March 2012: Flat-packed

In this LOFAR-themed episode, we talk to Dr Neal Jackson about the Low Frequency Array itself, Dr Tom Hassall tells us about using it to detect pulsars and Anna Kapinska discusses how it can be used to study active galactic nuclei. Megan rounds up the latest news and we find out what's in the March night sky from Ian Morison and John Field.

The News

In the news this month:

• Almost two hundred years after the massive star Eta Carina underwent a series of large eruptions, you might think that there would be little left to learn about what happened. But, like ships and submarines mapping the ocean floor using sonar, transmitting sound waves and listening for the echoes coming back from objects or rock features, astronomers can use reflections of light from gas clouds to learn something about an astronomical object, even when direct observations are impossible. This technique is particularly useful in studying explosions, like the nineteenth-century eruptions of Eta Carina which produced the spectacular Homunculus Nebula, but which happened before the invention of today's sensitive spectrometers; we can use the light from the echoes to help understand the nature of the progenitor object and the characteristics of the explosion.

  Eta Carina is a binary star and one of the most massive currently known in the Milky Way. In the mid-nineteenth century it underwent a series of large eruptions, throwing off huge amounts of material and becoming, for a short time, the second brightest star in the sky. Now, astronomers have used light echoes from the explosion to learn more about what happened, and the results have thrown up some interesting surprises. By taking a series of images of the Eta Carina region, spread out over several years, a team of astronomers were able to look for differences in the images as the light echo moved out from the explosion. Using the light from these echoes the team was able to determine the speed of the ejecta from the explosion, the temperature of the material, and the chemicals ejected from the star during the event.

  Led by Armin Rest of the Space Telescope Science Institute in Baltimore, the team published their results in the journal Nature on February 16th. By photographing the region around Eta Carina several times over many years, the team were able to detect the tiny variations caused by the "echo" of the light from the explosion moving outwards through the gas. Since this light has travelled along a longer path to reach us, bouncing off gas clouds in the nebula surrounding the star system, it arrives at Earth almost 200 years after the direct light from the explosion.

  From their observations, the team calculated that the temperature of the gas ejected during the 20-year period of eruption was about 5,000 degrees Kelvin, much cooler than expected based on models of eruptions which, up to now, were thought to have been similar in nature. The authors suggest that a different mechanism may have been responsible for the prolonged period of eruption in this case.

  Sometimes called supernova imposters due to their luminosity, giant eruptions of luminous blue variable stars involve a large increase in brightness which drives a loss of material from the star through a dense stellar wind. Such winds have temperatures of more than 7,000 degrees kelvin, significantly higher than that now observed in the light echo from the great eruption of Eta Carina.

  While other suggestions exist for the cause of Eta Carina's giant eruptions, including an explosion triggered by the accretion of material onto the smaller companion star, the actual mechanism is still unknown, but the data collected from these light echoes will help to test alternative models.

• February saw the publication of two papers investigating the nature of the first example of a particular type of object in the nearby Andromeda Galaxy. This mysterious class of extremely bright X-ray objects, known as ultra-luminous X-ray sources (ULXs), has been puzzling astronomers for some time, and there is more than one theory to explain their nature. This latest example is not only the first of its kind to be detected in the Andromeda Galaxy, also known as M31, but it is also the closest ULX source so far detected. The object, poetically titled CXOM31 J004253.1+411422, was first detected by the Chandra satellite in observations carried out in December 2009. It was subsequently also detected by X-ray cameras on board both the Swift and XMM-Newton satellites over the next two months as it decreased in brightness. Most known ULX sources are persistent, staying bright over long periods of time, but this one brightened suddenly, sharing some properties with Galactic X-ray binaries seen in our own Milky Way.

  Being comparatively nearby, this new object provides an excellent test for the two competing theories on the nature of ULX sources. The two usual models for ULX emission are: either a stellar-mass black hole accreting material from the surroundings at an extreme rate, or a so-called intermediate-mass black hole accreting material at a lower rate. Also known as IMBHs, these intermediate-mass black holes are thought to have masses larger than those created in supernova explosions but smaller than the supermassive black holes found in the centres of galaxies, but so far there is no direct evidence which proves their existence.

  The first of the two new research papers on this new ULX in M31 examines the X-ray observations and determines that the cause of the emission is likely to be an accreting black hole with a mass some 13 times that of the Sun, which underwent an outburst followed by an exponential decay in luminosity. The authors point out that this behaviour is similar to that seen in outbursts of Galactic novae.

  The second of these two papers also rules out an IMBH as the main cause of the emission, with the model that best fits the data being a stellar-mass black hole in a binary with a partially evolved (subgiant) companion star. The authors argue that this object shows that (still-hypothetical) IMBHs are not required to reach ULX luminosities, although this source (being transient in nature) is not necessarily typical of other ULX sources.

  Part of the problem with the study of ULX sources is that they are comparatively rare, with most galaxies hosting none at all. The discovery of this source came from an ongoing monitoring campaign which uses X-ray telescopes to regularly observe M31 looking for new and variable objects. In the same month as these two papers were published, a second example of a ULX in M31 was reported by the same monitoring programme. In a short telegram published online on February 13th, the team describes the detection of a second ULX which they plan to continue observing over the next two months, adding valuable information to the study of these rare and unusual objects.

• And finally: New results from the Planck satellite were presented at a meeting in Italy during February. Designed primarily to investigate the cosmic microwave background, the relic radiation left over from the Big Bang some 13.7 billion years ago, the spacecraft has been mapping the sky since its launch in 2009. The new results include the first all-sky map of carbon monoxide, a gas which is a significant constituent of the cold clouds of gas from which stars form. Such clouds contain large amounts of molecular hydrogen, but hydrogen in this form is very difficult to detect. Since molecular hydrogen and carbon monoxide form under very similar conditions, observations of carbon monoxide, which is much easier to detect, can be used to determine the distribution of molecular hydrogen in the Galaxy. Surveys for carbon monoxide using ground-based telescopes are very time-consuming, but Planck is designed to efficiently map the entire sky and so is capable of carrying out such a survey much faster.

  Such emission gets in the way of the much weaker background radiation from the Big Bang, and astronomers must carefully characterise these foregrounds and remove them from the data before they can map the cosmic microwave background.

  During its all-sky survey, Planck has also detected a mysterious haze of microwave radiation apparently coming from the centre our own Milky Way. This haze has several potential explanations, including supernovae, galactic winds, or the annihilation of dark matter particles, although so far none of these suggestions has been confirmed and the emission remains a mystery.

Interview with Dr Neal Jackson

Dr Neal Jackson from the University of Manchester talks us about a new array of low frequency radio telescopes called LOFAR. He discusses why LOFAR looks so unusual compared to other telescopes and why LOFAR could never have been done before the modern day. He proceeds to tell us about his own interests for LOFAR and gives an overview of some of the other key science projects.

Interview with Dr Tom Hassall

Libby caught up with Dr Tom Hassall a former PhD student from JBCA and now a postdoctoral researcher at the University of Southampton who is working on pulsars and radio transients. In this interview Tom talks to us about using LOFAR to detect pulsars, and how pulse profiles change with height above the pulsar.

Interview with Anna Kapinska

Anna Kapinska from the University of Portsmouth talks to us about radio galaxies and active galactic nuclei. She mentions the black holes in the centre of these galaxies, the jets being created and the lobes which can be observed. Similar jets to these may be produced in stellar-mass black holes, and Anna tells us the benefits of observing these objects. Anna also talks about the peculiar object SS433, which has a very interesting structure.

She also tells us about her newer work with LOFAR, automating imaging with the array in collaboration with The University of Bonn.

The Night Sky

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during March 2012.

Orion begins to set around sunset, with Taurus and the Pleiades Cluster above it in the southern sky. Above and left, the stars Castor and Pollux form the heads of Gemini, the Heavenly Twins. Nearby is Cancer, which contains the Beehive Cluster, M44. Further east lie Leo and the planet Mars, while Saturn rises in Virgo as the evening progresses. Ursa Major, high up in the north, hosts many nice objects, and, to its east, the bright star Arcturus rises in the constellation of Boötes.

The Planets

• Jupiter is some 45° above the south-western horizon at sunset, shining at magnitude -2.2 in Aries. Its angular size drops from 36 to 34" over the month, but small telescopes can still pick out its equatorial belts and dark barges.
• Saturn rises about four hours after sunset at the beginning of the month and crosses the southern sky during the night, reaching a maximum elevation of around 31° in the UK. It has a magnitude of +0.4, and is around 7° up and left of the first-magnitude star Spica. Its rings, now around 15° from edge-on, are visible enough to see the gaps between them.
• Mercury follows the setting Sun and can be seen in the hour after sunset. It reaches its greatest elongation (greatest separation from the Sun in the sky) on the 5th, at which point it is 12.5° above the western horizon half an hour after sunset. It then dims from magnitude -0.4 to +0.6 over the next five days, and disappears from view before the end of the month.
• Mars is due south around midnight as it reaches opposition (the opposite side to the Sun in the sky) on the 3rd, and gets to 46° above the horizon. It has a magnitude of -1.2 and is moving retrograde, the apparent backward motion resulting from Earth overtaking the planet in its orbit.
• Venus is near Jupiter and is the brightest of the naked-eye planets, which are all particularly good to observe in the first half of the month. It is about 44° from the Sun and 38° above the horizon at sunset, which is almost its highest possible elevation. Its angular size increases from 18 to 25" during the month, while its illuminated fraction drops from 64 to 49%, keeping its brightness virtually constant at magnitude -4.3 to -4.4.

Highlights

• The opposition of Mars means that it makes its closest approach to Earth of the last two years on the 5th, and so appears larger than usual around this date. Its angular size during these approaches varies by almost a factor of two due to the elliptical orbits of Mars and, to a lesser extent, the Earth: August oppositions afford the closest encounters as they coincide with Earth's greatest distance from the Sun and Mars's smallest. The current opposition is at the other end of the cycle, so the planet reaches a relatively small maximum size of 13.81". The north polar cap and Syrtis Major will nevertheless be visible after midnight.
• Jupiter, Venus and Mercury line up just after sunset on the 4th. Binoculars may be required to spot Mercury (don't use them until after the Sun has gone down), and they may also reveal Uranus in the same field of view, just down and to the left. It is 2° from Mercury and has a magnitude of +5.9.
• the 24th is a good time to observe Saturn and its major moons without hindrance from our Moon. Titan is visible through a small telescope, while an aperture of 8" (20cm) or more allows other moons to be seen.
• Jupiter and Venus appear together with a thin crescent Moon on the 25th, the latter exhibiting Earthshine. Venus moves above Jupiter in the sky during the month.

Southern Hemisphere

John Field from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during March 2012.

The summer constellations of Taurus, Orion and Gemini slide towards the western horizon as the nights lengthen. The Pleiades, or Matariki, will soon disappear from the evening sky and will reappear in the midwinter mornings. The bright, orange star Aldebaran is one of the eyes of Taurus the Bull and follows the Pleiades. It is a red giant and, from our viewpoint, sits among the more distant Hyades Cluster which forms the rest of the Bull's head. Cancer the Crab is almost due north in the early evening, and contains the Beehive Cluster at its centre. This cluster is larger than the Moon on the sky, and its age and velocity suggest a shared history with the Hyades. Leo the Lion rises at sunset and follows Cancer across the sky. Its brightest star, Regulus, has an 8th-magnitude companion that can be viewed with binoculars or a telescope.

Mars is currently in Leo and reaches opposition on the 3rd of March, when it is on the opposite side of the sky to the Sun and is making a close approach to Earth. Mars's day is slightly longer than Earth's, allowing different parts of the planet to be observed at the same time on successive nights, and features such as the north polar cap can be seen. Saturn rises around 10pm at the beginning of the month and around 8pm at the end, appearing as a bright yellow 'star' in the constellation of Virgo.

The Milky Way runs from north to south in the evening sky, becoming brightest near the southern horizon. Our eyes can pick out some 6000 of the Galaxy's 400 billion or so stars, and the Milky Way is the combined light from many of those that we cannot resolve. To the Māori in Aotearoa, it is Te Ikaroa - the Long Fish - and is the path taken by Tama-rereti as he placed the stars into the heavens. The Milky Way is mottled by dark clouds of interstellar material that block the more distant starlight. Crux, the Southern Cross, is in the southern region of the Milky Way. Between Crux and Canis Major lies the former constellation of Argo. In 1752, this great ship was broken into three parts: Carina, the Keel; Vela, the Sails; and Puppis, the Poop Deck. Malus, the Mast, became Pyxis, the Compass. Carina contains the night sky's second-brightest star, Canopus, shining at magnitude -0.72. In Greek mythology, Argo carried Jason and the Argonauts in their quest to find the Golden Fleece, and Carina, its hull, points towards the south celestial pole.

The 20th of this month marks the autumnal equinox, when day and night are of almost identical length and the Sun moves from due east to due west during the day. In Mā legend, Te-Rā - the Sun - had a summer wife call Hine-Raumati and a winter wife called Hine-Takurua. From the time of the summer solstice he begins moving from Hine-Raumati to Hine-Takurua, and begins to journey back again from the point of the winter solstice.

Odds and Ends

In January, stormy weather damaged the LOFAR station at Chilbolton, UK. This telescope is unusual as it was constructed by volunteer astronomers from nearby LOFAR-UK Universities. The Jodcast went along to help out on the repair day which included replacing the damaged antennas and securing the others.

Neutrinos may not travel faster than light after all, but it isn't a sure thing yet. The OPERA Collaboration caused a stir in September 2011 by announcing that they had measured muon neutrinos moving with superluminal speed, breaking the rule of Einstein's special theory of relativity that says that no particle with mass can travel at or above the vacuum speed of light. After an exhaustive search, the team has now come up with two possible sources of error. The first is a connection in the optical fibre used to carry a GPS signal to one of the clocks in the underground experiment; the signal is needed to synchronise the clocks at either end of the neutrinos' path, and an incorrect measurement of its travel time through the fibre may mean that the neutrinos weren't going faster than light after all. The second is an oscillator recording the GPS synchronisation times, but this erred in the opposite direction and may mean that the neutrinos really were breaking the light barrier.

The Hubble Space Telescope has been used to confirm that the planet GJ1214b is a so called water-world. GJ1214b was discovered by a ground-based telescope in 2009 and was found to have a diameter 2.7 times that of Earth. However, with a mass just 7 time that of Earth, it has a much lower density than our own planet. Water on this planet may behave in very unfamiliar ways due to the planet's temperature and atmospheric pressure, leading to exotic things like 'hot ice' and 'superfluid water'.

Show Credits