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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