# Why were pulsars not discovered in the visual spectrum?

In 1967, the first pulsar was discovered at a radio observatory in Cambridge.

Radio astronomy started around the 1930s, but astronomy in the visible part of the spectrum had been done for a long time by then.

Was it a coincidence that no observation of a pulsar had been made before?

Or are pulsars generally better suited for radio astronomy?

## 4 Answers

Astrophotography of distant objects traditionally was done by exposing photographic plates for a few minutes or even hours, and then developing them. With this procedure it is simply not possible to detect the intensity variations of pulsars (varying in the seconds or even milliseconds range).

I doubt if this is even possible now, by using CCD sensors instead of photographic plates.

• Photoelectric detectors were present in the 1960s and are capable of optically detecting pulsars (and such detections were reported in the 60s. But they are incapable of discovering pulsars. – Rob Jeffries Jun 28 at 18:57

I'm going to raise some of the same points the other answers have, but in slightly more detail.

There are two main types of visible light coming from a pulsar: thermal black body emission from the surface of the neutron star, and optical synchrotron radiation originating in the magnetosphere. The first problem with observing either of these lies in the fact that neither contributes significantly strong emission. The Crab pulsar appeared as a magnitude $$+17.7$$ V-band source in the initial optical observations (see Cocke et al. 1969), pushing the limits of what could be observed at the time.

The vast majority of neutron stars do not display substantial thermal emission ($$\lesssim1\%$$ of total optical radiation), meaning that optical synchrotron radiation is a much better thing to look for at visible wavelengths. However, given the dimness of the source, you need to understand the radio/x-ray pulsations of the neutron star. The confirmation of the first optical detection involved a complicated setup to synchronize with the target and time-average the signal (Nather et al. 1969), and this would not have been possible had the Crab not already been detected at radio wavelengths. The technology existed, but one wouldn't casually scan the sky at optical wavelengths with it; that would be fairly pointless unless you had a pulsating target.

To expand on a point made in Thomas's answer, systems for detecting optical pulsars don't naively use CCDs, but instead use a principle called photon counting, which, well, counts photons.

Optical pulsars aren't very bright, and we've only found 6 of them so far.

Wikipedia says:

There are very few of these known: the Crab pulsar was detected by stroboscopic techniques in 1969, shortly after its discovery in radio waves, at the Steward Observatory. The Vela pulsar was detected in 1977 at the Anglo-Australian Observatory, and was the faintest star ever imaged at that time.

As Thomas mentions, the long exposures typically used to capture images of faint objects cannot detect the pulsing of a pulsar, unless you use stroboscopic techniques. And you're not likely to try using a stroboscopic image capture unless you suspect that the object may be pulsing.

Good evening.

Pulsars can be detected optically, however it is somewhat challenging. As was pointed out, generally specially designed detectors are needed to detect this emission as they generally pulse very fast, faster than conventional silicon CCDs or HgCdTe and InSb infrared detectors (the workhorses of astronomy) are generally used.

The current technology that can do this are specially designed CCD imagers (with tiny fields of view to minimize the number of transfers between pixels needed to read out), EMCCD and CMOS imagers, and more exotic technologies like kinetic inductance detectors.

To see a cool video of a pulsar blinking, see this talk.

• Pulsars were detected at optical wavelengths long before CCDs. nature.com/articles/2211037a0 – Rob Jeffries Jun 25 at 10:53
• You are right, but the spirit of the question is why pulsars were not discovered optically, even as optical astronomy predates radio. The instrument in your paper uses stroboscopic techniques, locking an optical chopper to the known phase of the pulsar. This is a "specially designed detector" that would not have been useful for anything else, including discovering other pulsars. – Mike Jun 26 at 20:32
• Photoelectric detectors can detect optical pulsations. They have been around for decades before CCDs. Adding your comment to your answer would make it better. – Rob Jeffries Jun 28 at 18:55