Comet Hyakutake - Update (April 30, 1996)

Dust Jets (ESO NTT Image)

This is another Update about recent developments around this comet; the latest was published on the ESO Web on April 10 . It is based on information received directly by email and also from IAU Circulars and on other Hyakutake WWW pages.

NB! Note the request below for information about images of Comet Hyakutake, obtained on April 9, 1996.

Richard M. West (ESO)
April 30, 1996; 11:00 UT

The perihelion passage

Comet Hyakutake will reach perihelion tomorrow, May 1, 1996, at about UT 09:27 in the morning. At this time it will pass the (centrum of the) Sun at a distance of 34.44 million kilometres. Since the angular distance in the sky between the comet and the Sun will only be about 7 degrees, it will not be possible to observe the comet at this moment and during the next days. It is expected that observations will recommence from the Southern Hemisphere in the second half of May, probably also at the ESO La Silla observatory.

Brightness and tail development

Following its splendid display during the approach to the Earth in late March, it is the general impression by visual observers that the comet's performance has recently become somewhat less spectacular. Part of this is due to its very low position in the sky during much of April so that most observers have seen it in a rather bright sky, but it also appears that the intrinsic brightening, as it moves closer to the Sun, has been less than predicted. Indeed, the most recent brightness estimates, dating from April 24 - 26, are in the interval 2 - 3 mag, or about 2.5 magnitudes fainter than expected by direct extrapolation of its performace in March.

Most tail estimates made in the second half of April are below 10 degrees, and just a few degrees towards the end of the month. While this is undoubtedly also due to the difficult observing conditions, it does seem to be somewhat less prominent than foreseen, cf. the forecast by Rob L.W. van de Weg .

Although another outburst was observed at the Pic du Midi observatory on April 15, it is therefore evident that the cometary activity is developing slower than expected and, unless this trend reverses, the prospects for observations after perihel are less bright than thought. Still, it will most probably continue to be visible with the unaided eye until the end of May.

Nevertheless, an unexpected brightening is still possible, especially as the strain on the nucleus induced by the sunlight and the solar wind will be greatest near perihelion. Other comets have been known to break up at that time and since small fragments have already been observed to come off the nucleus of Comet Hyakutake, further events of this type are possible, leading to increased evaporation from the freshly exposed surfaces and therefore to a brightening.

You will find more information about the brightness of Comet Hyakutake in a short article by Charles Morris .

The X-ray mystery

As reported earlier, cf. IAUC 6373 (April 4), unexpectedly strong X-rays were observed from the coma of Comet Hyakutake. This is the first time that X-rays have ever been observed from a comet.

Efforts are underway since early April to explain this radiation, but it does not seem easy to find a plausible mechanism which may be responsible for this effect. The emission clearly comes from the part of the coma which is most strongly illuminated by the Sun and it is natural to assume that the Sun's X-ray radiation (originating in the solar corona) is involved in some way.

The purely morphological aspects (distribution of X-ray intensity over the coma) have been studied by two Indian scientists, S.K. Chakrabarti and K.S. Krishnaswamy, who have presented a description (an 'analytical' model) based on the density of ions in the coma at different distance from the nucleus. This model reproduces the observed X-ray image quite well, but does not say anything about the actual mechanism whereby the X-rays are produced.

On behalf of the Hyakutake X-ray team , Konrad Dennerl of the Max-Planck-Institute for Extraterrestrial Physics (Garching) has made an informative statement (April 29) about their recent work. Here is an excerpt, reproduced with permission:

The analysis of the ROSAT observations has confirmed the presence and variability of the X-ray emission from comet Hyakutake; the origin of the X-ray emission, however, is not yet understood.

What was regarded previously as a plausible mechanism, namely fluorescence scattering of solar X-rays in the coma, is now regarded to be less likely. This possibility was favoured by the fact that it provides a straightforward explanation of the observed crescent-shaped morphology under the assumption that the coma is optically thick to soft X-rays - and here is the problem: The effective K-alpha cross sections of oxygen and carbon are so small that the required amount of scattering matter seems to be unrealistically large. From the observed count rates we derive an X-ray luminosity in the 0.1 - 2.0 keV energy band of the ROSAT HRI of roughly 1.e16 erg s-1 (1 Gigawatt). To achieve this luminosity by fluorescent scattering, a mass of about 1.e15 g, comparable to the total mass of the nucleus, would be required.

Explaining the high X-ray luminosity seems to be a major challenge, also for the other models which invoke solar wind interactions with the coma and collisions between dust in the coma and zodiacal dust particles.

Currently we are working on a paper which describes the observational data and discusses possible models.

It is important to mention that the observational data collected with ROSAT certainly contain more information that we have been able to utilize so far, despite extensive efforts in data analysis. At the moment we are quite confident that we will be able to extend our observational basis significantly within the next few days - we will keep you informed.

It will be exciting to follow the further developments on this front. There is no doubt that Comet Hyakutake will keep the specialists busy for a while!

Spectral observations

Observations with the NASA Infrared Telescope Facility on April 8 (IAUC 6378; April 11) indicate the presence of two infrared emission lines of C2H2 (acetylene); the abundance is approx. 0.1 - 0.4 percent of that of water.

Of great interest are also the high-angular observations with the IRAM Plateau de Bure Interferometer in France (IAUC 6388; April 26). The measurements with four 15-metre antennas at 88 GHz on March 23, i.e. just before the time of closest approach, have allowed to measure (hyperfine) emission lines from HCN (cyanic acid) molecules from a very small area (a few hundred kilometres diameter) around the cometary nucleus. From the intensity and the distribution of this emission, there seems to be little doubt that it originates in 'parent' HCN molecules, directly released from the nucleus. Similar observations are also available of emission at 230 GHz from CO (carbon monoxide) parent molecules.

These observations allow a direct estimate of the amounts of these molecules which leave the nucleus, i.e. the actual evaporation rates. Thus they are extremely important for better understanding the physical processes that take place at the nucleus and also its chemical composition.

Request for images obtained on April 9

On April 9.89243 UT (21h 25m 06s), a gamma-ray burst was observed with two satellite observatories at a sky position only 2.6 degrees from the position of Comet Hyakutake at that moment (IAUC 6384; April 19). The registered location of the burst was RA = 2h 50.5m; Decl = +41d 55m (J2000.0) and the duration was 105 seconds. The positional uncertainty is about 2 degrees.

The origin of gamma-ray bursts is still unknown and it would be extremely important to learn whether a 'flash' at this position was also seen in visual light. Thus, all observers of Comet Hyakutake are requested to check whether they have available photographs or CCD-frames which were obtained at this instant and which also show the sky location where the burst was observed.

A map with the locations is available.

Observers who believe that they may have such images should contact T. Harrison (USA telephone: 505-646-3628 or email: tharriso@nmsu.edu).