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LT maintenance: mirrors realuminised and throughput doubled 05 August 2015

The middle of June saw the Liverpool Telescope (LT) go offline for two and a half weeks, to successfully undertake important scheduled maintenance. The main item in the to-do list was to realuminise the telescope's primary and secondary mirrors. This difficult task was performed outstandingly, and the results were clear to see, in that the throughput of the telescope was doubled - i.e. twice as much light now enters the telescope's detectors as before.

Why realuminise?
mirrors
Image © 2015 J. Marchant

"Normal" household mirrors that we're used to every day have their reflective (usually silver) layer deposited chemically on the back of a sheet of glass, and then another layer of paint is applied on top. This sandwiches the metal between glass and paint, protecting it and making sure it stays reflective for the lifetime of the mirror. This is what's known as a "second surface" mirror, because the reflecting layer is on the second surface away from the viewer, i.e. the back of the glass. The first surface on the other hand is the front of the glass.

For everyday use this kind of mirror is fine, but you wouldn't be happy using it in a telescope. That's because light has to pass through the glass twice, once on the way in and again on the way back out. The glass not only absorbs some of the light (which you don't want when looking at faint objects), but also causes multiple reflections which hamper sensitive measurements.

That's why astronomers prefer to use "first surface" mirrors in their telescopes instead. As you may have guessed already, this is where the metal is deposited on the front of the glass. The metal in this case is usually aluminium, and it's evaporated onto the glass in a huge vacuum chamber. The layer ends up around 350 atoms thick, which for aluminium is about 100 nanometres, about the same thickness as gold leaf and 160,000 times thinner than kitchen foil.

This fragile coating is exposed to the air and any contaminants that might fall on it, but that disadvantage is far outweighed by the fact that light does not have to pass through any glass in front, resulting in a much clearer and brighter image. We should not leave out the even more important fact that the glass substrate itself is carefully shaped and polished to a precise figure to focus the light in the exact manner required. This shaping is by far the most difficult part of telescope mirror manufacture.

Over time, dust (especially "calima" blown from the nearby Sahara Desert) builds up and adheres to the aluminium, degrading its reflectivity. Periodic cleaning is not 100% perfect, and eventually after several years it is better to realuminise the mirror instead. This involves taking the mirror out of the telescope, and removing the aluminium with acid. The glass "blank" is then carefully cleaned, and put inside a huge vacuum chamber where a new layer is evaporated onto it in a carefully controlled manner.

Very conveniently, one of the many telescopes on the mountaintop has their own realuminising plant. The William Herschel Telescope (WHT) stands just 500 metres away from the LT, and its vacuum chamber was built to accommodate their 4-metre diameter primary mirror. The WHT staff very kindly agreed to realuminise our mirrors, and provided expert crane operators and equipment to help us move our sensitive optics over to their site.

Moving the mirrors
primary mirror
LT Engineering Manager Stuart Bates by the primary mirror.
(Photo © 2015 M. Crellin)

The LT's primary mirror is a precision-crafted disc of glass two metres in diameter and 20cm thick, weighing in at 1.3 tonnes. Extracting such a large and heavy piece of sensitive equipment from the base of the LT, without scratching it or causing any damage at all, was a particularly tense time. As expected though, on-site personnel performed the task expertly. They were site manager and senior mechanical engineer Stuart Bates, mechanical engineer Mark Crellin, LT site engineer Dirk Raback, and crane operators and engineers from the WHT. The primary mirror was slid out from underneath the telescope in its mirror cell and immediately covered in lint-free tissue to protect it and prevent dangerous reflections. It was then carefully lifted off the mirror cell into a special padded transit box, and then hoisted out of the top of the open enclosure.

Meanwhile, the secondary mirror was also earmarked for realuminisation. This was the first time the secondary had been treated since the LT was built in 2003. The reason for the delay was that the secondary had been coated with a thin layer of silicate to protect the aluminium, and until recently, removing this layer to get at the metal without damaging the glass substrate underneath was a prohibitively expensive and difficult task.

primary mirror
Dirk Raback, Stuart Bates and Mark Crellin at the top of the telescope.
(Photo © 2015 N. Clay)

This year however, the very expert who applied the silicate coating all those years ago was available to remove it again in person. David Jackson is now retired, but was happy to come to La Palma and help. So, the telescope's top end ring was lifted off the and placed in a special holding rig. The mirror was then removed and placed in another transit box to be taken to the WHT's plant along with the primary.

Realuminising

At the plant, the primary's aluminium layer was carefully removed with powerful acids under strict safety supervision. After that, the glass was washed and carefully dried to ensure not a speck of dust remained on the surface. Finally, the glass "blank" was placed in the WHT's huge realuminising chamber.

primary mirror
David Jackson (left) and Jürg Rey cleaning the primary glass before realuminising. (Photo © 2015 N. Clay)

Meanwhile, under the expert guidance of David Jackson, Jürg Rey, head of operations at the Isaac Newton Group of telescopes, was able to successfully remove the silicate layer on the secondary mirror without damaging the glass underneath. Now the silicate was gone, the aluminium layer underneath could be removed in the same way as the primary. The secondary glass blank was then cleaned, and it took its place next to the primary in the WHT's cavernous aluminising chamber.

primary mirror
New primary being reinstalled.
(Photo © 2015 N. Clay)

Aluminising went without a hitch, and the next day the equally careful task of transporting the freshly-coated mirrors back to the Liverpool Telescope began. On-site, the secondary was replaced in its mounting and the top-end ring hoisted back onto the top of the telescope. The 1.3-tonne primary in its transit box was lifted back into the enclosure, then taken out of the box and placed to submillimetre precision back onto the pneumatic actuators in the mirror cell.

Increased throughput

Over the next two days the rest of the telescope was put back together and all instruments remounted, in time for LT Director Iain Steele, Operations Scientist Jon Marchant and PhD student Helen Jermak to fly out from Liverpool to undertake recommissioning.

The telescope had had its major optical components dismantled and reassembled, and all instruments had been removed and remounted. So from 27 June to 1 July, a full end-to-end test of the telescope's electrical, hydraulic, pneumatic and optical systems was made, along with similar tests for each instrument. This lengthy sequence of tests proved the telescope was operating nominally and that all of its instruments were in focus and working properly.

primary mirror Factor increase in throughput
by IO:O filter
(Image © 2015 J. Marchant)

One of the first things that became apparent during recommissioning was that the throughput of the telescope had improved by a greater margin than anticipated. Helen Jermak performed tests through IO:O filters to show that on average, twice as much light reaches detector sensors than before, depending on filter. The greatest increase (a factor of 223%) is in the SDSS-U filter, while the least increase (a factor of 163%) is in SDSS-Z. This general trend of greatest increase in the blue part of the optical spectrum is a sign that it was finally being able to realuminise the secondary mirror that caused most of the improved throughput.

Realuminising was not the only task on this maintenance visit. DevOps engineer Neil Clay also came to site and undertook timely and crucial maintenance on the telescope's entire IT system, as well as repairs and maintenance to the weather mast and its sensors. The 1-degree FOV Skycam Z was overhauled too, its 20cm primary mirror being realuminised along with the LT's. Being so relatively small, there was no problem finding room for it in the WHT's huge aluminising chamber.

Summing up, the three-week programme of site work and support from the UK was very successful. A big thank you goes to LT staff both on-site and on backup in the UK, Mark Crellin, David Jackson, Helen Jermak, and Jürg Rey and the WHT staff.

When stars collide: LJMU team identifies rare luminous red nova in Andromeda 1600 GMT 5 May 2015

M31 RN Light-curves obtained by imaging M31LRN 2015 over a number of weeks through seven different filters. Data from the LT (filled circles) are combined with observations from other telescopes (open circles). The x-axis shows time in days; the y-axis is the brightness of the nova, in magnitudes.

In January 2015 the discovery of a possible classical nova in the Andromeda Galaxy (M31) was announced by the Global MASTER Robotic Network, a Russian-led network of telescopes dedicated to time domain astronomy. Classical novae are not particularly rare events, with around 30 observed each year in M31 alone. However, as the LJMU team of Dr Steven Williams, Dr Matt Darnley, Prof. Mike Bode and Prof. Iain Steele were soon to realise, the object in M31 was a much more unusual object. By following the outburst with the Liverpool Telescope's new spectrometer SPRAT and its work-horse imager IO:O, Williams and co. demonstrated that the outburst - dubbed M31LRN 2015 - was not a classical nova, but was instead a luminous red nova (LRN), a much less common class of stellar transient.

Classical novae are thought to be associated with binary star systems. They result from a burst of nuclear fusion on the surface of a white dwarf (a very compact dense star about the size of the sun), as material spirals down onto the white dwarf from its larger companion star. By comparison, the nature of luminous red novae is still uncertain. A growing body of evidence suggests that they may be the result of two stars merging together, causing a sudden explosion and a very dramatic brightening of the system.

The study of these systems as a class of transient only really began with the 2002 outburst of V838 Mon in our Milky Way galaxy, when astronomers noticed that it behaved differently to classical novae. The characteristics of the V838 Mon outburst event were found to be similar to the luminous red nova identified in M31 in 1988, and to V4332 Sgr, a similar type of outburst which occurred in the Milky Way in 1994.

The first SPRAT spectrum of M31LRN 2015, taken three days after its discovery and before the outburst had reached peak brightness, shows that it initially exhibited strong hydrogen emission lines (labelled Hα below). These hydrogen lines weakened over time and the spectra then began to show various absorption features, its spectrum resembling a cool supergiant!

The outburst was also followed with IO:O using different coloured (B, V, i' and z'-band) filters. These observations showed that after reaching its peak brightness, the luminous red nova faded quickly in the bluer filters, but remained bright in the redder filters for several weeks.

An IO:O image of the outburst was used to precisely determine the position of the system. The team then searched the Hubble Space Telescope data archives and found an image of the object, taken in 2004, prior to the recent outburst. The data show that the likely progenitor star of M31LRN 2015 was consistent with a red giant. Interestingly, the object appears to show evidence of hydrogen emission many years prior to the outburst, although the source of this emission is not clear. Further observations of this and other systems are certainly warranted. Astronomers clearly have some way to go before these enigmatic objects are fully understood.

All of the LT observations of M31LRN 2015 and the archival study of its progenitor have recently been published in Williams et al. (2015).

Sprat spectrum

Top: SPRAT spectra of M31LRN 2015 taken on Jan 16, Feb 2, Feb 12 and Feb 28, 2015. The LRN was at its brightest between the first and second spectrum. Emission and absorption lines from a number of atoms and ions have been labelled. Bottom: Three spectra of a similar type of object, V838 Mon, displayed for comparison purposes.

The LT draws in the crowds at StarGazing Live 2015 1600 GMT 27 Mar 2015

Jon Marchant of the Liverpool Telescope group was joined by Astrophysics Research Institute astronomers Matt Darnley and Simon Prentice at this year's StarGazing Live event in Leicester. Hosted by the BBC, the event coincided with a spectacular partial solar eclipse witnessed by millions of people in the UK and across Europe.

The day was split into a morning session of eclipse watching, followed by an evening of star-gazing from the Racecourse at Leicester and the fields surrounding Jodrell Bank. Although the eclipse's path of totality missed mainland Britain, those watching were still able to enjoy a deep partial eclipse, with 85-95% of the Sun's diameter being covered by the Moon, depending on the viewer's location in the UK. Here in Liverpool the weather was kind: at around 9:30 am the skies darkened and a thin crescent "sun" could be viewed by projection or through protective glasses as the moon covered almost 91% of the solar disc.

Back in Leicester, when not marvelling at the eclipse, the team were able to share a range of exhibits and activities with members of the public. These included a fine collection of meteorites and a spectroscopy demonstration. A live feed to the telescope was also set up, although the weather in La Palma was poor so the robotic control system was unable to open the enclosure, even for Stargazing Live. Instead, some fine observations of galaxies and nebula acquired earlier in the week were presented to visitors alongside a live infrared view of the inside of the enclosure, the latter showing the telescope patiently waiting for conditions to improve. Better luck next year.

sg1 sg2 sg3

Members of the public visiting the LT exhibit at this years Stargazing Live event in Leicester and - centre - the stand before opening for business (click to enlarge).

Time domain astronomy at NAM-2015 1400 GMT 10 March 2015

The LT Group and Astrophysics Research Institute, Liverpool John Moores University have once again organised two sessions at the U.K.'s National Astronomy Meeting. Hosted by the Royal Astronomical Society, NAM-2015 will be held at Venue Cymru in Llandudno, North Wales. As part of the week-long festivities, two 90 minute sessions are being organised which will focus on time domain astronomy with robotic telescopes. These sessions will be held on Thursday, 9 July, at 9 am and 1.30 pm. Although the abstract submission deadline has passed, the meeting website is still open for registration. Further details are available at the official NAM-2015 website.

Time domain astronomy with robotic telescopes: the science that drives the technology

With the growing number of "triggers" induced by wide-field and all-sky surveys such as VVV, iPTF/zPTF, PanSTARRS, ASAS-SN and now Gaia, our sessions will review the current status of time domain astronomy and, specifically, the way in which robotic telescopes react to these triggers.

The first of two sessions will cover the science that forms the backbone of transient and time domain astronomy: Nova and supernova, GRBs, CVs and Be-type stars, variable YSOs, X-Ray transients, tidal disruption events, etc. In the second session we will review the new technologies that are being proposed and/or developed to address these science areas: developments at existing robotic facilities but also new concepts for future telescopes and instrumentation.

A broad range of oral and poster presentations have been received. The programme of talks will be posted here and at the NAM-2015 website when these become available. In the meantime, all those interested in the transient universe are encourage to visit the NAM-2015 homepage to register for the meeting. Note that it is possible to register for just one day if you can't manage the full week.

For further details on our two sessions please don't hesitate to contact a member of the Science Organising Committee. Note, however, that logistical information is only available from the official NAM-2015 website.

Science Organising Committee

Chris Davis, Iain Steele, Chris Copperwheat, David Bersier, Rob Smith and Paolo Mazzali. Contact details are available here.

EWASS
Robotic telescopes and instrumentation for time domain astronomy 1600 GMT 13 Feb 2015

The upcoming European Week of Astronomy and Space Science (EWASS) will include a Special Session (Sp6) entitled 'Robotic telescopes and instrumentation for time domain astronomy'. EWASS2015 is organised by the European Astronomical Society (EAS), and will be held in Tenerife, Spain, from June 22 to 26, 2015 (see the EWASS 2015 web-site for details).

In this special session we aim to address how current and future robotic facilities meet the scientific needs of the European time domain community. The session will feature both talks on scientific results as well as new and existing robotic facilities covering all areas of time domain astronomy. We also include papers introducing new instrumentation, as well as discussions on the technical and software challenges of robotic response.

Aims and scope of the meeting

The past decade has seen robotic telescopes increasingly employed for the study of the time variable sky. Arrays of small telescopes covering large fields-of-view have proved to be powerful tools for the discovery of explosive transients (for example the MASTER robotic network) and transiting exoplanets (SuperWASP). The rapid reaction and flexible, remote scheduling capabilities of larger aperture robotic facilities such as the 2-metre Liverpool Telescope, based on La Palma, makes them powerful tools for the exploitation of time variable objects such as supernovae, gamma ray bursts, exoplanets and binary stars.

The next decade will see time domain science becoming an even more prominent part of the astronomical agenda. On the discovery side of things, new European facilities such as the Next Generation Transit Survey (NGTS) will build on the success of the SuperWASP project. New goals for the time domain community include the discovery of electromagnetic counterparts to astrophysical gravitational wave sources, and proposed facilities such as GOTO and BlackGEM aim to use dedicated arrays of small, independently pointed telescopes to address the large positional uncertainty of any gravitational wave detection.

The diversity of new survey missions on the horizon provides unprecedented opportunities for robotic follow-up and scientific exploitation. In the field of exoplanet science, NGTS and the next generation of space missions such as PLATO will build on the work of Kepler by discovering more planets with bright host stars in order to maximise the potential of ground based follow-up. The STELLA Robotic Observatory on Tenerife is a precursor to a potential PLATO follow-up facility dedicated to stabilised hi-resolution echelle spectroscopy for radial velocity studies and the characterisation of exoplanet host stars. The GREGOR solar telescope is also in the process of being equipped with a high resolution spectrograph for night-time robotic operations. The first telescope of the Stellar Observations Network Group (SONG) was also recently inaugurated on Tenerife. This 1-metre telescope is the prototype of a modern, global network of robotic telescopes, and will address a wide range of time domain topics, with a particular focus on exoplanet follow-up and the study of the internal structure and evolution of stars via asteroseismology.

For transient science, the next generation of synoptic surveys such as LSST will discover huge numbers of targets and facilities such as LOFAR, SKA and CTA will probe transient phenomena at previously unexplored wavelengths. An increasingly important part of time domain science will be the software challenges of rapid classification in order to make best use of available follow-up facilities. This new era of transient astronomy will demand deeper and an even more rapid reaction capability, and to that end plans are underway for a 4-metre robotic successor to the Liverpool Telescope, to come into operation on La Palma in approximately 2020.

Programme

In this session we aim to address how current and future robotic facilities meet the scientific needs of the European time domain community. Examples of the topics to be covered include (but are not limited to):

  • New robotic facilities for time domain science
  • Upgrades for existing facilities
  • Automated transient classification
  • Linking transient discovery with rapid follow-up
  • Robotic telescopes in the era of multi-messenger astronomy
  • Novel detector technologies for rapid reaction

Abstract submission has now closed and the programme of talks has been fixed. Due to high demand, the meeting has been extended to THREE SESSIONS, all of which will be held on Friday, Jun 26th. See the EWASS 2015 web-site for further details.

The scientific organising committee:

M.F. Bode (Liverpool John Moores University, UK)
C.M. Copperwheat (Liverpool John Moores University, UK)
C.J. Davis (Liverpool John Moores University, UK)
L.J. Goicoechea (Universidad de Cantabria, Spain)
I.A. Steele (Liverpool John Moores University, UK)
K. Strassmeier (Leibniz-Institut fuer Astrophysik Potsdam, Germany)

Black hole caught having a snack 1600 GMT 18 November 2014

We don't as yet know very much about black holes, but one of the things we do know is that it's not a good idea to get too close to one of them! Their powerful gravitational pull can rip apart anything that passes nearby. Yet a star may have survived such a close encounter, an encounter that was recently observed by LJMU's David Bersier and colleagues using the Liverpool Telescope.

TDE artist's impression An artist's impression of the passage of a star close to a black hole, causing a a Tidal Disruption Event (image courtesy CXO/NASA).

TDE Light curve Light curves showing how the brightness (magnitude) of the stellar debris steadily changes with time during the star's encounter with the black hole. The event was observed over a considerable period of time in a variety of imaging filters. The u, g, r, i and z-band data were secured at the LT; the other data were observed by Swift. (MJD is the Modified Julian Date.)

Only a few such stellar disruptions have been seen before. Close encounters are thought to be rare, and to date their discovery has largely been by accident. In order to catch such an uncommon event, astronomers need to look at a large fraction of the sky, and look often. This is what the All-Sky Automated Survey for Supernovae (ASAS-SN, pronounced "assassin") is designed to do. Its six small telescopes - four in Hawaii and two in Chile - scan the sky every night, looking for variable sources, transient objects, and sudden outbursts. ASAS-SN discoveries often trigger rapid follow-up observations on larger telescopes, particularly robotic facilities like the LT.

On January 25, 2014, an otherwise anonymous galaxy located a mere 650 million light years away in the constellation that contains the "Big Dipper", looked significantly brighter than usual. This object, nicknamed ASASSN-14ae, was initially thought to be a supernova, the explosion of a massive star, albeit an unusual one.

Several telescopes, including NASA's Swift observatory and the Liverpool Telescope, were immediately used to obtain more data. PhD student Thomas Holoien of Ohio State University led the effort and coordinated the observing campaign.

As the story unfolded it became clear that ASASSN-14ae was not a supernova, but was instead something entirely different: a Tidal Disruption Event, or TDE. Such an event is believed to occur when a star gets a little too close to a black hole, an object with a mass several million times that of our Sun. Luckily, the star in this case seems to have survived the encounter, with only a small chunk of matter being ripped off!

The amount of energy released during the event allowed researchers to calculate that only one thousandth of the mass of our sun - about the mass of the planet Jupiter - had been sucked into the black hole.

Light curves, showing how the brightness of the debris ripped from the star varied during the encounter, are shown to the right. As the debris falls towards the black hole it settles into an "accretion disk", where it gets hot and thus shines. The steady decline in brightness of this material, seen over a period of many weeks with Swift and the LT, matches what is expected of a TDE.

The Liverpool Telescope is the perfect machine to follow an event such as this. Although Holoien and his team needed access to a telescope for only ten minutes or so each night, observations were needed over a long period of time. The fact that the LT is entirely computer-controlled means that the observations could be scheduled remotely and all in one go: a very lengthy stay at an overseas observatory was thus not required. This is perhaps a less romantic way of observing, but is none-the-less a lot more efficient.

Monitoring the whole night sky every other night, the ASAS-SN survey has a good chance of detecting more of these events, and perhaps even more exotic cosmic catastrophes that we haven't thought of yet! In the meantime, the LT will be ready and waiting to react to ASAS-SN triggers and secure the observations needed to better understand these remarkable cosmic phenomenon.

The observations described here have recently been published in volume 445 of the Monthly Notices of the Royal Astronomical Society by Holoien, Bersier and their collaborators. A copy of the article is available here.

Rapid SPRAT confirmation of a Gaia transient: it's a dwarf nova! 1200 GMT 16 Oct 2014

SPRAT spectrum
A SPRAT spectral image of Gaia14aat with wavelength on the horizontal axis increasing to the right (coverage approx. 400-790 nm) and offset along the spectrograph slit on the y-axis. The vertical lines are emission from the atmosphere (the two brightest sky lines are labelled). The horizontal line is the black-body continuum emission from the binary system; superimposed onto this continuum are emission lines at very specific wavelengths. The hydrogen Balmer recombination line, H-alpha, is labelled, though other lines are also evident in the fully-reduced data.

One of the secondary goals of the Gaia Space Telescope is to survey the whole sky for variables and transients, objects that suddenly increase in brightness. The Gaia Photometric Science Alerts programme hosted by Cambridge University in the U.K. has recently gone public, and one of the first alerts released has been robotically observed by the Liverpool Telescope. As part of a campaign of rapid follow-up observations with the newly-commissioned SPRAT spectrograph, a group of LJMU astronomers have just released the first Astronomer's Telegram based on a Gaia transient alert.

The transient Gaia14aat was detected by the Gaia Photometric Science Alerts programme with a magnitude of 15.7 on 10th October. The team, all part of the Liverpool Telescope group at LJMU, measured the object's position precisely. They then identified the progenitor of the outburst in archival Sloan Digital Sky Survey (SDSS) images as an object with an r-band (red) magnitude of 18.9. The target had thus suddenly brightened by over three magnitudes; that's an increase in luminosity of more than 15 times.

The question then was: what is Gaia14aat?

Using the SPRAT spectrograph installed on the Liverpool Telescope, the group obtained a 10 minute spectrum of the object on October 15th. The spectrum covers the wavelength range of 400 to 790 nanometres and exhibits emission lines from hot atomic hydrogen: a bright H-alpha line at 656 nm and fainter H-beta and H-gamma lines at 486 and 434 nm.

Observing with SPRAT involves first taking an image (so that the target can be identified and moved onto the spectrograph slit). This "white light" acquisition image can also be used for science, however, and was in this case used to estimate the r-band magnitude of the target, which by the date of the LT observations had faded to about 18.5, close to the SDSS value. The object had already returned to its quiescence state in the 5 days since the Gaia detection. Clearly, time is of the essence when observing Gaia transients!

Based on the duration and brightness of the transient and the emission features in the SPRAT spectrum, the team believe that Gaia14aat is a dwarf nova outburst in a hydrogen-rich cataclysmic variable. Dwarf novae are binary systems in which a white dwarf star accretes matter from a companion; cataclysmic variables are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state.

Gaia14aat will undoubtedly be the first of many transients discovered by the Gaia Space Telescope and subsequently observed by the LT. These early observations illustrate the power of SPRAT for categorising faint transients, and the importance of rapid response and robotic operations. Exciting times lie ahead.

The LJMU team of Andrzej Piascik, Iain Steele, Chris Copperwheat and Chris Davis would like to acknowledge the ESA Gaia mission and particularly the DPAC Photometric Science Alerts Team.

LT discovers the sixth eruption of a remarkable Recurrent Nova in M31 1700 GMT 14 Oct 2014

M31 RN A quasi-true colour LT image of the region of M31 around M31N 2008-12a. The yellow backdrop is the unresolved stellar population of the disk of M31, where the dark patches are regions of high extinction. The majority of the point sources seen in the image are in the foreground and belong to our own Galaxy. The image has been enhanced by the use of a 'H-alpha' image to highlight regions of excited Hydrogen gas. The nova can be seen in eruption in the centre of the image (marked by the white arrow).

The LT has in recent weeks been doing what it does best: making exciting discoveries in time domain astronomy! A team led by Dr Matt Darnley of the Astrophysics Research Institute at LJMU has detected the latest eruption of a remarkable Recurrent Nova (RN) in the nearby galaxy M31. This object is particularly noteworthy because of the frequency of its eruptions. Most RNe undergo an outburst once every 10-100 years; the RN in M31 seems to erupt annually.

Darnley and his team were the first to spot the latest eruption of the nova and, thanks to the LT's robotic capabilities, have been able to monitor the event with images and spectra obtained every few hours/days over a period of a few weeks. They have certainly not let the grass grow under their feet, having made full use of the recently-commissioned optical spectrograph, SPRAT.

Novae are associated with nuclear explosions on the surface of a white dwarf, which results in a sudden brightening of the star. Recurrent nova outbursts are caused by the accretion of material from a companion star, usually a red giant, onto the white dwarf through an accretion disc.

As reported in an LT news item earlier this year, the true recurrent nature of the nova system in M31, designated M31N 2008-12a, was characterised following its fifth detected optical eruption in 2013. An international study co-led by Darnley and Dr Martin Henze of the European Space Astronomy Centre in Spain, along with independent work by the Intermediate Palomar Transient Factory (iPTF), uncovered the progenitor system of M31N 2008-12a and inferred the presence of an extremely high mass white dwarf as well as a high mass accretion rate. These are the tell-tale signs that M31N 2008-12a may one day evolve into a Type Ia Supernova explosion.

Such a high mass white dwarf leads to a very rapid evolution of the 'optical lightcurve' of each eruption. The nova fades very rapidly post-eruption. Consequently, despite five optical eruptions and three separate X-ray detections of the event in recent years, very little was known about the behaviour of the system during its eruptions - until now.

In anticipation of a sixth eruption towards the end of 2014, Darnley has been leading a campaign on the Liverpool Telescope (LT) to monitor M31N 2008-12a to detect any changes in its behaviour. This LT campaign was also designed to react rapidly following a newly detected eruption, to obtain as much data on the system as possible.

Nightly monitoring of M31N 2008-12a by the LT began towards the end of July 2014, and just before 10pm (GMT) on 2nd October a sixth eruption was detected. As planned, intensive photometric monitoring of the eruption using the IO:O optical imaging CCD camera on the LT was immediately implemented. In addition, and for the first time, the team deployed the newly commissioned SPRAT (SPectrograph for the Rapid Acquisition of Transients) instrument on the LT, a low-resolution though high throughput spectrograph designed specifically for the classification of transients like novae.

Remarkably, SPRAT has been mounted on the LT for less than a month before Darnley et al. used it to obtain the first spectra of an extragalactic nova ever taken with the LT. These data have led to spectroscopic confirmation of the nature of the eruption and have allowed the team to determine the expansion velocity of its ejecta.

Sprat spectrum

A SPRAT spectrum of M31N 2008-12a taken on the night of 3rd October 2014. The spectrum shows the tell-tale Hydrogen, Helium, and Nitrogen emission lines expected of a 'He/N' nova in eruption.

As well as Matt Darnley and Martin Henze, the international collaboration also includes; Mike Bode (LJMU), Steve Williams (LJMU), Allen Shafter (San Diego State University, USA), Jan-Uwe Ness (ESAC), and former LJMU PhD student Rebekah Hounsell (Space Telescope Science Institute, USA). Iain Steele, Rob Smith, and Andrzej Piascik, all from the LT Group at LJMU, were instrumental in obtaining and analysing the SPRAT spectroscopic observations.