salonibhatiaini17: have we been able to observe the hawking's radiation?
jaidepp: how does radiation escape the gravity of a black hole?
salonibhatiaini17: is it radiated from outside the event horizon?
jaidepp: so the gravity doesn't act there
Patrick Das Gupta: The Hawking radiation has not been observed because it is a tiny effect. Only for blackholes of mass less than 1000 kgs would Hawking temperature be high enough for the evaporation to be observable.
jaidepp: how does radiation escape the gravity of a black hole
Patrick Das Gupta: The Hawking radiation takes place outside the event horizon and furthermore it is a quantum mechanical process. So, the radiated particle tunnels out in a quantum mechanical process. So, if only classical mechanics was there then blackholes would only grow in mass by swallowing matter. Because of quantum mechanics, blackholes also radiate and hence there is a possibility that eventually blackholes get completely evaporated away.
salonibhatiaini17: does this radiation get redshifted because of gravity?
Patrick Das Gupta: Good question. Indeed the gravity of the blackhole does cause redshifting of Hawking radiation.
salonibhatiaini17: can a binary blackhole exist?
Patrick Das Gupta: Yes, in principle two blackholes can form a binary system. But then how will one observe such a system, since the system will not radiate. Astronomers have observed several binary systems in which one of the objects is blackhole the other is a star. The gas from the star spirals on to the blackhole companion and forms an accretion disc that becomes very hot and radiates X-rays copiously. Two blackholes binary system can be dtetected using gravitational wave detectors. So, in the future the LIGO experiments in USA may see many such double blackhole binaries, Since close blackhole binaries will spiral in by emitting gravitational waves.
salonibhatiaini17: so we can observe the gravitational radiation
Patrick Das Gupta: Yes. Although indirectly one has observed graviational waves from the Hulse-Taylor binary pulsars in the sense that they spiral in at the rate predicted by Einstein's formula of gravitatioanl radiation, so far direct detection of Gws has not been possible.
salonibhatiaini17: in a quasar the redshift is supposed to be cosmological but we also consider a blackhole so can't it be graviational also due to the acretion disc.
Patrick Das Gupta: Good question again. In order for a high graviational redshift the emission has to be from very near the event horizon of the blackhole. But we know that the quasar emissions are from regions that 100 to 1000 times further from the event horizon of the supermassive blackholes. Only the X-yay emissions from quasars are from near the event horizon. But optical and radio emissions are far from the central blackhole of a quasar. So, the redshift is essentially due to the expansion of the universe.
salonibhatiaini17: how do we know that the x rays are emitted from near the event horizon , wont this be redshifted?
Patrick Das Gupta: It is the time variation of X-rayss that tell us the size of the emitting region. X-rays from quasars fluctuate on time scales of few hours. That means, the emitting region has to be of size 3 times 10 ^10 cm/sec times 36oo secs.
jaideep_gupta: what is the meaning of being redshifted
Patrick Das Gupta: This is few times the Schwarzschild radius of the supermassive blackhole. Surely, the X-rays get redshifted. But it has a continuous spectrum so one cannot determine the redhifting from the spectrum. Redshift means that the frequency of light when it is emitted is more than the frequency of light that is detected. In case of Doppler shifts, a source that is moving away from the observer leads to Doppler redshifting. Einstein showed that if radiation starts from strong gravity region towards weak gravity regions it gets redshifted. Meaning if you shine blue light from strong gravity region towards weak gravity region, your friend there will observe the colour to be red.
salonibhatiaini17: what is meant by time variation of x rays and how do we study it ?
Patrick Das Gupta: The X-ray detectors see varying X-ray flux from a quasar. The time variation of X-Ray flux from quasars typically are of few hours durations. While radio emissions from quasars vary over a period of months.
salonibhatiaini17: why do they vary?
Patrick Das Gupta: These are still active areas of research. The consensus among astronomers is that the fluctuation in the radiation occurs due fluctuation in the hot plasma in the accretion disc around the supermassive blackholes.
salonibhatiaini17: why is the temperature of the accretion disc high?
Patrick Das Gupta: The accretion disc has gradients in the velocity. Gas rotating in the inner radii of the disc move faster than the gas at the outer radii (in accordance with Keplers law). So, inner radii gas rubs against the outer radii gas due to difference in speeds causing frictional heating of the gas. It is also believed that magnetic viscosity also cause heating of gas in the accretion disc. But proper theoretical modelling is still an active area of research. Lot of magnetohydrodynamics is involved in this field.
Rathnasree: Thank you, Prof. Patrick Das Gupta and thank you all the viewers for being with us today. Saloni and Jaideep, your questions were good and made this interaction today enjoyable
salonibhatiaini17: Thankyou it was amazing.
jaideep_gupta: thank you. what are those blue stars that are said to live long?
Rathnasree: Jaideep, we will take up your question in the website discussions or in the next lecture.
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