In 1967 there was an extraordinary event for radio astronomy: the discovery of neutron stars (PULSAR: Pulsating Radiosource, or briefly PSR) by J. Bell, a student of A. Hewish at the Observatory of Cambridge.

The story begins in 1965, when Hewish started the construction of a rectangular plane array composed from 2048 half wave dipoles positioned horizontally at some wavelengths of height from the ground, operating at a frequency of 82 MHz.

The physical surface of the instrument covered an area of about 20000 square meters and it had the possibility to move in declination (a few degrees) its main lobe modifying the phase of the signals collected by the various groups of elements that arrived at the receiver (by varying the lengths of the lines of coaxial cable that constitute the transmission lines of the antenna).

The Earth's rotation ensured scanning in right ascension. The system was designed to study the compact radio sources by analyzing the quasar phenomenon of scintillation produced by their irregular structure of the interplanetary medium.

While the researchers were engaged in this work, they noticed in their recordings substantial and regular fluctuations of the signal.

Optimizing the time constant of the receiver (very short) they were able to highlight a series of pulses characterized by regular repetition period, theoretically generated by objects in the final stage of evolution with radius between 10 and 50 km and mass between about 0.5 and 2 solar masses: they were the PULSAR or neutron stars.

The width of the pulses of the first pulsar discovery was of the order of a few hundredths of a second.

It's groped an estimate of the size of the issuer object knowing that these are always less than or, at most equal to the product of the speed of propagation of the radio signal in a vacuum (speed of light) and the pulse radio duration: the resulting that the size of the pulsar result to be of the order of a few thousand km (comparable to the diameter of the Moon).

This unexpected result led to the hypothesis that the signals were artificial, from some planet inhabited by intelligent beings animated by the desire to interstellar contact.

With a mixture of irony and belief, the letters assigned to the first four discovered pulsars were LGM 1, 2, 3 and 4, where LGM was short for Little Green Man. A careful examination of the signals revealed that they carried a huge amount of energy if the source of the pulses had been the distance of the Sun from the Earth: this fact made the idea of man-made signals sent from an extraterrestrial civilization hardly plausible.

 

Characteristics of received signal from a Pulsar

Pulsars are objects of small dimensions that, by rapidly rotating (model T. Gold), emit high energy radiation pulses at regular intervals, specific for each object, the range of values from 33 milliseconds to 3.75 seconds (the shortest is relative to the Crab Nebula pulsar, remnant of a supernova - see table below).


Signal received from a pulsar

 

Characteristics of the main pulsar measured at a frequency of 408 MHz most of the "modern" names assigned to the pulsar are built using the approximate equatorial coordinates preceded by the letters PSR.

 

Features of the main pulsar

 

Periodic signal emitted by a Pulsar as appears at the output of a receiver using a short time constant of post-detection.


The pulses received, with different shape according to the radio source, have very small width compared to the repetition period (a few percent) and are of complex shape, with details of the duration of 0.2 milliseconds or less.

Objects known today are about a thousand.

The PSR 0950+08, for example (see table), has a period of 0253 seconds with a rise time of approximately 5 milliseconds: an object capable of irradiating pulses so fast must necessarily have small dimensions, since, in a short period of time, no large object may propagate information through its structure with rapidity such as to maintain the phase of the pulsation. As the radiation travels only 1500 km in 5 milliseconds, this distance represents the maximum diameter of the object 0950+08.

The analysis of the signals coming from other pulsar has shown how these are not larger than about 30 km, with an average diameter of a few kilometers.
The neutron stars are observable over a wide band of frequencies, even if their emission is more intense toward the longer wavelengths, with a rapid decrease in intensity toward the higher frequencies.