Comet Hyakutake - Update (May 11, 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 30 . It is based on information received directly by email and also from IAU Circulars and on other Hyakutake WWW pages. I am most grateful to those colleagues who have sent me information about their observations, quoted below.

Richard M. West (ESO)
La Silla, May 11, 1996; 04:00 UT

New Orbit and ephemeris

A new orbit and ephemeris have been made available by Brian Marsden. Note also the associated urgent request for further astrometry in order to enable the HST and other observers to acquire the comet without problems.

Post-perihelion observations by SOHO

Comet Hyakutake was observed from the SOHO spacecraft on the date of the perihelion passage, May 1. When, on May 9, there had still been no official information about any further post-perihelion observations from the ground or from space, I posted a corresponding note on the ESO WWW Hyakutake page. In response. I received two emails from Phillippe Lamy (LAS, Marseille) and Guenter Brueckner (SOHO LASCO PI), informing that more observations had indeed been made by the LASCO instrument on the SOHO spacecraft during the following days (see below), showing interesting developments in the tail structure.

However, as of this moment (May 11), no post-perihelion ground-based observations have yet been reported. Upon my arrival at the ESO La Silla observatory, I was informed about one unsuccessful attempt by visual observers to locate the comet from this site in the evening of May 10. It is therefore apparent that the total magnitude is fainter than magnitude 1, most probably 2.

On May 9, 16:40 UT, P. Lamy wrote: Using the LASCO/C3 coronagraph aboard the SOHO spacecraft, LASCO scientists from the Naval Research Laboratory, Washington and the Laboratoire d'Astronomie Spatiale, Marseille, monitored the perihelion passage of comet Hyakutake from May 1 to 5. Both plasma and dust tails extended over more than 4 deg. The plasma tail exhibited considerable time variability and split into 2 components on May 5. As the broad, fan-shaped, dust tail rotated, it uncovered a narrow spike close to the projection of the orbital plane suggesting a population of large dust trailing the nucleus. As the main dust tail will further swing, this spike will very likely appear as an anti-tail when the comet will be again visible to ground-based observers.

On May 10, 14:45 UT, G. Brueckner added: On your comet Hyakutake WWW page I find a note, that the comet was last seen by SOHO on 1 May 96. That is not so. Hyakutake entered the f.o.v (field-of-view) of the C3 coronagraph on SOHO on 30 April 96 at 18:08 and left on 06 May at 02:39. All this applies to the head of the comet.

During its passage in the f.o.v., there were no signs of a break up. We cannot comment on a possible dimming of the coma, because a precise photometry has not been carried out yet.

However Hyakutake developed a third tail, which lined up precisely with the orbit of the comet. This tail grew in brightness and length, while Hyakutake was in the f.o.v. of the coronagraph. Presumably this tail is made up of 'new' dust or heavy particles, which are not subject to radiation pressure or the solar wind. This indicates a mass loss of Hyakutake, while it approaches the Sun.

The other two tails - the dust and the ion tail - are rotating in the plane of the sky, as Hyakutake swings around the Sun. There is a hint of a partial disconnection in the ion tail.

He adds that LASCO images of comet Hyakutake can be obtained from the LASCO Comet Hyakutake Homepage .

There is therefore little doubt that Comet Hyakutake has now developed a so-called 'anti-tail' pointing towards the Sun. As mentioned above, the usual interpretation is the release of relatively large (heavy) dust particles in the orbital plane which are less influenced by the pressure of the solar light than the lighter ones. This feature is particularly well visible, whenever the Earth crosses the plane of the comet's orbit.

More about the X-ray observations

There have been exciting developments during the last few days concerning the mystery of comet Hyakutake's X-rays. Here is a short summary by Konrad Dennerl and Jakob Englhauser (Max-Planck-Institute for Extraterrrestrial Physics, Garching), received on May 9, 10:00 UT:

As you may have seen already (IAUC 6393), the Proportional Counter Array onboard the Rossi X-ray Timing Explorer (XTE) observed comet Hyakutake by chance during a slew from one target to the next, for only 24 seconds, on Mar 31, just three days after the ROSAT observations. It did not detect X-ray emission from the comet, but the upper limit can be used to constrain the spectral shape, under the assumption that the comet's overall X-ray luminosity was similar as during the ROSAT observations. XTE is sensitive to X-rays above 2 keV whereas ROSAT covers the range from 0.1 to 2 keV. The comparison of the ROSAT HRI flux with the XTE upper limit indicates a soft X-ray spectrum, with a 'temperature' kT But this is just one part of the news!

Attached to the main X-ray telescope on ROSAT and aligned with it is a smaller instrument for measurements in the extreme ultraviolet (sometimes abbreviated XUV, sometimes EUV) region. This instrument has less angular resolution and sensitivity than the X-ray telescope, but an increased field of view (5 deg diameter versus 40 arcmin diameter of the HRI); it is therefore usually called the 'Wide Field Camera' (WFC). The spectral bandpass extends from 0.09 to 0.2 keV. In this region interstellar absorption is quite severe, and only few cosmic X-ray sources can be investigated with this instrument. This is the reason that the WFC is not as well known as the main X-ray telescope. The WFC is a UK contribution to the ROSAT project, and the data are processed independently of the X-ray data in the UK.

When we were analysing the HRI data of the comet Hyakutake observations, we realized the importance of the WFC data, and notified our collegues from UK about the extraordinary scientific value of this data. They expedited the processing of the WFC data (which is usually delayed by several months), so that we were able to analyse them over the last weeks. The first preliminary results are now available, and the corresponding IAUC (6394) appeared last night. Since the WFC observations had taken place simultaneously with the HRI observations and cover a wider field of view, at an almost complementary energy range (which does, obviously, not suffer from interstellar absorption in this case!), they provide, together with the HRI data, very important observational constraints on the X-ray emission processes which are currently discussed. A preliminary comparison of the fluxes indicates a very soft X-ray spectrum (e.g. with kT = 0.1-0.4 keV for thermal bremsstrahlung emission) and it seems that pure line-fluorescence can now been ruled out .

More information is now available on the related Homepage at the Leicester University .