millisecondpulsars, distributed all over the sky tokeep accurate time, hence the expeirment
is called a "Pulsar Timing Array"

The Gravitational waves interact with the pulsars, stretching and squeezing space-time,
which causes changes in the time of arrival of the pulses to earth, and as we very
accurately have been keeping time, this change can be noted over many years ofobservation, hence detecting the presence of the Gravitvational Waves!
Neel Kolhe

THE UNIQUE FEATURE OF THE INDIAN PULSAR TIMING ARRAY

The same pulse at various radio frequencies
High frequency

Low frequency

Very low Frequency

Outer space isn't truly empty!
There are free electrons, protons
and other material between us andthe pulsars we observe. Lower
frequency radio light interacts
differently with this medium thanlight at higher frequencies.
Neel Kolhe

InPTA observes pulsars at both a higher frequency and alower frequency simultaneously due to a unique feature of
the uGMRT which let's us split the telescope dishes into
two sets, this lets us study the medium between us andthe pulsar exquisitely and also helps us correct our pulsar'clocks' better, thus helping PTAs improve their precision.

DETECTING GRAVITATIONAL WAVES USING PULSARSAs the Gravitational Wave Backgroundshould affect all the pulsars we observe ina similar way, we should see common
effects in pairs of them, this gives rise to aunique signature:urledveC alasthehe lli n g s a n d d o w

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InPTA teamed up with the European Pulsar
Timing Array (EPTA) and we combined our data,to generate a more sensitive dataset.
Us, along with other PTAs such as NANOGrav and
PPTA now see tantalizing hints of this uniquesignature.
A global effort deemed the International Pulsar
Timing Array (IPTA) is afoot to chase theseelusive waves

Neel KolheA truly global venture, some of whose constituents have been operational for decades,
the IPTA exemplifies the international spirit of science, bringing scientists from allwalks of life together for a cosmic goal with more PTAs such as the African PTA andChinese PTA joining in every year.
Neel Kolhe

WHAT COMES NEXT?
OBTAIN A STRONGER SIGNAL AND UNDERSTAND THE GWB
PTAs get more sensitive, the longer they observe a larger number of pulsars,
over the next few years we'll probe the nature of the current signal much deeper.

SHOOT FOR INDIVIDUAL BINARIES, WORK WITH VLBI NETWORKS
The stochastic background is a mish-mash of gravitational wave signals from all over thesky, PTAs expect to detect individal merging binaries at the cores of galactic nuclei, finding
and studying candidates with VLBI experiments such as the EHT, EVN, VLBA will be crucial.

MULTIMESSENGER ASTRONOMY AND COSMOLOGICAL DISTANCES
With the detection of individual binaries, we will be able distances to these mergers, andpredict the expansion rate of the universe at an early epoch. Merging black hole's circumbinarydisks are also potential candidates for neutrino detections.

PULSAR TIMING ARRAYS ARE STRONGER TOGETHER!
With PTAs such as the chinese pulsar timing array and the african pulsar timing array joining theinternational effort and with telescopes such as the SKA and the extended GMRT on the horizon,
pooling in data and searching for all of the above together shall be a fruitful effort!

A NEW WINDOW IN GRAVTITIONAL WAVE ASTRONOMY IS NOW OPEN!