It's summer and the Jodcasters are widely dispersed. Despite the lack of a studio, Jen and Dave managed to meet up at Milan's central station to record an episode.
In the news this month:
- Neutron stars are created when massive stars explode as supernovae at the end of their lives. The violent explosion compresses the core of the star resulting in a very dense object spinning very fast. While these neutron stars have strong magnetic fields hundreds of thousands of times stronger than any man-made magnet, there is a subclass of stars with magnetic fields hundreds of times stronger still. Known as magnetars, only fourteen of these objects have been discovered so far. But on the 22nd of August 2008, X-rays reached Earth from a massive outburst of an unknown object. The burst of high energy photons triggered an automatic sensor on board the Swift satellite and, within hours, the XMM-Newton satellite was also monitoring the event. The outburst lasted for four months and hundreds of smaller bursts were recorded over this period. The astronomers studying the satellite data soon realised that what they were seeing was probably due to the outburst of a previously undiscovered magnetar. It is thought that an outburst like this occurs when the unstable magnetic field pulls the crust of the magnetar, allowing matter from inside the star to escape in an exotic volcanic eruption. This material can interact with the magnetic field, changing its configuration and releasing yet more energy. This particular object, known as SGR 0501+4516, is located roughly 15,000 light years away in the constellation of Auriga, and is the first new magnetar candidate in a decade. Although all known magnetar candidates are located a long way from Earth, their flares are so energetic that they can supply as much energy to our detectors as is recorded by solar flares from our own Sun. The magnetic fields on a magnetar are so strong that if one were as close as half the distance to the Moon, it would wipe the details from every credit card on Earth. While the exact mechanism that forms a magnetar is not yet certain, the analysis of SGR 0501+4516, published in the Monthly Notices of the Royal Astronomical Society during June, does provide strong evidence that the two classes of magnetar (known as Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters) are actually different phenomena from one type of object, rather than two physically distinct classes of magnetar.
- The extreme environment surrounding supermassive black holes in active galaxies is difficult to see directly due to both the large distances of these galaxies from the Earth, and the high levels of obscuration by the surrounding accretion disk and dusty torus. One method of probing this region is trough observing the shape and time variability of spectral lines emitted by the gas in the region around the black hole. Spectral lines are caused by the emission of photons at specific wavelengths, each atom and molecule emit lines at different sets of wavelengths resulting in a sort of chemical fingerprint which can be used as a diagnostic tool when studying the interstellar medium as well as the atmospheres of stars and planets. In the case of active galaxies, there is a large amount of high energy X-ray radiation which can be studied by satellites such as the European Space Agency's XMM-Newton. This X-ray continuum can knock electrons out of atoms from orbits close to the atomic nucleus, if the incoming photon has sufficient energy. When this happens, an electron from a higher orbit can drop down to fill the hole, emitting an extra photon as it does so. These emitted photons have specific energies corresponding directly to the difference between the energy states of the two orbits. One such X-ray emission line, known as the iron-K line, has been detected in the central regions of several active galaxies over the last fifteen years, but a paper published in Nature last month describes how, for the first time, both the iron-K and iron-L lines have been detected. The observations of both of these spectral features in one galaxy has allowed the researchers to investigate in detail the region immediately around the supermassive black hole at the centre of the galaxy known as 1H 0707-495. Led by Andrew Fabian at the University of Cambridge, the team have studied the shape of the lines which are distorted in characteristic ways by the speed of the orbiting iron atoms, the energy needed for the X-rays to escape the black hole's gravitational field, and the spin of the black hole itself. The fact that both the K and L iron lines are present suggests that the region around the black hole is much richer in iron than the rest of the galaxy. Modelling shows that iron is nine times more abundant than in the Sun. The team also measured how the brightness of the L line varied with time. Careful analysis of the data showed a time lag of 30 seconds between changes in the X-rays seen directly, and those seen in reflection from the disk. The delay of this echo lets the size of the reflecting region to be determined and leads to an estimate of 3 to 5 million solar masses for the mass of the black hole. The observations also showed that the black hole is spinning very rapidly and swallowing the equivalent of two Earths an hour - close to its theoretical limit.
- Images from the Cassini spacecraft showed spectacular plumes of gas erupting into space from fractures in the surface of Enceladus, one of Saturn's many moons. A popular theory explaining these plumes is that they come from a sub-surface ocean kept liquid beneath the frozen icy surface by tidal heating generated by the moon's eccentric orbit around Saturn. Two papers published in the journal Nature on the 25th June use very different techniques to investigate the chemistry of the plumes to try and pin down details of the model. Using the Keck telescope in Hawaii and the Anglo-Australian Telescope in Australia, a team led by Nick Schneider at the University of Boulder in Colorado made sensitive observations looking for spectral signatures of sodium in both the plumes and Saturn's E-ring, thought to contain significant material from the plumes. A sub-surface ocean in contact with the moon's rocky core would be expected to contain significant amounts of sodium in the form of salt - as is the case with Earth's ocean. So, if these plumes are caused by geysers from water just below the surface, then the sodium signal should have been easily detected. Despite more than 13 hours of observing, the team saw no significant spectral lines due to sodium, apparently ruling out an ocean immediately below the surface. Another team, led by Frank Postberg at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, used data from the Cosmic Dust Analyser experiment on board Cassini. They analysed the mass spectra of thousands of E-ring dust particles and found a small but significant population of grains rich in sodium salts. At first, these two results seem contradictory. Where do the sodium-rich grains come from if the plumes themselves contain no detectable trace of sodium? The suggestion is that the plumes actually come not from a sub-surface reservoir, but from an ocean much deeper beneath the surface. The proposed explanation is that a deep salty ocean exists many kilometres below the surface. Evaporation from this ocean releases pure water as jets of steam into the E-ring, leaving a salty residue behind. The sodium-rich grains found in the E-ring could be carried to the surface by gas bubbles in these jets - creating salty droplets which freeze once they reach the surface, dragged along with the flow and out into the E-ring. In this model, the amount of sodium carried to the surface in these bubbles is too small to be detected by ground-based spectroscopy, so is consistent with the results reported by Scheider's team. Further observations are planned, as Cassini is due for four further fly-bys of Enceladus before mid-2010.
- The Herschel satellite, launched last month from French Guiana, has taken its first images. On the 14th June, one month after launch, the command to open the cryostat lid was sent to the spacecraft. Although the telescope has not yet reached its final orbit at L2 and is still undergoing testing and commissioning, the team operating the satellite attempted a sneak preview test image on the same day the cryostat lid was opened. From a list of potential targets for this first imaging test, the Whirlpool galaxy, M51, was chosen partly because of the comparable data available from other infra-red instruments such as the MIPS camera on Spitzer. Despite being at a very early stage in commissioning, the resulting images are spectacular, showing that the telescope optics are performing extremely well so far. Further commissioning is underway before full science operations begin later in the year.
Prof Vik Dhillon (University of Sheffield) talks to us about high-speed astronomy.
NASA launched two robotic missions to the Moon on the 18th June as part of their Lunar Precursor Robotic Program. The Lunar Reconnaissance Orbiter (LRO) is now in orbit around the Moon, surveying the surface for possible landing sites for future manned missions. The Lunar CRater Observation and Sensing Satellite (LCROSS) is sending one of its two components to crash into the lunar surface and the other part in orbit will analyse the ejecta in the hope of finding water.
Buzz Aldrin (aka Doc Rendezvous!) has released a rap song called Rocket Experience with the help of Snoop Dogg and Talib Kweli! The video is on funnyordie.com and the song is available to buy on iTunes - some of the profits from the sales will go to Buzz Aldrin's ShareSpace Foundation.
The Night Sky
Ian Morison tells us what we can see in the night sky during July 2009.
As the Sun is setting, Leo is low in the west with the planet Saturn. A little higher, towards the south west, is the rather sparse constellation of Virgo with a bright star Spica. High up in the south is the constellation Bootes with one very nice bright star - Arcturus. Just to the left of Arcturus is a rather lovely arc of stars called Corona Borealis - the Northern Crown. To the south, and higher up, is the constellation Hercules. The four brighest stars make up the key-stone and two-thirds of the up the right-hand side of the keystone, binoculars will show a slightly fuzzy object. With a telescope you see the wonderful globular cluster M13. Below Hercules is Ophiuchus. Below Ophiuchus we have Scorpius and Sagittarius (containing the Teapot). From the northern latitudes of the UK we don't see these constellations well and a better view is seen from further south. Looking towards the south east, rising through the night, is the wonderful part of the sky containing Cygnus, Lyra and Aquila - the Summer Triangle. If you have binoculars, go from Altair up towards Altair about a third of the way. There you will see a dark region called the Cygnus Rift containing Brocchi's+Cluster - the Coathanger.
Jupiter is now in Capricornus, becomes more easily visible this month as its elevation in the pre-dawn sky is getting higher - about 23 degrees above the horizon by beginning of the month. One problem with observing Jupiter with a telescope when it is so low in the sky is refraction in the atmosphere. This shifts the different colours of light in Jupiters image by differing amounts, so giving a blurred image. Using a green filter will help considerably in giving a cleaner image.
Saturn is seen low below Leo - but somewhat below the feet of the Lion. You'll be able to see it in the first couple of weeks of July.
Mercury reached "Western Elongation" in the middle of June which is when it lies furthest in angle from the Sun and seen before sunrise. In the first few days of July you'll see it very low in the east just before dawn.
Mars is becoming more prominent and is at an elevation of 25 degrees due east as the Sun rises on 1st July. On a particularly good night, a small telescope might be able to make out some of the features but we really have to wait a few months to see it at its best.
Venus is now easily visible in the pre-dawn sky. It is about 20 degrees above the horizon as the Sun rises on the first of July, so will be easier to spot later in the month. It is at magnitude -4.1.
The highlights this month:
- On July 10th there is a very nice line up of Jupiter's Galilean satellites. At the same time, Jupiter is as close as it gets to Neptune in the sky. Neptune is much fainter - magnitude 7.8 - but between them will be a star Mu Capricornis. Given a transparent sky at a dark site, 8 x 40 binoculars should allow you to see Jupiter and Neptune together.
- In the early morning of July 13th, Venus is only 9 arcminutes from the star Epsilon Tauri.
- In the morning of July 18th, the Moon occults the star Merope in the Pleiades Cluster.
- July is also a very good time to spot Noctilucent Clouds!
Towards the north you'll see Leo setting in the north west. Above that is the constellation of Virgo. You then pass, going towards the east, the constellation Libra and then the lovely region of Scorpius and Sagittarius. Fairly low in the north is Arcturus and to its lower right Hercules. Towards the south is a wonderful view. Towards the south east is Sagittarius - the Teapot - and if you follow the line the water would take coming out of the teapot you will find a lovely star cluster M7. Just above the teapot is the Lagoon Nebula. Low in the south are the Small and Large Magellanic Clouds.
Odds and Ends
|Interview:||Prof Vik Dhillon and Stuart Lowe|
|Night sky this month:||Ian Morison|
|Presenters:||David Ault and Jen Gupta|
|Editors:||Stuart Lowe and Dandan Xu|
|Intro script:||David Ault|
|Segment voice:||Danny Wong-McSweeney|
|Cover art:||First light image from ESA's Herschel observatory Credit: ESA and the PACS Consortium|