Topic: Late January Astronomy Bulletin

[Clive]

TERRESTRIAL GAMMA-RAY FLASHES COMMONER THAN THEY WERE THOUGHT TO BE
NASA

New research merging Fermi data with information from ground-based
radar and lightning networks shows that terrestrial gamma-ray flashes
arise from an unexpected diversity of storms and may be more common
than currently thought. The outbursts, called terrestrial gamma-ray
flashes (TGFs), were discovered in 1992 by the Compton Gamma-Ray
Observatory, which operated until 2000. TGFs occur unpredictably and
fleetingly, with durations less than a millisecond, and remain poorly
understood. In late 2012, Fermi scientists employed new techniques
that effectively upgraded the satellite's Gamma-ray Burst Monitor
(GBM), making it 10 times more sensitive to TGFs and allowing it to
record weak events that were overlooked before. As a result of the
enhanced discovery rate, they were able to show that most TGFs also
generate strong bursts of radio waves like those produced by
lightning. Previously, TGF positions could be roughly estimated from
Fermi's location at the time of the event. The GBM can detect flashes
within about 800 km, but that is not precise enough to associate a TGF
definitively with a specific storm.

Ground-based lightning networks use radio data to pin down strike
locations. The discovery of similar signals from TGFs meant that
scientists could use the networks to determine which storms produce
gamma-ray flashes, opening the door to an understanding of the
meteorology powering such extreme events. Scientists sifted through
2,279 TGFs detected by Fermi's GBM to derive a sample of nearly 900
events accurately located by the Total Lightning Network operated by
Earth Networks in Germantown, Maryland, and the World-Wide Lightning
Location Network, a research collaboration run by the University of
Washington in Seattle. Those systems can pinpoint the location of
lightning discharges -- and the corresponding signals from TGFs -- to
within 10 km anywhere on the globe. Scientists suspect that TGFs
arise from strong electric fields near the tops of thunderstorms.
Updraughts and downdraughts within the storms cause rain, snow and ice
particles to collide and acquire electrical charge. Usually, positive
charge accumulates in the upper part of the storm and negative charge
accumulates below. When the storm's electrical field becomes strong
enough to break down the insulating properties of air, a lightning
discharge occurs. Under the right conditions, the upper part of an
intra-cloud lightning bolt disrupts the storm's electric field in such
a way that an avalanche of electrons surges upward at high speed.
When the fast-moving electrons are deflected by air molecules, they
emit gamma rays and create a TGF. About 75% of lightning stays within
the storm, and about 2,000 intracloud discharges occur for each TGF
that Fermi detects. On the basis of current Fermi statistics,
scientists estimate that some 1,100 TGFs occur each day, but the
number may be much higher if low-altitude flashes are being missed.


SPACECRAFT APPROACHES CERES
NASA

The Dawn spacecraft has entered the approach phase in which it will
continue to close in on Ceres, the first-discovered and largest
asteroid (now 'dwarf planet'), which has not previously been visited
by a spacecraft. Dawn was launched in 2007 and is scheduled to enter
Ceres orbit in March. The next couple of months promise to give us
continually improving views of Ceres, prior to Dawn's arrival. By the
end of January, the spacecraft's images and other data should be the
best ever taken of the object. Dawn is currently approaching Ceres at
around 725 km/h. The spacecraft's arrival will mark the first time
that a single spacecraft has successively orbited two solar-system
objects. Dawn previously explored Vesta for 14 months in 2011/12,
capturing detailed images and data about that body.

The two planetary bodies are thought to differ in important ways.
Ceres may have formed later than Vesta, and with a cooler interior.
Current evidence suggests that Vesta retained only a small amount of
water because it formed earlier, when radioactive material was more
abundant and would have produced more heat. Ceres, in contrast, has a
thick ice mantle and may even have an ocean beneath its icy crust.
Ceres' average diameter is 950 km, Vesta's is 525 km, and Vesta is
also the second-most-massive body in the asterod belt. The spacecraft
uses ion propulsion to traverse space far more efficiently, though
with much smaller acceleration, than if it used chemical propulsion.
In an ion-propulsion engine, an electrical charge is applied to xenon
gas, and charged metal grids accelerate the xenon atoms out of the
thruster. The corresponding reaction force on the accelerating grids
propels the spacecraft. Dawn has now completed five years of
accumulated thrust time, far more than any other spacecraft.


NASA TO REPAIR MARS ROVER
BBC News

The rover Opportunity, which has been on Mars for more than 10 years,
is suffering from memory problems. The six-wheeled vehicle -- not to
be confused with Curiosity, which was launched in 2011 -- keeps
resetting unexpectedly. The problem seems to be that its non-volatile
memory is suffering from a fault, probably related to the hardware's
age. The effect is that when the rover tries to save telemetry data
to the flash memory it fails, and so it then writes them to the
volatile memory instead. When the rover powers down, the information
is then wiped. The problems are becoming more severe, with the memory
issue causing the rover to reset itself, and in some cases stop
communicating with mission control altogether. The Opportunity team
believes that it has found a way to hack the rover's software to
disregard the faulty part. The process will take a couple of weeks,
however, and Opportunity is ageing and could be nearing the end of its
useful life. Even if the rover fails now, it will have greatly
exceeded the initial goal of spending three months on Mars.
Opportunity has covered 41.8 km on the surface and sent back
interesting information about the planet's biological make-up.


VANISHING NEUTRON STAR MEASURES SPACE-TIME WARP
Faculty of Science, University of British Columbia

A team of astronomers has measured the masses of both stars in the
binary pulsar system J1906, which is about 25,000 light-years away.
The pulsar emits a beam of radio waves and spins every 144
milliseconds. It orbits its companion star in a little under four
hours. By tracking the motion of the pulsar, the team was able to
measure the gravitational interaction between the two very compact
stars. Each of the stars is more massive than the Sun, but they are
over 100 times closer together than the Earth is to the Sun. The
resulting extreme gravity causes many remarkable effects. According
to general relativity, a neutron star wobbles like a spinning top as
it moves through the gravitational well of a massive, nearby companion
star. Orbit after orbit, the pulsar travels through a space-time that
is curved, which affects the star's spin axis. Through the effects of
the immense mutual gravitational pull, the spin axis of the pulsar has
now wobbled so much that the beams no longer hit the Earth, and the
pulsar is now all-but invisible to even the largest telescopes. This
is the first time that such a young pulsar has 'disappeared' through
precession. It is expected to wobble back into view in due course,
but it might take as long as 160 years. The masses of only a few
double neutron stars have ever been measured, with J1906 being the
youngest.


MILKY WAY CORE DRIVES 3-MILLION-KM/H WIND
Space Telescope Science Institute (STScI)

At a time when our earliest human ancestors had recently mastered
walking upright, the heart of our Milky Way Galaxy underwent a titanic
eruption, driving gases and other material outward at 2 million mph.
Now, at least 2 million years later, astronomers are witnessing the
aftermath of the explosion: billowing lobes of gas towering about
30,000 light-years above and below the plane of our Galaxy. The
enormous structure was discovered five years ago as a gamma-ray glow
in the sky in the direction of the Galactic Centre. The balloon-like
features have since been observed in X-rays and radio waves. Now
astronomers have used the Hubble telescope to measure the velocity and
composition of the lobes. They now seek to calculate the mass of the
material being blown out of our Galaxy, which could lead them to
determine the outburst's cause from several competing possibilities.

Astronomers have proposed two possible origins for the bipolar lobes:
a firestorm of star birth at the Milky Way's centre or an eruption of
its super-massive black hole. Although astronomers have seen gaseous
winds, composed of streams of charged particles, emanating from the
cores of other galaxies, they are getting a unique, close-up view of
our Galaxy's own fireworks. When we look at the centres of other
galaxies, the outflows appear much smaller because the galaxies are
farther away, but the outflowing clouds astronomers are seeing in our
Galaxy are 'only' 25,000 light-years away. We can study the details
of those structures and look at how big the bubbles are and can
measure how much of the sky they are covering. The lobes, dubbed
Fermi Bubbles, were initially observed with the Fermi Gamma-ray space
telescope. The detection of high-energy gamma-rays suggested that a
violent event in the Galaxy's core aggressively launched energized gas
into space. To provide more information about the outflows, Hubble's
Cosmic Origins Spectrograph (COS) was used to observe the ultraviolet
light from a distant quasar that lies behind the base of the northern
bubble. Imprinted on that light as it travels through the lobe is
information about the velocity, composition, and temperature of the
expanding gas inside the bubble, which only COS can provide.
Scientists found that the gas on the near side of the bubble is moving
toward us and the gas on the far side is travelling away. COS spectra
show that the gas is rushing from the Galactic Centre at roughly 3
million km/h. That is exactly the signature of a bipolar outflow and
is the closest sightline we have to the Galaxy's centre where we can
see the bubble being blown outward and energized. The COS observa-
tions also measure, for the first time, the composition of the
material being swept up in the gaseous cloud. COS detected silicon,
carbon, and aluminium, indicating that the gas is enriched in the
heavy elements produced inside stars and represents the fossil
remnants of star formation.

COS measured the temperature of the gas at approximately 10,000 C,
much cooler than most of the super-hot gas in the outflow, thought to
be at about 10 million C. This is the first result in a survey of 20
distant quasars whose light passes through gas inside or just outside
the Fermi Bubbles. An analysis of the full sample should yield the
amount of mass being ejected. The astronomers can then compare the
outflow mass with the velocities at various locations in the bubbles
to determine the amount of energy needed to drive the outburst and
possibly the origin of the explosive event. One possible cause for
the outflows is a star-making frenzy near the Galactic Centre that
produces supernovae, which blow out gas. Another possibility is a
star or a group of stars falling onto the Milky Way's super-massive
black hole. When that happens, gas superheated by the black hole
blasts deep into space. Because the bubbles are short-lived compared
to the age of our Galaxy, it suggests that that may be a repeating
phenomenon in the Milky Way's history. Whatever the trigger is, it
probably occurs episodically, perhaps only when the black hole gobbles
up a concentration of material. It looks as if the outflows are a
hiccup, and there may have been repeated ejections of material that
have blown up, and we are catching the latest one. By studying the
light from the other quasars in the program, we may be able to detect
the fossils of previous outflows. Galactic winds are common in
star-forming galaxies, such as M82, which is furiously making stars in
its core. It looks as if there may be a link between the amount of
star formation and whether or not such outflows happen. Although the
Milky Way overall currently produces a moderate one to two stars a
year, there is a high concentration of star formation close to its
core.


RECORD-BREAKING BLACK-HOLE OUTBURST DETECTED
Amherst College

Last September, a team of scientists used the Chandra X-ray
Observatory to observe and record the largest-ever flare in X-rays
from the super-massive black hole at the centre of the Milky Way. The
astronomical event puts the scientific community one step closer to
understanding the nature and behaviour of super-massive black holes.
Super-massive black holes are the largest of black holes, and all
large galaxies are believed to have one. The one at the centre of our
Galaxy, the Milky Way, is called Sagittarius A* (Sgr A*), and
scientists estimate that it contains about four and a half million
times the mass of our Sun. Scientists working with Chandra have
observed Sgr A* repeatedly since the telescope was launched into space
in 1999. The team was originally using Chandra to see if Sgr A* would
consume parts of a cloud of gas, known as G2. Last September the team
detected an X-ray outburst that was 400 times brighter than the usual
X-ray output from Sgr A*; it was nearly three times brighter than the
previous record-holder, seen in early 2012. A second enormous X-ray
flare, 200 times brighter than Sgr A* in its quiet state, was observed
with Chandra on 2014 October 20.

Astronomers have two main ideas about what could be causing Sgr A* to
erupt in that extreme way. One hypothesis is that the gravity of the
super-massive black hole has torn apart a couple of asteroids that
wandered too close. The debris from such a 'tidal disruption' would
become very hot and produce X-rays before disappearing for ever across
the black hole's point of no return (the 'event horizon'). If an
asteroid were torn apart, it would go around the black hole for a
couple of hours -- like water circling an open drain -- before falling
in. That's just how long the brightest X-ray flare lasted, so that is
an intriguing clue to be considered. Another, different, idea is that
the magnetic field lines within the material flowing towards Sgr A*
are packed incredibly tightly. If that were the case, the field lines
would occasionally reconnect and reconfigure themselves. When that
happens, magnetic energy is converted into the energy of motion, heat
and the acceleration of particles -- which could produce a bright
X-ray flare. Such magnetic flares are seen on the Sun, and the Sgr A*
flares have a similar pattern of brightness levels to the solar
events. At the moment, it is impossible to distinguish between those
two very different ideas. In addition to seeing the giant flares, the
team also collected more data on a magnetar -- a neutron star with a
strong magnetic field -- located close to Sgr A*. The magnetar is
undergoing a long X-ray outburst, and the Chandra data are allowing
astronomers to understand a bit about the unusual object. As for the
G2 gas cloud, astronomers estimate that it made its closest approach
-- still about 15 billion miles away from the edge of the hole -- in
the spring of 2014. The researchers estimate the record-breaking
X-ray flares were produced about a hundred times closer to the black
hole, making it very unlikely that the Chandra flares were associated
with G2.


PHILAE COMET LANDER ELUDES DISCOVERY
BBC News

The most recent imaging search by the over-flying Rosetta 'mother
ship' can find no trace of the probe. Philae touched down on Comet
67P/Churyumov-Gerasimenko on November12 , returning a swathe of data
before going silent when its battery ran flat. ESA scientists are now
waiting for Philae itself to reveal its position when, as the comet on
which it is sitting gets nearer the Sun, it gets enough power to start
transmitting again. Researchers have a pretty good idea of where the
robot should be, but pinpointing its exact location is difficult. On
touchdown, Philae bounced twice before coming to rest in a dark ditch.
That much is clear from the pictures it took of its surroundings. Its
location, the mission team believes, is just off the top of the 'head'
of the duck-shaped comet. The orbiting Rosetta satellite photographed
that general location on December 12, 13 and 14, with each image then
scanned by eye for any bright pixels that might be Philae. But no
positive detection was made. Rosetta has now moved further from 67P,
raising its altitude from 20 to 30 km, and there is no immediate plan
to go back down.

Even if they cannot locate it, scientists are confident that the
little probe will eventually make its whereabouts known. As 67P moves
closer to the Sun, lighting conditions for the robot should improve,
allowing its solar cells to recharge the battery system. The latest
assessment suggests communications could be re-established in May or
June, with Philae distributing enough electricity to its instruments
to resume operations around September. That would be at perihelion --
the time when the comet is closest to the Sun (185 million km away)
and at its most active. Scientists continue to pore over the data
Philae managed to send back before going into hibernation. Highlights
include a clearer idea of the nature of the comet's surface.
Researchers say that the surface appears to be covered in many places
by a soft, dusty 'soil' about 15-20cm deep. Underneath is a very hard
layer, which is thought to be mainly sintered ice. High-resolution
pictures from Rosetta's Osiris camera system reveal an array of
rounded features that the Osiris team has nicknamed 'dinosaur eggs'.
The features have a preferred scale of about 2-3m and may be evidence
of the original icy blocks that came together 4.5 billion years ago to
build the comet. Early interpretations of the general surface of the
comet indicate that many structures are probably the result of
collapse over internal voids. Although 67P is a small body just 4 km
across, its gravity is nevertheless strong enough to shape depressions
and arrange fallen boulders. A good example is in 'Hapi' valley --
the gorge that forms the 'neck' of the comet. It contains a string of
large blocks at its base, which one Osiris team-member argued were
very likely to have fallen from the nearby vertical cliff dubbed
'Hathor'. All the surface features on 67P have been given names that
follow an ancient Egyptian theme. Hapi was revered as a god of the
Nile; Hathor was a deity associated with the sky.


LEAP SECOND ON JUNE 30
CNN

Atomic clocks keep more accurate time than the Earth's rotation, which
can speed up and slow down because of tides, accumulation or melting
of ice at different latitudes, and changes within its core. To get
clocks back into synchronization with the Earth, an extra second is
periodically added to Coordinated Universal Time (UTC), the world's
benchmark time standard. Scientists at the International Earth
Rotation and Reference Systems Service have announced that a leap
second will be required on June 30th. The previous one was in 2012,
but that didn't go entirely smoothly. Some of the software platforms
that underpin a lot of web sites did not know how to cope with the
extra second. The issue has become an international sticking point.
Some countries, including the United States, want to get rid of leap
seconds altogether, saying that they are too disruptive to precise
systems used for navigation, communication and other services. But
others, like Britain, have argued that it is risky to allow a
divergence to develop between the time kept by atomic clocks and that
of the Earth's rotation. An international radio-communication
conference in 2012 put off a decision on the matter until this year.
In any case, fewer leap seconds are being added nowadays than a few
decades ago. Between 1972 and 2000, a total of 22 leap seconds was
invoked, but since the millennium we have only needed four. Relative
to the 1970s the Earth has speeded up a little bit.