On
August 17, 2017, scientists seeking the holy grail of gravitational wave (GW)
astronomy struck gold. The elusive
and long sought after GW signals from merging binary neutron stars were found
and multi-messenger observations provided tell-tale signs of this merger to clinch
the issue without any qualms. Two of the GW detectors in the US picked up the
signal and a third, working in Europe, confirmed it. Several of the satellites in
the sky detected signals from this event across various bands of the electromagnetic
spectrum, and a vast array of optical and radio telescopes worldwide trained
their vision into this new phenomenon, finding a variety of corroborating
signals.
The AstroSat scientists, who pitched in with
their efforts, today stand shoulder to shoulder with a few thousand scientists
across the globe (including three Nobel Prize winners and a few scores of other
Indian scientists) to announce this
momentous discovery and an `open sesame' moment of staring at the huge
cache of scientific discovery that this new era of `multi-messenger,
time-domain astronomy' opens up.
Gravitational Wave Astronomy: the last frontier
Any accelerated electronic charge emits electromgnetic
radiation: scientists routinely use this to generate and send electromagnetic
waves like radio waves, optical light, and X-rays. Any moving mass disturbs the
space time and a `quadropole' moment in the moving mass should generate
gravitational waves: theorised Albert Einstein'a hundred years ago. Einstein's
words are treated as Veda Vakya or Gospel Truth, and
astronomers routinely use this to understand the dynamics of compact large
masses in the cosmos. Russell A. Hulse and Joseph H. Taylor, Jr discovered two
radio pulsars going around each other, slowly hurtling towards each other, and they
invoked Einstein's gravitational wave theory to understand their behavour: they
were duly awarded a Nobel Prize for this work. This opens up an interesting
question - shouldn't astronomers, who use every branch of electromagnetic
radiation from radio to gamma-rays to prise open the secrets of the Universe,
use gravitational waves to understand exotic features of the cosmos - like the
ripples of the Big Bang or merging of black holes when galaxies collide ?
Well, they should, but the catch
lies in the fact that the gravitational force is extremely weak compared to the
electromagnetic force, and common sense deems that even the most sensitive
detectors that humans can build cannot detect the most exotic gravitaional wave
sources that we can imagine. However, during the past few decades, a huge number of
dedicated scientists have built the most sophisticated detectors capable of
measuring infinitesimal movement of mass corresponding to a tiny fraction of a nanometer
in kilmoter sized objects so that they would be sensitive to the gravitational
waves from outer space. Year after year, they kept looking for signs of merging
neutron stars, but the quest was in vain !
Mother Nature usually likes to
keep surprises up her sleeve! When the GW detectors with highly improved sensitivity
were switched ON in 2015, they found something: not a neutron star-neutron star
merger, but a totally unexpected event of two massive black holes merging and
spewing out energy equivalent to the complete burning out of mass corresponding
to two Suns. This is indeed a momentous discovery, and the architects of this
humongous human effort, Kip Thorne, Rainer Weiss, and Barry Barish, duly got
this year's Nobel Prize.
What about the elusive case of the
merger of two neutron stars anticipated from the discovery of Messers Hulse and
Taylor? During the past two years, four GW events were discovered, however, they
were all due to mergers of black holes. The problem with merging black holes is
that they are, as apparent from the name, `black'; i.e., apart from the GW events,
there are no tell tale signs of the merger in any other branch of
electromagnetic radiation. So, we cannot determine where they are coming from, or
what are their progenitors. This is not the case for neutron star mergers. It
was firmly believed that when GW events are discovered from neutron star
mergers, they would be accompanied by huge amounts of electromagnetic radiation,
which will help us pin down the sources of these events.
The whole scientific community was
eagerly waiting for this much anticipated event.
CZT Imager of AstroSat pitches in
AstroSat was launched on September 28, 2015 and the
CZT Imager (CZTI) instrument of AstroSat was the first instrument to be made operational. On
October 6, 2015, the first day of operation, CZTI detected a gamma-ray burst (GRB) and proved to be an efficient
GRB detector. The scientists working with the CZTI data realised that it would be a wonderful instrument to detect
any gamma-ray events accompanying the GW sources.
The problem with detecting such gamma-ray
events is that they are rare, unpredictable, and can come from any direction in
the sky. Hence, the detectors need to have all sky sensitivity, and generally,
there is a trade off in their observing capabilities. Currently, there are three sensitive
operating GRB monitors, along with a few more less sensitive detectors, each
having their own capabilities and limitations. The most sensitive GRB monitor currently
operating is the Swift satellite,
however, it can observe only one tenth of the sky at any given time. CZTI and the Fermi satellite, on the other hand, are sensitive to much larger
regions in the sky, but have very limited capability to localise these events.
The anti-coincidence shield of the INTEGRAL
satellite, too, can act as a GRB monitor.
Each of these instruments played
their part in the race to detect gamma-ray signals accompanying the GW events.
During the very first GW event on September 14, 2015 (before the launch of AstroSat), Fermi claimed that it had detected a GRB like event within 0.4 s of
the GW event. Observations from the INTEGRAL
satellite, however, disagreed: the consensus was that this could be some
unrelated spike in the background. During another GW event detected in January
2017, optical astronomers saw, the very next day of the event, some source
gradually diminishing in brightness. Could this be the tell-tale signs of
something happening in the GW source? CZTI
chipped in with a firm No! It had detected a GRB, 21 hours after the GW event.
The fading optical source was shown to be this GRB, unrelated to the GW event.
Aug 17, 2017: a red letter day
On August 17, 2017, the much
anticipated event occurred.
The GW detectors in US registered a
very long series of signals, or `chirps', closely resembling what the
scientists have simulated for decades to be coming from neutron star
coalescence. Even before they could announce this discovery, the Fermi satellite had detected a GRB at
the same time: in fact within a couple of seconds of the GW event. Could this
also be an unrelated background fluctuation event? Very unlikely, because, at exactly
the same time, the anti-coincidence shield in the INTEGRAL satellite had also detected this GRB. What about Swift and CZTI? They didn't detect any! The event should probably be outside
the narrow field of view of Swift.
What about CZTI? It was active and
operating and the GRB should have been detected. The only way to reconcile was
to assume that the source was blocked by the Earth: this helped to narrow down
the possible source regions of the GW event.
Soon, the GW detectors from Europe
too pitched in, and the region of the sky responsible for the GW event and GRB
was narrowed down to a small region. Optical telescopes around the world
scanned each and every galaxy in this region and, lo and behold, there indeed
was a bright optical object, not seen before, near a galaxy called NGC 4993.
The rest, as they say, is history.
Soon, infra-red and ultraviolet emissions were seen from this source. Nine days
later, an X-ray source was detected, and fifteen days later, radio emission was
also observed. From such vast multi-wavelength data, the physics of colliding
and merging neutron stars were studied in depth. An exciting find is that the
material ejected in the event is rich in heavy elements, so much so that,
colliding neutron stars can account for the entire supply of precious metals, like
gold, platinum and silver, in the universe. Production of these elements have been difficult
to understand, and now the source has been found!
The story of GW170817 bears testimony to
the amazing outcome possible when all the world's best instruments are combined
for a single purpose. The collaborative efforts of a number of teams worldwide
lends an added credibility to this exciting and substantial discovery and
ushers in a new era in multi-messenger, time-domain
astronomy!
Scientific curiosity: never satiated
The GW
detectors are taking a year off to return with an improved sensitivity. Neutron
star merger events and the accompanying `kilonova' should be fairly common
observations during the next run.
There should be more of black hole merger events as well. Scientists are
already dreaming about the rich future harvests:
Can we get any tell-tale signatures of black
hole mergers to identify where they are coming from? Perhaps more sensitive all sky detectors would
help with an answer.
Can these events be used as a tool for
distance measurement to refine cosmology? A massive
collaboration between GW theorists and kilonova observers should be able to do
it.
Can we learn anything about the regions
close to black hole? Possible.
Are there some strange stars among the
neutron stars? Certainly
more such objects will tell us. Finally,
has Mother Nature more surprises up her sleeve? Only the future will tell
us!
What next? Significant next steps will involve making
detectors more sensitive, improving localisation capability and most
importantly, continued collaboration of observatories worldwide spanning all
electromagnetic bands, neutrinos and gravitational waves.
In the
Indian space science context, the capability of CZTI would certainly be
improved through better algorithms and simulations: it should be possible to
independently confirm and localise gamma-ray events for future GW associations.
Perhaps, even a much improved CZTI like all sky monitor could be designed and flown!
Multi-messenger studies of GW170817
incorporating the contribution of AstroSat CZTI are published in the journals Science and Astrophysical Journal Letters.