AIR Talk on pulsars given in 1999
Pulsar –The cosmic Lighthouse
In the daily grind and struggles for designer lives, we rarely glance at the skies and the variety of exciting and peace giving objects the sky has to offer. We need also to get away from city lights and cloudy nights to look at the myriad of objects which do their bit towards humbling us – The Moon, planets, stars, nebulae, galaxies and their clusters. These objects, we can see with our naked eyes, or through using sophisticated optical telescopes, i.e., telescopes which can gather visible light. What differentiates visible light from other radiations like Radio, X-ray or gamma Ray radiation? This quantity is the wavelength, a typical length characterising a wave. Radiation at radio wavelengths is characterised by much longer wavelengths than the visible radiation, while X-ray
And gamma rays are at much shorter wavelengths than visible light. For instance, my voice is reaching you after being carried along by radio waves of wavelength ...., you might be familiar with the frequency of this carrier being .... The two quantities happen to be related.
The heavens are also filled with many exotic objects which might be detected at wavelengths other than the visible light. Celestial objects emitting radio waves are studied using radio telescopes like the large single dish one at Arecibo,
Puerto Rico, or using an array of antennas like the Giant Meter Radio Telescope at Narayangaon, in Maharastra. At the Jodrell Bank Radio Telescope in England Jocelyn Bell, a research student was observing radio data to study interplanetary scintillations, something akin to the atmospheric twinkling of star light, only, here, radi data twinkles when it passes through a cloud of plasma that is fluctuating.
While engaged in this she noticed some unusual looking data from a given region in the sky, that was recorded on a chart on the evening of August 6th, 1967. She labelled this as a ‘little bit of scruff’. The scruff reappeared in the same region of the sky at about the same time the next night. The scruff turned out to be a series of regularly spaced radio pulses separated by about 1.337 301 1512 seconds! No known celestial source ever showed regular periodicity on such rapid time scales. It was also unprecedented that one could quote the period to an accuracy of ten decimal places. What was the source of these highly regular radio pulses? There was some thought then as to whether extraterrestrials, jokingly labelled ‘little green men’, were behind these signals.
It was deduced very soon that these were natural signals from a galactic radio source – a Pulsar. Rotation was deduced to be the underlying cause of the pulses that we observe from these objects. A star, only a little more massive than the sun, is compressed into a ball of about 10 km radius. In that process, it starts to rotate extremely rapidly – once every second or even faster, it also gains an enormous magnetic field - some million million times the magnetic field strength of earth. These gigantic magnetic fields rotating at such enormous speeds act like a giant generator, generating enormous electric fields. Charged particles accelerated to relativistic speeds in these huge fields produce the radio radiation which we see.
But, why pulses? This would mean that the sight of radiation is localised on a small region on the surface of these stars. As the star rotates, this localised region and the emission from it becomes momentarily visible and then moves out of sight. The star acts as a cosmic light, or rather, as a radio house sending out beams of radiation into the interstellar space. If we, on earth, happen to intercept the beams from any of these objects, we will detect a radio pulsar in that direction.
Let us listen to an audio of data from some of these pulsars. An audio tone has been inserted in the radio data from these pulsars to make it audible. We will listen to a slow pulsar first and then go on progressively to faster ones.
The bulk of data showered by these objects onto our planet has been detected by us in the form of radiation at radio wavelengths, although activity at optical, Xray and gamma ray wavelengths is also recorded from some of these objects. The peculiarity of the activity from these objects happens to be their constancy and inconstancy put together – They are inconstant in the sense that they give pulsed radiation, series of radio pulses with spacing between the pulses varying from a millisecond to a couple of seconds. In their constancy they manifest themselves as the universe’s bestknown clocks, some of them with accuracy better than a tenth of a microsecond! Their periods do change, but these changes are at a scale of something like a millionth of a billionth second per second!
An understanding of why these objects pulse has been simpler than the rationalization of how they shine. Pulsating nature of these objects is understood in a simple and elegant picture of these objects as rotating cosmic light (rather, radio) houses sending their radiation beams into outer space in some fixed directions. If we, on earth, happen to intercept the beams from any of these objects, we will detect a radio pulsar in that direction.
This then is the picture – a star, about as massive as the sun, is rotating rapidly (its beam intercepts us every second or millisecond!), there is some localized activity on the surface of this object where radiation beams are directed outward from the star (this localized region is identified with the magnetic poles of the star). This object, we call it a neutron star, is highly dense – the densities which prevail in this object are like the densities in the nucleus of an atom. A spoonful of neutron star matter would weigh as much as all of humanity! Its magnetic field is intense, with intensities ranging from a billion to hundred thousand billions of a typical units of magnetic fields – a Gauss.
The surface gravity on these objects is so intense that anyone trapped in the vicinity of this object needs to travel at about half the speed of light in order to escape from the gravitational clutches of this monster. Such highly dense, fast rotating stars with intense magnetic fields are thought to be stellar remnants which survive after a massive star at the end of its lifetime as an ordinary star explodes as a supernova. The explosion takes place when the interior nuclear fuel of a star, which burns through its lifetime, is exhausted and the core of the star collapses to nuclear densities, becoming a neutron star. The core would have to be larger than 1.4 times the sun’s mass in order that the collapsed object comes to rest as a neutron star. This is the celebrated Chandrasekhar’s limit. For lower core masses the star is able to come to terms with the relentless gravity that tends to compress it, by the quantum mechanical behaviour of electrons in an object we refer to as the white dwarf. In neutron stars, it is the quantum mechanical behaviour of neutrons which resists further gravitational collapse of the object. At even higher masses, even this would not be sufficient to hold off gravity and the object would collapse to the ultimate exotic object which occupies zero volume, the Blackhole.
This picture of pulsars has gradually been unfolded and accepted by the scientific community in the time immediately following the discovery of pulsars, the year 1968. Amazingly, the possibility of the existence of such objects was predicted in a remarkably far seeing work when Baade and Zwicky remarked in 1934, ‘with all reserve we advance the view that supernovae represent the transitions from ordinary stars into neutron stars, which in their final stages consist of extremely closely packed neutrons .’ The association of supernovae with a central neutron star became confirmed when pulsars were discovered in the centers of the eerie remnants of some historical supernovae like the Crab nebula.
We understand that these are rapidly spinning, highly magnetized objects. The small rotation rates and magnetic fields observed in ordinary massive stars are enough to explain this enormous increase in the spinning rate and the field strength of these objects when we invoke the conservation principles provided to us by nature, as the stellar remnant is compressed to high densities. With such high magnetic fields in these objects it is easy to accept that in the higher magnetic intensity regions of the magnetic poles relativistic particles are accelerated to high velocities and emit radiations that we ultimately detect from these objects. The detailed mechanisms which give rise to these radiations are as yet controversial. One of the beautiful manifestations of the radiation from pulsars which has considerably increased the understanding of the geometry of their emission has been the seminal observation by Radhakrishnan and Cooke (1969) that as the radiation proceeds along the pulse of emission its plane of polarization (the plane in which oscillations in electric and magnetic fields take place, manifesting themselves as observable radiation) changes in a way which seems to reflect the changing magnetic field structure as the beam of the pulsar sweeps past us.
We talked about the regularity of pulsars as the best known clocks in the universe. The clock is slowing down as a result of an extremely slow rate of decrease in the spins of the neutron stars.
The slow down itself is very steady and seems predictable when lo! Suddenly one of these objects jerks itself and speeds up in a sudden glitch of increased spinning rate. A simple way of understanding such a sudden increase in spinning rate has been to think of the solid outer crust of these stars undergoing upheavals due to seismic activity. A break in the crust and inward moving results in an increased spinning the same way that pulling our hands inward while on a turn table would tend to make us rotate faster. Yet another exotic understanding of this activity involves the possibility of the existence of a macroscopic quantum state – that of a superfluid which essentially offers zero resistance to flow and rotation, in the deep interiors of these objects. Such a superfluid will not slow down the same way as the outer solid crust and in general would rotate faster. A sudden deposition of angular momentum from interior to the crust could also be at the base of the observed jumps in rotation rates. These as well as a better understanding of the origin of radiation from pulsars are issues to be settled with the ever-increasing body of observations and theoretical understanding of these cosmic radiation houses.
--- By N. Rathnasree, Nehru Planetarium, New Delhi