===================================================================
1996 March 23 (6350) Daniel W. E. Green
COMET C/1996 B2 (HYAKUTAKE)
C. Lisse, Goddard Space Flight Center; and K. Dennerl and J.
Schmitt, Max-Planck-Institut fur Extraterrestrische Physik,
Garching, communicate: "Comet C/1996 B2 will be observed with the
x-ray satellite ROSAT. The scheduled observing intervals are as
follows: Mar. 26.493-26.529 UT, 26.626-26.634, 26.638-26.662,
26.708-26.729, 26.89-26.902, 27.554-27.591, 27.687-27.695, 27.703-
27.724, 27.772-27.789, 27.841-27.856, 28.417-28.44. Simultaneous
observations at other wavelengths are encouraged."
===================================================================
1996 April 4 (6373) Daniel W. E. Green
COMET C/1996 B2 (HYAKUTAKE)
C. Lisse, M. Mumma, and R. Petre, NASA Goddard Space Flight
Center; and K. Dennerl, J. Schmitt, J. Englhauser, and J. Truemper,
Max-Planck-Institut fur Extraterrestrische Physik, Garching, report
the discovery of x-rays from comet C/1996 B2 with ROSAT -- the
first such detections from any comet: "The observations took place
on nine different occasions during Mar. 26-28 (see IAUC 6350). The
brightest parts of the comet's x-ray image are diffuse but crescent
shaped, offset sunward by about 6' from the nucleus (projected
distance about 30 000 km), and extend to about +/- 8' (+/- 40 000
km) in the direction perpendicular to the sun-comet direction. The
observed radiation shows a clear x-ray (rather than ultraviolet)
signature and is strongly time variable on the order of hours. An
image can be found under
http://www.xray.mpe.mpg.de/~jer/rda/comet/hyakutake.html.
Preliminary estimates yield peak surface brightness countrates of
about 0.01 count sE-1 arcminE-2, with the comet being at Delta =
0.13 AU. A probable mechanism for the observed radiation is
scattering of solar x-rays by material in the comet's coma.
Another possibility is that the radiation is derived from energy
deposition by the solar wind."
===================================================================
1996 May 8 (6393) Daniel W. E. Green
COMET C/1996 B2 (HYAKUTAKE)
F. E. Marshall, C. Lisse, M. Mumma, and R. Petre, NASA Goddard
Space Flight Center; and K. Dennerl, J. Englhauser, and J. Trumper,
Max-Planck-Institut fur Extraterrestrische Physik, Garching, report:
"The x-ray emission from comet C/1996 B2 is primarily at energies <
2 keV. No evidence for emission in the band 2-10 keV was seen
during a fortuitous scan lasting 24 s across the comet on Mar.
31.2836 UT with the Proportional Counter Array onboard the Rossi X-
ray Timing Explorer. The 90-percent-confidence upper limit for the
flux in the band 2-10 keV, assuming a power-law spectrum with
photon index 2.0, is 1.6 x 10E-11 erg sE-1 cmE-2. The limit is 1.7
x 10E-11 erg sE-1 cmE-2 for an index of 3.0. During this
observation, the comet was 0.22 AU from the earth and 0.91 AU from
the sun. Assuming the comet's luminosity in the band 0.1-2 keV was
the same as during the non-flaring intervals of the ROSAT
observation on Mar. 26-28 (IAUC 6373), we can rule out spectral
shapes harder than a power law with photon index 2.0 or thermal
bremsstrahlung emission with a temperature of 2.5 keV. For both
these assumed spectral shapes, > 50 percent of the 0.1- to 10-keV
luminosity is at energies < 1.25 keV."
===================================================================
1996 May 9 (6394) Daniel W. E. Green
COMET C/1996 B2 (HYAKUTAKE)
J. P. Pye and R. G. West, Leicester University; M. Harden and
M. Ricketts, Rutherford Appleton Laboratory; K. Dennerl, J.
Englhauser, J. Schmitt, and J. Trumper, Max-Planck-Institut fur
Extraterrestrische Physik, Garching; and C. Lisse, M. Mumma, and R.
Petre, NASA Goddard Space Flight Center, report the discovery of
extreme-ultraviolet (EUV) emission from comet C/1996 B2 with the
ROSAT Wide Field Camera (WFC), simultaneously with the x-ray
measurements reported by Lisse et al. on IAUC 6373: "The WFC
measurements were made with the S1A filter, nominal bandpass 90-206
eV (14-6 nm). An image can be found at
http://ledas-www.star.le.ac.uk/rosat-goc/comet/. Bright, diffuse
emission is seen, roughly coincident with the x-rays and sunward of
the nucleus, and strongly time-variable in a similar manner. The
peak EUV surface brightness is of order 0.0004 count sE-1 arcminE-2
as measured, or of order 0.003 count sE-1 arcminE-2 when corrected
to WFC 'at launch' efficiency. The WFC images also show fainter
diffuse emission extending beyond the edge of the HRI field-of-view
(i.e., at about 15'-40' from the center of the comet nucleus),
forming an arc around the bright central region. The ratio of WFC
to HRI countrates shows that the spectrum in the EUV/x-ray band is
rather 'soft' and steeply increasing towards low photon energies.
This would appear to rule out soft-x-ray line-fluorescence as the
dominant emission mechanism. For assumed spectral shapes of a
power law or thermal bremsstrahlung emission, the ratio indicates a
photon index in the approximate range 2.0-3.0, or a temperature of
about 0.1-0.4 keV."
===================================================================
1996 May 21 (6404) Daniel W. E. Green
COMET C/1990 N1 (TSUCHIYA-KIUCHI)
K. Dennerl, J. Englhauser, and J. Trumper, Max-Planck-Institut
fur Extraterrestrische Physik, Garching, communicate: "X-ray
emission from comet C/1990 N1 (= 1990i = 1990 XVII) was detected in
data obtained with the Positional Sensitive Proportional Counter
(PSPC) of ROSAT during the all-sky survey. Between 1990 Nov. 18.76
and 20.36 UT (when Delta = 1.08 AU, r = 1.38 AU, and visual m1
about 7.0), the PSPC was scanning across the comet, yielding a
total exposure of 500 s. The x-ray image, corrected for the
comet's motion, shows an extended source at the expected position,
with a radial extent of at least 10', or 470 000 km. This PSPC
observation allows, for the first time, spectroscopic studies of
the x-ray radiation of a comet. The spectrum can be modelled by
thermal bremsstrahlung emission with kT = 0.4 +/- 0.1 keV.
Inclusion of the 525-eV oxygen-fluorescence line improves the
confidence of the fit. Pure line emission due to oxygen and carbon
fluorescence can be ruled out. The 0.1- to 2.4-keV x-ray
luminosity was 2.5 x 10E16 erg/s and thus comparable to that of
comet C/1996 B2 (cf. IAUC 6373). The optical luminosity of C/1990
N1, however, was lower by a factor of 15 than that of C/1996 B2.
This second x-ray detection of a comet indicates that x-ray
emission is a class property of such objects."
===================================================================
1996 June 4 (6413) Daniel W. E. Green
X-RAY DETECTIONS OF COMETS
K. Dennerl, J. Englhauser, and J. Trumper, Max-Planck-Institut
fur Extraterrestrische Physik, Garching, communicate: "X-ray
emission from several comets has been detected in data obtained
with the Position Sensitive Proportional Counter (PSPC) of ROSAT
during the all-sky survey, in addition to the discovery of x-rays
from comet C/1990 N1 (IAUC 6404). We list here all of the
significant comet detections obtained so far, with the
corresponding all-sky survey observing intervals: C/1990 K1 (Levy),
1990 Sept. 6.50-7.10 UT, 1991 Jan. 10.82-11.96, 1991 Jan. 17.49-
17.96; C/1990 N1 (Tsuchiya-Kiuchi), 1990 Nov. 18.76-20.36, 1991 Jan.
12.07-13.67; 45P/Honda-Mrkos-Pajdusakova, 1990 July 31.20-31.86.
All comets were at heliocentric distances < 2 AU, with visual m1
between 4.2 and 11.6. For a comparison of the x-ray and optical
properties, we are looking for images of these comets taken at or
near the specified times; please contact K. Dennerl (e-mail
kod@mpe.mpg.de)."
===================================================================
1996 July 12 (6433) Daniel W. E. Green
COMET C/1996 B2 (HYAKUTAKE)
C. Lisse, M. Mumma, and R. Petre, NASA Goddard Space Flight
Center; and K. Dennerl, J. Englhauser, J. Schmitt, and J. Truemper,
Max-Planck-Institut fur Extraterrestrische Physik, Garching,
communicate: "Comet C/1996 B2 was observed after its perihelion
passage with the ROSAT High Resolution Imager from June 22.10 to
23.77 UT (when Delta = 1.16 AU, r = 1.35 AU, and visual m1 = 7.1)
for 8900 s. The image, corrected for the proper motion of the
comet, shows an extended source with a radial extent of at least 8'
(400 000 km). The peak surface brightness was at 0.002 count sE-1
arcminE-2, approximately a factor of 5 lower than during the
previous ROSAT observations in March (IAUC 6373). The
reacquisition of C/1996 B2 after perihelion confirms the continuous
behavior of x-ray emission in the comet. ROSAT will continue to
observe the comet until Sept. 8. The scheduled observation times
will be posted at URL
http://www.xray.mpe.mpg.de/~jer/comets/; simultaneous
observations at other wavelengths (especially extreme ultraviolet
and radio) are encouraged."
===================================================================
1996 October 9 (6486) Daniel W. E. Green
EXTREME-ULTRAVIOLET DETECTIONS OF COMETS
J. P. Pye and R. G. West, Leicester University; and K. Dennerl,
Max-Planck-Institut fur Extraterrestrische Physik, Garching, report
the discovery of extreme-ultraviolet (EUV) emission from two comets
in data acquired by the Wide-Field Camera (WFC) during the ROSAT
all-sky survey. Accompanying x-ray emission has previously been
reported by Dennerl et al. (IAUC 6413). Significant EUV detections
have been made in the following observing intervals: C/1990 K1
(Levy), 1990 Sept. 6.50-7.43 UT, 1991 Jan. 9.96-11.96, and Jan.
17.36-18.02; C/1990 N1 (Tsuchiya-Kiuchi), 1990 Nov. 18.16-20.70.
All detections were made in the WFC S1 filter, nominal passband 90-
206 eV (14-6 nm). The upper-limit countrates derived for the WFC
S2 filter (passband 62-110 eV; 20-11 nm), when compared with the S1
measurements, yield spectral parameters consistent with those
previously reported for some comets (IAUC 6394, 6404).
===================================================================
1996 September 11 (6472) Daniel W. E. Green
COMET C/1991 A2 (ARAI)
K. Dennerl, J. Englhauser, and J. Trumper, Max-Planck-Institut
fur Extraterrestrische Physik, Garching, communicate: "X-ray
emission from comet C/1991 A2 (= 1991b = 1990 XXVI) was detected in
data obtained with the ROSAT Position Sensitive Proportional
Counter during the all-sky survey. The comet was observed from
1990 Nov. 18.09 to 21.43 UT (Delta = 1.2 AU, r = 1.5 AU), six weeks
before it was discovered (IAUC 5157), when it would have had
projected visual m1 about 12 (or fainter) and would be optically
the faintest comet ever detected in x-rays."
===================================================================
1996 October 19 (6495) Daniel W. E. Green
COMET C/1996 Q1 (TABUR)
K. Dennerl, J. Englhauser, and J. Truemper, Max-Planck-
Institut fuer Extraterrestrische Physik, Garching; and C. Lisse,
Goddard Space Flight Center, NASA, communicate: "X-ray emission
from comet C/1996 Q1 was detected with the ROSAT High Resolution
Imager. The observations were performed on several occasions
between Sept. 26 and Oct. 6, when Delta = 0.51-0.42 AU and r =
1.09-0.99 AU. The x-ray emission was highly variable on time
scales of hours (or less), both in intensity (at least by a factor
of five) and morphology, with episodes of considerable deviation
from the crescent shape of C/1996 B2 (Hyakutake) observed under
similar phase angles (IAUC 6373). The brightest parts of the
emission were always concentrated in an area with a typical extent
of 5' (90 000 km) and displaced sunward from the nucleus by about
100" (30 000 km). They reached a peak surface brightness of 0.01
count sE-1 arcminE-2, similar to that of comet C/1996 B2 during its
close approach to the earth in Mar. The rapid morphological
changes seem to rule out any scattering of solar x-rays by material
in the comet's coma as the dominant mechanism for producing the
bright parts of the x-ray emission."