Astronomy Picture of the Day |
APOD: 2001 April 13 - GRB010222: Gamma Ray Burst, X Ray Afterglow
Explanation:
A
fading afterglow from one of the most powerful explosions
in the universe is centered in this
false
color image from the spacebased
Chandra X-ray Observatory.
The cosmic explosion, an enormously bright
gamma-ray
burst (GRB), originated in a galaxy billions
of light-years away and was detected by the
BeppoSAX
satellite on February 22.
GRB010222
was visible for only a few seconds at
gamma-ray energies, but its afterglow
was
followed for days by x-ray, optical, infrared and radio instruments.
These Chandra observations of the GRB's
x-ray glow hours after
the initial explosion suggest an expanding fireball of material
moving at near light speed has hit a wall of relatively dense gas.
While the true nature of gamma-ray bursters remains unknown,
the mounting evidence from
GRB afterglows does indicate that
the cosmic blasts may be hypernovae -- the
death explosions of very massive, short-lived stars
embedded in active star forming regions.
As the hypernova
blasts sweep up dense clouds of material in the
crowded star forming regions they may also trigger more
star formation.
APOD: 2000 October 12 - HETE-2 Rides Pegasus
Explanation:
The
Stargazer,
a modified Lockheed L-1011 aircraft, soared into
the skies above
Kwajalein Atoll in
the pacific on October 9th.
A small satellite observatory known as
the High Energy Transient Explorer - 2
(HETE-2) was tucked into
Stargazer's winged
Pegasus
rocket, slung beneath the large trimotor jet's
fuselage.
Dropped
from its mother ship, the Pegasus then successfully flew
HETE-2 into orbit.
HETE-2's mission
is to hunt gamma-ray bursts, brief, random
flashes of high energy photons from the distant cosmos.
Gamma-ray bursts are impressive, believed to be
the most powerful explosions in the Universe, but
so few have been well located and studied that the
nature of the
bursters themselves is still shrouded in mystery.
HETE-2's x-ray and gamma-ray instruments will be able to rapidly
alert ground-based observatories
to point toward ongoing, bright gamma-ray bursts.
Communications antennae and solar panels neatly folded,
HETE-2 is seen
here being carefully enclosed in the Pegasus nose fairing.
APOD: 2000 March 24 - A Mystery in Gamma Rays
Explanation:
Gamma
rays are the most energetic form of light, packing a million
or more times the energy of visible light photons.
What if you could
see gamma rays?
If you could, the familiar
skyscape of steady stars would
be replaced by some of the
most bizarre objects
known
to modern astrophysics -- and
some which are unknown.
When the EGRET instrument on the orbiting
Compton Gamma-ray Observatory surveyed
the sky in the 1990s, it cataloged 271
celestial sources of high-energy gamma-rays.
These sources are very different from the powerful
gamma-ray bursters
that flash and fade rapidly from view, and
researchers identified some
with exotic black holes,
neutron stars, and distant
flaring galaxies.
But 170 of the cataloged sources, shown
in the above all-sky map, remain unidentified.
Many sources in this gamma-ray mystery map likely
belong to the already known classes of gamma-ray emitters and are simply
obscured or too faint to be otherwise positively identified.
However, astronomers
recently called attention to the ribbon
of sources winding through the plane of the
galaxy, projected here along the middle of the map, which
may represent a large unknown class of galactic gamma-ray emitters.
In any event, the unidentified sources could remain a mystery
until the planned launch of the more sensitive
Gamma-ray Large
Area Space Telescope in 2005.
APOD: November 4, 1999 - Gamma Ray Bursting
Explanation:
Using graphics and data from NASA's
Compton Gamma Ray Observatory,
this animation illustrates one of the most
exciting mysteries of modern astrophysics,
gamma-ray bursts.
Incredibly
gamma-ray bursts, sudden flashes of radiation with over 100,000 times
the energy of visible light photons, occur several times a day.
They typically last from fractions of a second to many minutes
and appear from random directions,
unexpectedly triggering
space-based gamma-ray instruments.
At left a burst suddenly appears, flickers and fades in a
false-color gamma-ray all-sky map,
briefly overwhelming all other sources of
celestial gamma-rays.
The graph at right shows the corresponding response of
an orbiting gamma-ray detector as its
counting rate suddenly climbs and falls recording the passage
of the mysterious burst.
Originating far across the Universe,
gamma-ray bursts are now known to be the most
powerful explosions since the big bang
and may yet prove to be
useful tools for exploring
the distant cosmos.
Future space and
ground-based observatories
will also work to discover
the nature of the bursters
and the source of their
extreme energy.
APOD: May 27, 1998 - Magnetar
Explanation:
What do you call a neutron star with a super-strong magnetic field?
You guessed it ... a Magnetar.
Imagine a star with more mass than the sun,
the density of a neutron, and
a magnetic field about a thousand trillion (a one followed by
15 zeroes) times stronger than Earth's.
It sounds exotic and theoretical, but
strong evidence for the existence of magnetars has recently
been announced based
on data from orbiting
X-ray and
Gamma-ray observatories.
Neutron stars are formed in the violent crucibles of stellar explosions.
Some become pulsars
with relatively weak magnetic fields,
spinning and emitting pulses of
electromagnetic radiation as their rotation slows.
However, astronomers now believe that some become magnetars,
with magnetic fields so intense that the solid neutron star crust
buckles and shifts under its influence.
The resulting star quakes could repeatedly
generate brief flashes of hard X-rays and soft gamma-rays
giving rise to the rare but
mysterious "soft gamma repeaters"
(not to be confused with "
gamma-ray bursters"!).
This still frame from an
animation illustrating a spinning, flashing magnetar
emphasizes the looping magnetic field lines embedded in the X-ray hot
neutron star surface.
APOD: May 7, 1998 - A Powerful Gamma Ray Burst
Explanation:
Gamma-ray bursts are thought to be the most powerful explosions in
the Universe, yet the cause of these high-energy flashes
remains a mystery.
Blindingly
bright for
space-based gamma-ray detectors the burst sources are
so faint at visible wavelengths that
large telescopes and sensitive cameras are required to search for them.
The faint optical flash from
a relatively intense gamma-ray burst
detected on December 14th of last year seems to have originated in
the galaxy indicated in
this Hubble Space Telescope image - taken
months after the burst had faded from view.
Astronomers have recently announced that
this galaxy's spectrum, recorded using the large
Keck telescope atop Hawaii's Mauna Kea,
indicates that it lies at a distance of about 12 billion light-years.
The energy required to produce the observed flash of gamma-rays from
this distance would be staggering!
Some estimates suggest that in a few seconds the
burster released the equivalent energy of several hundred
supernovae (exploding stars).
The eruption of such a large amount of energy in such a short time is
so extreme that even
exotic theoretical models
of the bursters are being challenged.
Could the bursts be caused by
the cataclysmic merger of
neutron stars
with black holes ... or something as yet unknown?
APOD: December 18, 1997 - Gamma-ray Burster
Explanation:
Gamma-ray bursts
seem to be the most powerful explosions in the Universe.
Yet their sources continue to elude
researchers who stand in awe and
frustration at the bursts' transient, enigmatic behavior.
The blinking gif above illustrates
the latest hard-won result in the
quest to identify and understand
the nature of the bursters.
These Apache Point Observatory optical images from Monday and Tuesday
this week have helped identify a faint, fading object (red arrow) near
the position of a gamma-ray burst.
The gamma-ray burst triggered
satellite observatories on Sunday, December 14th.
Faint stars near the constellation Ursa Major (the Big Dipper) also appear
in these "negative" images of the sky.
Though thousands of bursts have been detected by satellites sensitive
to gamma rays, it is likely that this object represents only the third
known optical counterpart to a
gamma-ray burst.
APOD: March 19, 1997 - Gamma-Ray Burster
Explanation:
What and where are the Gamma-Ray Bursters?
Since their
discovery
in the early 1970s, nobody has been able to
explain the cause of
mysterious flashes of
gamma rays
that come from seemingly random directions on the sky.
Worse yet, it is even unclear whether these
high energy explosions originate
in our own Galaxy or in distant galaxies across the Universe.
Until late last month, these bursters were known only by their
gamma-ray flashes - no counterpart had been seen at any other wavelength.
But on February 28, an Italian/Dutch satellite known as
BeppoSAX
detected what may well be
X-rays from a burster,
eight hours after the
gamma-ray flash.
The discovery image is shown above. Still hours later, using
the position provided by this X-ray image,
ground-based telescopes recovered an even better located
variable optical source which also seems to be related to the burster.
Dramatically, this optical transient has faded now.
In its place lies a steady source that appears to be
a dim, distant galaxy.
Did this Gamma-Ray Burst originate in the distant galaxy?
If so, it answers one facet of one of modern
astronomy's greatest controversies.
If not, this would not be the first
fortuitous coincidence to mislead astronomers.
Future satellite and ground-based observations will tell.
APOD: January 2, 1996 - The X-Ray Sky
Explanation:
What if you could see
X-rays?
If you could, the
night sky would be a strange and unfamiliar place.
X-rays are about 1,000 times more energetic
than visible light photons and are produced in violent and high
temperature astrophysical environments. Instead of the familiar
steady stars, the sky would seem to be filled with
exotic binary star systems
composed of white dwarfs,
neutron stars, and
black holes, along with
flare stars, X-ray bursters,
pulsars,
supernova remnants and
active galaxies.
This X-ray image of the entire sky was constructed with
Skyview,
using data from the first
High Energy Astronomy Observatory (HEAO 1),
and plotted in a coordinate system centered on the galactic center
with the north galactic pole at the top.
Sources near the galactic center are seen to dominate in this
false color map which shows regions of highest X-ray intensity in yellow.
Astronomers' ability to observe the sky at X-ray energies
will be greatly enhanced by the recently launched
X-ray Timing Explorer (XTE) satellite.