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First Optical Identification of an Extragalactic Pulsar

03 Giugno 1993

The recent identification of the optical image of a pulsar in the Large Magellanic Cloud is a fine illustration of astronomy as a high-tech international science. It is the first extragalactic pulsar to be so identified and only the third radio pulsar, after those in the Crab and Vela nebulae in the Milky Way, for which this has been possible.

The conclusive observations were made in early 1993 by astronomers from Ireland, Denmark and ESO [1] with TRIFFID [2], a new, powerful instrument of their own design, used together with the ESO MAMA [3] detector system and attached to the 3.5-metre New Technology Telescope (NTT) at the ESO La Silla observatory in Chile. Earlier observations by Italian astronomers with the same telescope were crucial for the success of this research project.

A spinning neutron star in the Large Magellanic Cloud

The Large Magellanic Cloud (LMC), a satellite galaxy to the Milky Way galaxy in which we live, is one of the most studied objects in the sky. In addition to several millions of stars it also contains a great number of nebulae of gas and dust. Some of these have been found to be the remains of gigantic supernova explosions in the past when heavy stars in the LMC became unstable and blew up. The most recent happened as late as in February 1987, when Supernova 1987 A became the first naked-eye supernova in 400 years.

One of these nebulae has a circular shape with a diameter of about 6 arcseconds; it is believed to be the remnants of the penultimate LMC supernova which exploded some 760 ± 50 years ago; this age is deduced from the expansion rate of the nebula. In 1984, a group of American astronomers studied the data from the Einstein X-ray satellite observatory and found that pulsed X-rays are emitted from the direction of this nebula. The measured pulsation frequency is unsually high and has now been refined to 19.838 Hz (pulses per second).

This is explained by the presence of a pulsar somewhere inside the nebula, that is an extremely compact neutron star which weighs as much as the Sun, but has a diameter of 10 -15 kilometres only. It was created by enormous pressures during the supernova explosion. The observed pulses indicate that it is now spinning around its axis once every 0.05 seconds. The nebula in which it is imbedded contains the rest of the material that was thrown out during the explosion.

The new object received the designation PSR 05-10-693 (the numbers indicate its approximate position in the sky). Because of its many similarities with the Crab pulsar and nebula, it has also been nicknamed the Crab Twin.

Due to the large distance to the LMC, of the order of 160,000 light-years, it has not been possible, until recently, to measure the very faint radio emission from this pulsar with southern radio telescopes. The X-ray observations only fix the pulsar position within a circle with a diameter of about 4 arcseconds (the "X-ray error circle"), and since detailed radio observations cannot be made of this faint radiosource, it is impossible to determine the position of PSR 05-10-693 with sufficient accuracy to permit identification of its optical image. Variations of the optical emission with the above X-ray frequency were measured in the mid-1980's from the general area of the nebula, but the image sharpness achievable with the astronomical telescopes available at that time did not allow the detection of a star-like object inside the relatively bright nebula.

Narrowing down the choice

This was the situation in early 1992 when a group of Italian astronomers [4] obtained images of the field around PSR 0540-693 with the ESO NTT.

Thanks to excellent weather conditions, the remarkable optical quality of the NTT and the fine performance of the ESO SUSI high-resolution CCD camera, they were able to record the most detailed images ever made of this region. Although the comparatively strong light from the nebula tends to "wash out" any details within its confines, they detected for the first time the presence of two, star-like objects inside the nebula. Both were much fainter than the nebula itself; they are located in the South-West area of the nebula (near the edge of the X-ray error circle) and are separated by about 1.3 arcseconds.

Because all of the exposures necessarily lasted much longer than the 0.05 second pulse interval, these observations did not permit to decide if any of the two had a variable light intensity and might therefore be the pulsar. Still, when the Italian astronomers published their new results [5], they remarked that the northernmost of the two objects was more likely to be the pulsar; this image was somewhat more star-like (sharper) than the other one and a jet-like symmetrical structure appeared to be exactly centered on it.

Catching individual photons

The observations which have now led to the unambiguous identification of the pulsar image were performed during 3 nights in January 1993 at the ESO NTT by the Irish/Danish/ESO group. Their new instrument, TRIFFID, together with the ESO MAMA detector, has the ability to record individual photons as they arrive at the telescope. The observational data consist of the exact position and time of arrival of each photon that comes from the sky field towards which the telescope is pointed. Common CCD images "add" together the photons during the exposure and therefore only permit to measure positions. It is exactly the information about the photon arrival times recorded by TRIFFID/MAMA [6] which is of crucial importance for the identification of objects with rapidly varying light intensity.

To the astronomers' great dismay, the weather was not very good during the observations. The atmospheric turbulence was rather high and the recorded images were not as sharp as they had hoped for. As planned, the astronomers spent most of their time at the telescope time recording the photons from the most recent LMC supernova, SN 1987A. They did not detect the elusive pulsar in that object, but were nevertheless able to set very stringent limits on its brightness.

In order to test their new equipment they carefully observed the Crab pulsar and also PSR 0540-699. The latter was observed about 90 minutes during two nights. But while the observations lasted only a short while, the subsequent data analysis at University College Galway [7] turned out to be very time-consuming.

The technique consisted of analysing the arrival times of photons in different parts of the field. To begin with, all photons detected in each square of a coarse area grid were put together and a time series analysis was performed to see if any of the squares displayed any periodicities. Already at this stage, and despite the very few photons and the statistical nature of light, the 19.838 Hz frequency could be clearly detected in the general area of the nebula. By successively diminishing the grid size, it was found that this photon periodicity was most significant in one small area with a diameter of less than 1 arcsecond. The positional centre of the corresponding photons could be determined with an accuracy of about 0.4 arcsecond.

The independent data obtained on two observing nights showed a positional coincidence better than 0.2 arcseconds and also corresponded within this uncertainty with the position of the northernmost object in the Italian astronomers' picture. The perfect coincidence, in position as well as in the pulse frequency, is the basis for the definitive identification of the pulsar's optical image. After more than three months of hard work the group was able to make the important announcement about their discovery on Circular 5786 of the International Astronomical Union on May 10, 1993.

The magnitude of the optical image of PSR 0540-693 is about 22.5 and its relative faintness is illustrated by the fact that only about 2 photons per second were detected from it during the NTT observations.

Why is the identification of the pulsar so important?

Being only the third radio pulsar for which the optical image has been found, PSR 0540-693 now belongs to a most selected and interesting group of objects. It is now possible to concentrate future observations to the corresponding optical object and to better exclude the disturbing background light from the surrounding nebula. There is no doubt that it will be very thoroughly studied by large optical telescopes during the coming years and the groups have already applied for additional observing time at ESO as well as with the Hubble Space Telescope.

When the direction to the pulsar is known with the highest achievable precision, it is possible to take correctly into account all the various effects introduced by the Earth's uneven motion and thereby to combine observations of individual pulses accurately over long periods. This will allow to properly measure the progressive lengthening of the pulsar period, the exact value of which is of key importance for understanding the pulsar mechanism. Of the 550 pulsars presently known, this measurement has only been made for the two youngest ones, the pulsar in the Crab Nebula and PSR 1509-58. The age of PSR 0540-693, as deduced from the lengthening of its rotation period, is not yet well determined, but it is likely to be the third youngest known.

Contatti

Richard West
ESO EPR Dept
Garching, Germany
E-mail: information@eso.org

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