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Thursday, 20 November 2014

A Swift glance at red dwarfs

November 20, 2014 is a huge day for NASA’s Swift spacecraft, as it marks the tenth anniversary of its launch. Currently orbiting our planet, Swift is scanning the skies for potential sources of events known as Gamma-ray bursts (GRB’s). Each burst is a huge, but relatively brief, flash of very high energy radiation coming from interstellar space. Astronomers believe they happen fairly frequently (we detect around one per day according to NASA), and last from a few milliseconds up to a few minutes. Swift’s role is to detect these events using the Burst Alert Telescope (BAT), in order to find out what exactly is causing them. This telescope has a very wide angle lens, which can see about a sixth of the sky at any one time – about the same as one of our eyes can see - and scans around 88% of the entire sky each day[1].

Gamma-ray bursts can happen at any moment, so Dr Kim Page and her team at the UK Swift Science data centre (UKSSDC) based in the University of Leicester take turns to be ‘on-call’ for when BAT is triggered. There are two ways BAT can be provoked into action; by a quick rapid spike in high-energy waves or by a cumulative increase in a particular part of the sky over a longer period of time. These two mechanisms are known as the “rate trigger” and “image trigger” respectively, named for the specific way in which they pick up radiation. If the telescope is triggered it sends a message to the various cameras onboard Swift to “switch on”, which then turn and face the area of sky from where the original signal was first detected. At the same time, it sends an automated SMS to the on-call team members at the UKSSDC, whilst also writing each team member an email containing the relevant data about the location, time and intensity of the trigger source. The X-ray, UV and optical cameras on Swift can then investigate the radiation burst further. The Swift team have found this to be extremely effective and usually the phenomenal increase in energy has a Gamma-ray burst at the centre. But, as we will see, this is not always the case.

Artist's impression of the NASA Swift spacecraft
The Swift spacecraft has been orbiting the Earth since 2004, collecting data about the locations of over 800 gamma-ray bursts; extremely high energy events which occur deep in space. From understanding more about the causes and nature of GRB’s, astronomers hope to understand the early universe better. Image credit: NASA E/PO, Sonoma State University/Aurore Simonnet

Occasionally, an event which isn’t a GRB will cause the detectors to swing round and peer into the depths of space. On April 23, 2014, BAT picked up a huge influx of energy coming from a small constellation known as Canes Venatici (part of the Ursa Major group). The image trigger alerted Dr Page with a text message at around 10pm, telling her and her team that there was a potential GRB in this constellation. Within two minutes the Swift cameras had collected as much data about the position of the source as they could. This data was then cross-referenced with catalogues of stars and galaxies, to see if this patch of sky had produced GRB candidates on previous occasions - making it more likely to be a “false positive”. As a matter of fact it had – and the team found that the patch of sky they were looking at contained a binary red dwarf system, about 60 light years away. So, by then we knew what was setting BAT off, says Dr Page, Red dwarf stars are well known for their highly energetic flares. But the fact that this system could be expected to produce powerful flares didn’t prepare Dr Page and her team for the sheer enormity of the flares they were seeing this time.

Tuesday, 11 November 2014

Comet Chemistry

Dirty snowballs, snowy dirtballs and the leftovers of solar system formation. These names are commonly used to describe the often visually stunning ice extravaganzas that streak across our skies from time to time - comets. These somewhat simplistic names mask the importance of comets as repositories for material from which the planets formed. Comets contain volatile chemicals - materials that vapourize at relatively low temperatures, such as water, carbon dioxide, ammonia, and methane. This means comets have very limited lifetimes - they get vapourized and lose a lot of their mass each time they pass close to the Sun - so we know that each one we see is relatively new to the inner solar system, giving astronomers a window through which to observe the elemental and isotopic evolution of the early solar system.

The Sun emits light across much of the spectrum, from the blues, greens and reds we can see down into the infrared and up into the ultraviolet. When that light reaches an object, like a comet, some wavelengths of light get absorbed while others are reflected, depending on which materials are present. Similarly, certain molecular interactions also emit light but only at certain wavelengths. This leads to the creation of “spectral lines”. However, even the Sun’s spectrum contains thousands of absorption lines, known as the Frauenhofer lines.

Comparison of images for the continuous electromagnetic spectrum, emission and absorption spectral lines, and the solar spectrum.
Different elements and chemicals absorb and emit light at different wavelengths, so by analysing these spectral lines we can determine which materials are present. (Public domain image)

Current methods for determining the composition of a comet from the relative comfort of the Earth rely on the analysis of these spectral lines, but this approach is hampered in a number of ways. Firstly, there is the problem of the sheer number of spectral lines detected, and the superposition of these lines in relation to one another. Identifying the distribution of one molecule amongst the thousands of lines that can be detected in one observation is a complex and difficult process. This can also limit the detection of rarer chemicals as they can be masked by more prevalent compounds. Interference from the Earth's atmosphere is also an issue when ground based telescopes are used. The atmosphere can absorb photons at a number of different wavelengths, so some lines of certain species (molecular oxygen and water for example) are not detected at all.

On top of all this, as a fast moving object hurtling towards the Sun, the only volatiles available to study are those located in the comet's coma - the cloud of gas surrounding the solid ‘nucleus’ of a comet. The composition of the coma is assumed to be different from that of the nucleus because the volatile elements vapourize more quickly, so the coma is expected to have more of them and may not reveal some non-volatile elements present in the nucleus. As the relationship between the chemicals observed in the coma and those contained in the nucleus is unknown, abundances in the protoplanetary disk that seeded our solar system cannot be accurately inferred either.

Annotated photograph of comet 103P Hartley 2. Copyright Nick Howes.
Annotated photograph of Comet Hartley 2, showing the solid nucleus in the centre, the gaseous coma surrounding it, and a faint tail. Comet 103P Hartley was once described as a weird little comet by NASA, due to its high levels of activity.
Photograph ©Nick Howes, used with permission. All rights reserved.

At present, knowledge of a comet's interior is limited mostly to theoretical models. Models suggest that comets are composed of a non-volatile component, usually referred to as dust, and a volatile one consisting of ice[1]. To date, only two comets have been comparatively well studied; Halley's comet from its 1986 sojourn around the solar system and comet Tempel 1. Comet Tempel-1 was the main focus of the successful NASA Deep Impact mission which provided the first opportunity to look inside a comet.