PLEASE NOTE:


*

CCNet, 25 November 1999
------------------------------


     PRE-MILLENNIUM JITTERS OF THE DAY

     "Considering the known neutron stars inside our own galaxy, a
     case can be made that evolutionary disjunctions in Earth's past
     may have been caused not only by asteroid impacts, but also by
     gamma-ray bursts from merging neutron stars a few thousand     
     light years distant in our galaxy."
          --Science Week, 22 November 1999


     "Finally, some news to soothe the millennial jitters. Not. The
     odds are higher than you might expect that before Y3K, an
     asteroid will plunge into the ocean and trigger a powerful surge
     of water called a tsunami that inundates shorelines somewhere,
     researchers say."
          -- ScienceNow, 23 November 1999



(1) FIVE LUNAR IMPACTS NOW CONFIRMED
    Joan and David Dunham <dunham@erols.com>

(2) DROWNING IN ASTEROIDS
    Michael Paine <mpaine@tpgi.com.au>

(3) GAMMA RAY BURSTS AND POSSIBLE TERRESTRIAL DISASTERS
    Science-Week <prismx@scienceweek.com>

=============
(1) FIVE LUNAR IMPACTS NOW CONFIRMED

From: Joan and David Dunham <dunham@erols.com>

Subject: Two more confirmed lunar impacts - now 5

David Palmer reports two more lunar impacts that he videorecorded at
his home in Greenbelt, Maryland at 3:49:41 and 4:08:00 UT of 1999
November 18.  The times are estimated to be accurate to +/-3 seconds
since they were obtained just by calibrating the VCR clock with time
from the CNN cable TV broadcast. The flashes are also in the video
recording that I made at Mount Airy, about 60 km to the northwest,
bringing the total now to five confirmed lunar impacts, four of them on
my tape and also on other videotapes made by others, and the other, the
first one reported, confirmed with Brian Cudnik's timed visual
observation.

Brian Cudnik reports that the flash he saw was yellowish-orange in
color, redder than nearby psi1 Aquarii.  All of the videorecordings are
black-and-white.  A third probable untimed visual confirmation of that
event has been provided by Steve Hendrix, who watched the dark side of
the Moon with a 4.5-inch Meade telescope from Cameron, Missouri from
4:40 to about 4:55 UT.  It was the only flash that he saw during that
period and it matched Brian Cudnik's description.  Before hearing about
Cudnik's and my description of the flash, Hendrix was hesitant to share
his observation since he had "never seen anything like this before and
didn't want to appear over zealous".

A summary of the five confirmed lunar impacts are given in the table
below.  This is an ASCII plain text table that must be viewed with a
fixed-space font such as Courier for the columns to line up properly. 
For the time being, we are naming these with letters in the order of
discovery.  The UT date is 1999 November 18. In each case, the events
were confirmed on my videotape made at George Varros' backyard in Mount
Airy, Maryland, and the timings are from my tape.

            Accuracy, Approx.  Discovered  Selenographic
Name  U.T.    sec. Mag1  Mag2    by       Long. Lat. Description
   h  m   s
D  3:49:40.5   0.4   3    7  David Palmer  48W   1N 175km SW of Kepler
E  4:08:04.1   0.6   5    8  David Palmer  70W  15S 175km S of Grimaldi
A  4:46:15.2   0.1   3    8  Brian Cudnik  71W  14N  50km ENE of Cardanus
B  5:14:12.93  0.05  7    8  Pedro Sada    58W  15N 200km WNW of Marius
C  5:15:20.23  0.05  4    7  Pedro Sada    59W  21N  75km S Schiaparelli

Mag1 is the approximate magnitude of the flash estimated from my tape on
the half-frame on which it first appears.  Mag2 is the estimated
magnitude a half-frame, or 1/60th second, later.  In all cases except D
I can't see any evidence of the flash in the half-frame 1/30th second
after the first one, except for D, where it seems to appear there at
about 9th mag. The selenographic locations for D and E are very
approximate, based on rough estimates rather than measurements, and
could be in error by 5 deg. or more.  The others should be accurate to
within about 2 deg. or 50 km.  All of these are in the western part of
Oceanus Procellarum (Ocean of Storms) except E, which is in highlands
area a short distance west of the western shore of Oceanus Procellarum.
The times of B and C have been determined by Don Stockbauer, Victoria,
Texas, after creating an accurately time-inserted copy using an IOTA-
Manly video time inserter.  He also determined the time of A, but for
technical reasons to less accuracy; it will be possible to refine it
later.  D and E have been timed from the tape just using a stopwatch.

Several have asked me how large the impacting meteors are, and if the
new crater they form might be seen.  I need help from an expert in
impact dynamics on this - I don't have expertise in that field.  I have
heard one estimate that the impactors, to produce flashes this bright,
are meter-size, but another estimate is that they may be just 100 grams
or so. In any case, I believe that the "splash" that these objects made
are less than 100m across and will not be visible with Earth-based
telescopes. In 2003, the Japanese Selene spacecraft plans to map the
Moon from low orbit in detail, and coparison of its images with those
of Lunar Orbiter, Apollo, and/or Clementine will hopefully reveal some
small new craters.

Ray Sterner and I digitized our images of B and C yesterday and we
hoped to get them posted on our Web site at http://iota.jhuapl.edu, but
that might not be possible now until Monday.  In the meantime, we will
post the latest information about these flashes at the main IOTA site at
http://www.lunar-occultations.com/iota
including Sada's images of B and C, and Palmer's images of D and E. I
don't plan to make any more mass-mailings like this one about these
events, but will distribute future updates to a few who are especially
interested.  Otherwise, check the Web site for further updates, but I will
try to answer specific questions about these events.

David Dunham, IOTA, 1999 November 24

PS - After sending this, I will not be at this address, but will be
reachable at david.dunham@jhuapl.edu, phone 1-240-228-5609.

Joan and David Dunham
7006 Megan Lane
Greenbelt, MD 20770
(301) 474-4722
dunham@erols.com

==============
(2) DROWNING IN ASTEROIDS

From Michael Paine <mpaine@tpgi.com.au>

Dear Benny,

Tsunami modeller Steve Ward brought this article to my attention.
Note that I indirectly covered the issue of the apparent disagreement
between the experts in my Explorezone article Asteroid tsunami: good
news and bad 
<http://explorezone.com/columns/space/1999/september_tsunami.htm>

I understand that the debate is still alive and well.

Michael Paine
The Planetary Society Australian Volunteers

-------------
From: ScienceNow, 23 November 1999 7:00 PM
http://sciencenow.sciencemag.org/cgi/content/full/1999/1123/1
(subscription needed)

Drowning in Asteroids

Finally, some news to soothe the millennial jitters. Not. The odds are
higher than you might expect that before Y3K, an asteroid will plunge
into the ocean and trigger a powerful surge of water called a tsunami
that inundates shorelines somewhere, researchers say.

The odds are vanishingly small that an asteroid will strike a
particular spot on Earth. However, after any impact, collateral damage
can range far and wide due to fires, dust clouds, and tsunamis, which
can travel thousands of kilometers from impacts in the sea. Tsunamis
are composed of a series of broad waves, causing repeated floods far
inland, so even a 2-meter-tall tsunami, a mere pup, can damage
low-lying areas far more extensively than normal storm waves.
Researchers have used computer models to show that tsunamis from rare
kilometer-sized asteroids could wipe out entire coasts, but no one had
studied the hazards posed by smaller impacts.

A new analysis by geophysicist Steven Ward and planetary scientist Erik
Asphaug of the University of California, Santa Cruz, concludes that the
biggest tsunami hazard arises from asteroids between 30 and a few
hundred meters across, which may strike the ocean every 1000 to 100,000
years. The team assessed many factors that determine risk, from impact
rates to wave size and how energetic a wave remains after traversing
the ocean. Their results, to appear in the journal Icarus, show that a
typical coastal site facing a broad expanse of ocean has a 1 in 14
chance of experiencing a 2-meter-tall tsunami in the next 1000 years.
The chances fall to 1 in 35 for 5-meter tsunamis and 1 in 345 for
devastating 25-meter waves. Ward and Asphaug also assessed the specific
hazards for six major cities. For instance, Tokyo has a 1 in 24 chance
of a 5-meter tsunami in the next millennium, while New York City faces
1 in 47 odds of similar waves.

"This is a more realistic assessment than any I have seen," says
planetary scientist Alan Hildebrand of the University of Calgary in
Alberta, Canada. He notes that certain coastal and seafloor shapes can
amplify tsunamis, increasing the hazard at some sites. However,
planetary scientist David Crawford of Sandia National Laboratories in
Albuquerque, New Mexico, cautions that his own supercomputer
calculations of ocean impacts produce tsunamis up to 10 times smaller
than those in Ward and Asphaug's analysis. "We've agreed to disagree,"
Crawford says.

--ROBERT IRION

copyright 1999, ScienceNow

==============
(3) GAMMA RAY BURSTS AND POSSIBLE TERRESTRIAL DISASTERS

From Science-Week <prismx@scienceweek.com>

****************************************************************
This is SW BULLETIN, a free publication published each Monday by
the Editors of SCIENCE-WEEK, the weekly Email research digest.
For information about ScienceWeek, see the end of this file or
visit the SW website at: www.scienceweek.com (At the end of this
file you will find the table of contents of the current issue
of ScienceWeek.)
****************************************************************

SW BULLETIN - November 22, 1999
---------------------------------------------

This Week's Report:

Medical Biology: Gamma Ray Bursts

---------------------------------------------

[The following originally appeared in ScienceWeek 16 July 1999]

GAMMA RAY BURSTS: THE LARGEST EXPLOSIONS IN THE UNIVERSE

Gamma rays are  extremely high energy electromagnetic radiation with
wavelengths of less than approximately 0.01 nanometers. X-rays are
radiation of wavelengths approximately 0.01 to 10 nanometers, shorter
than ultraviolet radiation but longer than gamma rays. Gamma ray bursts
are intense flashes of gamma rays and x-rays detected at energies up to
10^(6) *electron volts. They were discovered by US Air Force satellites
in 1967 but not declassified until 1973. The detection of these bursts
averages approximately 1 per day, and measurements indicate the
distribution of bursts is isotropic, i.e., they are uniformly
distributed across the sky. The nature of gamma ray bursts remains
mysterious. Astronomers have obtained rigorous distance estimates only
recently, placing gamma ray bursts definitely in the realm of
cosmology. *Redshift measurements suggest extremely large distances,
making gamma ray bursts the most powerful catastrophic energy releases
known to mankind. ... ... Dieter H. Hartmann (Clemson University, US)
presents a review of current research concerning gamma ray bursts, the
author making the following points:
    
1) Gamma ray bursts are short flashes of almost pure high-energy
emission (x-rays and gamma rays) that occur randomly on the sky, and
from loci which apparently do not emit more than once. Typical
durations are of the order of seconds, but can range from a few
milliseconds to over 1000 seconds. The bursts are extremely bright,
outshining all other objects on the gamma ray sky, but their spectra
are featureless and reveal little about the underlying physical
processes. Integrating burst spectra over energy and time yields large
fluences (received energy per unit area), but does not determine the
total burst energy until the distance is known.
    
2) Although the statistical properties of gamma ray bursts long
supported the idea that bursts occur at cosmological distances, this
distance scale was finally established by a burst on 8 May 1997, for
which a faint extended object was optically identified as the host, the
object showing clear evidence of absorption lines that indicated a
lower redshift limit of z = 0.835. On 14 December 1997, another burst
showed absorption lines at z = 3.42, and then a third burst on 3 July
1998 had associated absorption lines at z = 0.966 -- all of this
indicating that gamma ray bursts, along with *quasars, are the most
distant objects in the Universe. Such large distances imply large
energies, and in fact the assumption of isotropic emission implies
burst energies in excess of 10^(53) ergs, comparable to *supernova
energies but released predominantly in the gamma ray band. The optical
afterglows of gamma ray bursts are much brighter than supernova, hence
the name "hypernova" has been proposed.

3) Studies of gamma ray burst host galaxies suggest they are normal
star-forming galaxies, and not galaxies with *active nuclei. The
estimated star formation rates in these hosts, together with other
evidence from x-ray spectra and photometry of the gamma ray burst
afterglows suggests that gamma ray bursts may be directly associated
with star-forming regions. If that turns out to be correct, astronomers
would have a powerful new tool for the study of structure formation in
the Universe, a tool that could reach further back in time than
quasars.

4) Despite recent breakthroughs in gamma ray burst observations, many
questions remain about the nature of the underlying processes and the
evolutionary sequences leading up to the creation of the central engine
driving these outbursts. The ultimate goal of understanding this engine
may be accomplished through simultaneous optical observations, and such
is the objective of dedicated experiments under development throughout
the world.
-----------
Dieter H. Hartmann: Afterglows from the largest explosions in the
Universe.
(Proc. Natl. Acad. Sci. US 27 Apr 99 96:4752)
QY: Dieter H. Hartmann, Clemson University, Clemson SC 29634 US.
-----------
Text Notes:
*electron volts: An electronvolt is defined as the energy acquired by
an electron falling freely through a potential difference of one volt,
and is equal to 1.6022 x 10^(-19) joule.

*Redshift: Redshift (symbol: z) is a lengthening of the wavelengths of
electromagnetic radiation from a source caused either by the movement
of the source (Doppler effect) or by the expansion of the universe
(cosmological redshift). Redshift is defined as the change in
wavelength of a particular spectral line divided by the unshifted
wavelength of that line. Large redshifts imply large radial velocities
(which imply large distances, according to current cosmological
theory), but at redshifts greater than about 0.2 there is a
relativistic divergence from a linear relation. A redshift of 4.0
corresponds to an object receding with a radial velocity 92% that of
the velocity of light. The largest astrophysical redshifts so far
observed are of the order of z = 5.

*quasars: (quasi-stellar objects) Extremely luminous sources radiating
energy over the entire spectrum from x-rays to radio waves, and which
are apparently among oldest and most distant objects in the universe.

*supernova: A violent explosion in which certain stars end their lives.
The star may become more than 10^(9) times as bright as the Sun and may
outshine its host galaxy for weeks.

*active nuclei: (active galactic nuclei) Central regions of galaxies in
which considerable energy is generated by processes other than those
operating in ordinary stars. The energy may result from the accretion
of material into a massive black hole situated at the core of the
galaxy. (See Report #3 this issue.)
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 16Jul99
-------------------
Related Background:

ASTROPHYSICS: SUPERNOVAE AND GAMMA RAY BURSTS

Supernovae are violent explosions marking the terminal stage of certain
stars. They are classified into two broad types, Type I and Type II. A
Type II supernova shows hydrogen in its spectrum, while a Type I
supernova shows no hydrogen in its spectrum. Type I supernovae are
further classified as Type 1a, Type 1b, and Type Ic. A Type 1a
supernova is believed to be due to the explosion of a *white dwarf star
in a binary star system, the result of matter falling onto it from the
companion star. When the mass of the white dwarf exceeds the
*Chandrasekhar limit, the white dwarf undergoes runaway carbon burning
and explodes. Type Ib and Ic supernovae are thought to result from the
collapse of the cores of massive stars which have lost their hydrogen
envelopes. Type II supernovae arise from the explosion of stars of more
than 8 solar masses. In this case, the explosion involves a violent
blow-off of outer-layer material after the massive star has collapsed
into a *neutron star or a black hole. Despite the existing
classification scheme, Type Ib and Type Ic supernovae are more closely
related to Type II supernovae than to Type Ia supernovae. Gamma ray
bursts are intense flashes of *gamma rays detected at energies up to
10^(6) *electronvolts. They were discovered by US Air Force satellites
in 1967 but not declassified until 1973. The detection of these bursts
averages about 1 per day, and measurements indicate the distribution of
bursts is isotropic, i.e., they are uniformly distributed across the
sky. The current consensus is that gamma ray bursts are produced by the
merger of two neutron stars, and up to this point, the bursts that have
been noted apparently originate outside our own galaxy.

In 3 contiguous reports in the same journal, 3 research teams now
report an association of the gamma ray burst of 25 April 1998
(GRB980425) with the supernova SN1998bw, which exploded at
approximately the same time as the gamma ray burst. Although in general
the properties of supernovae are very different from those of gamma ray
bursts, the apparent new consensus is that supernova SN1998bw
establishes a second class of gamma ray burst which is distinctly
different from the cosmological kind. It is suggested that in some
supernovae the outer layer of the exploding star is given sufficient
energy to cause it to expand at speeds approaching the speed of light,
and that this initially produces a burst of gamma rays and a subsequent
radio emission. If this suggestion is correct, gamma ray bursts may be
produced by two substantially different mechanisms. [Editor's note: A
collection of previous SW reports on gamma ray bursts can be found in
the SW Focus Report "Astrophysics: Gamma Ray Bursts" which is available
at URL <http://scienceweek.com/swfr012.txt>]
-----------
S.R. Kulkarni et al (9 authors at 5 installations, US AU)
Radio emission from the unusual supernova 1998bw and its
association with the gamma-ray burst of 25 April 1998.
(Nature 15 Oct 98 395:663)
QY: S.R. Kulkarni <srk@astro.caltech.edu>
-----------
T.J. Galama et al (50 authors at 21 installations, NL US CL IT JP
UK DE AU)
An unusual supernova in the error box of the gamma-ray burst of
25 April 1998.
(Nature 15 Oct 98 395:670)
QY: T.J. Galama <titus@astro.uva.nl>
-----------
K. Iwamoto et al (27 authors at 9 installations, JP IT CL DE NL
US)
A hypernova model for the supernova associated with the gamma-ray
burst of 25 April 1998.
(Nature 15 Oct 98 395:672)
QY: K. Nomoto <nomoto@astron.s.u-tokyo.ac.jp>
-----------
Text Notes:

*white dwarf star: White dwarf stars are extremely dense and compact
stars that have undergone gravitational collapse. They are the final
stage in the evolution of low-mass stars after they have lost their
outer layers. White dwarf stars are about the size of Earth, but with a
mass about that of the Sun.

*Chandrasekhar limit: The remnant mass after the blow-off during the
terminal stage of the life of a star determines the ultimate fate of
the star. If the remnant mass is less than 1.44 solar masses (the
Chandrasekhar limit for a star with no hydrogen content), the star
collapses into a white dwarf. If the remnant mass is greater than 1.44
solar masses, depending on the remnant mass, the star collapses into
either a neutron star or a black hole. Named after Subrahmanyan
Chandrasekhar (1910-1995), who first proposed the modern theory of
stellar gravitational collapse, and who received the Nobel Prize in
Physics 1983.

*neutron star: If, following its terminal stages, the remnant mass of a
star is between 1.4 and 2 to 3 solar masses, the star will collapse
into a neutron star, a body with a radius of 10 to 15 kilometers, with
a core so dense that its component protons and electrons have merged
into neutrons. The average density of a neutron star is 10^(15) grams
per cubic centimeter, and the weight of an object on the surface of a
neutron star would be 10^(11) its weight on the surface of the Earth.
Neutron stars apparently have an outer shell of iron, but it is iron
like no Earth iron, an iron of 4 orders of magnitude greater density.

*gamma rays: Gamma rays are radiation of high energy, from about 10^(5)
electronvolts to more than 10^(14) electronvolts -- radiation with the
shortest wavelengths and highest frequencies, the gamma ray region of
the electromagnetic spectrum merging into the adjacent lower energy
x-ray region.

*electronvolts: (eV) A unit of energy defined as the energy acquired by
an electron in falling through a potential difference of 1 volt. 1
electronvolt = 1.602 x 10^(-19) joule.
-------------------
Summary & Notes by SCIENCE-WEEK <http://scienceweek.com> 30Oct98
-------------------
Related Background:

ANALYSIS OF A GAMMA RAY BURST FROM A HIGH REDSHIFT GALAXY

Gamma rays are radiation of high energy, from about 10^(5)
electronvolts to more than 10^(14) electronvolts -- radiation with the
shortest wavelengths and highest frequencies, the gamma ray region of
the electromagnetic spectrum merging into the adjacent lower energy
x-ray region. Gamma ray bursts are intense flashes of gamma rays
detected at energies up to 10^(6) electron volts. Knowledge of the
properties of gamma-ray bursts has increased substantially following
recent detections of counterparts at x-ray, optical, and radio
wavelengths. But the nature of the underlying physical mechanism that
powers these sources remains unclear. An important question is the
total energy in the burst, for which an satisfactory estimate of the
distance is required, and until now the best estimate is that the
bursts lie at cosmological distances. ... ... Kulkarni et al (16
authors at 9 installations, US IN IT) now report identification of the
host galaxy of a previously optically detected burst (GRB971214), with
a determination of the galaxy redshift at z = 3.42. When combined with
the measured flux of gamma-rays from the burst, this large redshift
implies an energy of 3 x 10^(53) ergs in the gamma-rays alone, assuming
the emission is isotropic. This is much larger than the energies
previously considered, and the authors suggest it poses a challenge for
theoretical models of the bursts.
QY: S.R. Kulkarni (srk@surya.caltech.edu)
(Nature 7 May 98 393:35) (Science-Week 29 May 98)
-------------------
Related Background:

GAMMA RAY BURST FIREBALL MODEL MAY NEED REVISION

The current consensus is that gamma ray bursts are produced by the
merger of two neutron stars, and up to this point, the bursts that have
been noted apparently originate outside our own galaxy. Castro- Tirado
et al (27 authors at 15 installations, ES DE SE DK IT UK US) report an
optical transient from a gamma ray burst (GRB 970508) imaged 4 hours
after the event, displaying a strong ultraviolet excess and reaching
maximum brightness 2 days later. The optical spectra did not show any
emission lines, and no variations on time scales of minutes were
observed for 1 hour during the decline phase. The authors suggest the
observations are incompatible with the fireball and afterglow models of
gamma ray bursts, and that another physical mechanism may be
responsible for the constant phase seen the first few hours after the
burst occurs. QY: T. Broadhurst, Univ. of Calif. Berkeley, Dept.
Astronomy 510-643-8520 (Science 13 Feb 98)
-------------------
Related Background:

OPTICAL STUDIES OF A GAMMA-RAY BURST SUGGEST FIREBALL MODEL

Studies of the mysterious gamma-ray bursts seen in every part of the
sky daily continue to be reported. This week we have the results of
observations of gamma ray burst (GRB) GRB970508, which occurred on May
8, 1997 (hence the name). Optical studies of the source of the burst by
M. R. Metzger et al (California Institute of Technology, US; National
Radio Astronomy Observatory, US; Institute of Space Astrophysics,
Frascati IT; University of Ferrara, IT) using data from the recently
orbited Italian-Dutch satellite BeppoSAX indicate the source of the GRB
is extra-galactic at a distance of 5 billion parsecs (1 parsec = about
20 trillion miles). Taking into account the recorded energy and its
loss by intervening absorption across that distance, we are considering
an initial energy burst with a magnitude equal to the total radiation
from our Sun during the entire age of the universe. The computed energy
figure is 10^(51) ergs of gamma-rays. A consensus among astrophysicists
is forming that these GRBs involve "relativistic fireballs" produced by
colliding neutron stars, either two neutron stars colliding with each
other, or single neutron stars colliding with black holes. The various
radiant energy data are coming in so rapidly now, there is a feeling
the physical nature of GRBs will soon be completely understood.
(Nature 26 Jun 97)
-------------------
Related Background:

GAMMA RAY BURSTS AND POSSIBLE TERRESTRIAL DISASTER

Neutron stars are one of the possible end-products of stellar
evolution. If, following its terminal stages, the remnant mass of a
star is between 1.4 and 2 to 3 solar masses, the star will collapse
into a neutron star, a body with a radius of 10 to 15 kilometers, with
a core so dense that its component protons and electrons have merged
into neutrons. The average density of a neutron star is 10^(15) grams
per cubic centimeter, and the weight of an object on the surface of a
neutron star would be 10^(11) its weight on the surface of the Earth.
Neutron stars apparently have an outer shell of iron, but it is iron
like no Earth iron, an iron of 4 orders of magnitude greater density.
Theory predicts that a neutron star should rotate very rapidly,
be extremely hot, and have an intense magnetic field. Leonard and
Bonnell (US National Aeronautics and Space Administration, US), in a
review of gamma ray bursts and the important data on these phenomena
collected during 1997, point out the following: 1) The current
consensus is that gamma ray bursts are produced by the merger of two
neutron stars; 2) up to this point, the bursts that have been noted
apparently originate outside our own galaxy; 3) considering the known
neutron stars inside our own galaxy, a case can be made that
evolutionary disjunctions in Earth's past may have been caused not only
by asteroid impacts, but also by gamma- ray bursts from merging neutron
stars a few thousand light years distant in our galaxy.
QY: Peter J.T.
Leonard, NASA Goddard Space Flight Center,
Greenbelt, MD US (Sky & Telescope February 1998)
-------------------
Related Background:

EVIDENCE FOR DISTANT SOURCE OF GAMMA RAY BURSTS

Gamma Ray Bursts have been much in the news recently. They were first
accidentally discovered some 30 years ago by military satellites, and
then interest was rekindled when the Compton Gamma Ray Observatory was
launched by NASA in 1991. The Compton orbiting device has been
detecting Gamma Ray Bursts in all parts of the sky on a daily basis.
One controversy among astronomers is whether the source is within our
galaxy or extra-galactic. Now Mark R. Metzger, leader of a team at the
California Institute of Technology (Pasadena CA US), reports that the
bursts detected here are coming to us through a stellar gas cloud about
7 billion light-years away, which means the source of the bursts must
be at least that far, certainly extra-galactic, and travelling to us
for about half the age of the universe. At least one part of a 30 year
old puzzle has apparently been solved -- the source of the bursts.
(UPI 15 May 97)
---------------------
DETECTION OF X-RAY AFTERGLOW ASSOCIATED WITH GAMMA-RAY SOURCE

Gamma ray bursts (GRBs) continue to tantalize astrophysicists. The
distribution of these bursts is isotropic across the sky, but
inhomogeneous in space, and with a deficit of faint bursts. The problem
is that present gamma ray telescopes have poor imaging capabilities,
and the phenomenology is unusual in that the bursts last only from a
fraction of a second to hundreds of seconds. At present, it is not
clear whether the bursts are produced in our own galaxy or at
cosmological distances. This week Costa et al (a team of 26 researchers
in IT and NL) report an analysis of a GRB of 28 February 1997
(GRB970228). The major discovery is that of an associated x-ray
afterglow which fades within a few days according to a power-law decay
function (empirical). The authors suggest that for the first time since
the discovery of GRBs, it will now be possible to correlate gamma-ray,
x-ray, optical, and radioastronomy observations. (Nature 19 Jun 97)
---------------------
AN HISTORIC MEETING DEVOTED TO GAMMA RAY BURSTS

Gamma ray bursts (GRBs) have been much in the news the past 10 months,
principally because of correlative data from x-ray, optical, and radio
instruments. Last month saw the Fourth Huntsville Symposium on Gamma
Ray Bursts (15-20 Sep 1997, Huntsville AL US), and the meeting is being
called "historic". There is apparently now a consensus that GRBs are
cosmological rather than galactic in origin, in other words from
outside our Milky Way galaxy. So that part of the 30-year puzzle
concerning GRBs is evidently solved. The other part of the puzzle
concerns the physical events producing the bursts, and for that part of
the puzzle there is apparently no consensus yet. It has recently been
proposed that GRBs are associated with the cataclysmic end of massive
stars, and if this is true, it is believed the appearance of GRBs
should provide data concerning the rate of formation of such stars, a
critical parameter that has evidently been established by observation.
In any case, the gamma ray burst field has apparently now shifted to
data analysis at new wavelengths of the electromagnetic spectrum, with
the emphasis now on x-ray, optical, and radio observations from several
instrument sources, including the valuable BeppoSAX satellite, the
Hubble Space Telescope, and the Burst and Transient Source Experiment
(BATSE) aboard the Compton Gamma Ray Observatory.
QY: Bohdan Paczynski
<bp@astro.princeton.edu>
(Nature 9 Oct 97)

[SW Bulletin 22 Nov 99]

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1. On Proteolytic Enzymes
2. Zoology: A Polarized Light Compass Organ in Spiders
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In Focus: On Prokaryotic and Eukaryotic Cells

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