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Liverpool Telescope frame of the AGN 0957+561 on 5th December 2011. This typical 120 second exposure in SDSS-R band shows the two images of the gravitationally lensed AGN, which are separated by ~6 arcsec (marked with a blue arrow).

A long-term photometric monitoring study with the Liverpool Telescope (LT) was recently used to unveil the nature of the accretion flow and jet connection of a distant active galactic nucleus (AGN) for the first time. This project was conducted by the GLENDAMA research team (Rodrigo Gil-Merino, Luis J. Goicoechea, Vyacheslav Shalyapin and Vittorio F. Braga) at the Universidad de Cantabria (Spain), and the LT has been the key facility to carry it out.

AGNs generate spectacular X-ray, ultraviolet (UV) and optical continuum luminosities by matter accretion onto their central rotating supermassive black holes. However, the precise geometry and origin of this huge energy production is still largely unknown, and direct spatial resolution of the emitting regions from such objects is not currently possible. Fortunately, there is a time-domain technique to probe the accretion physics for AGNs. This is the so-called "continuum reverberation" (or echo) mapping, which relies on the analysis of time-delayed responses of different continuum emitting regions to original fluctuations in a driving source.

A successful reverberation analysis requires multiwavelength monitoring during a period of strong variability. At redshifts z > 1, this task is much easier to plan for a gravitationally lensed AGN (the gravitational field of foreground galaxies produces two or more images of the same background galaxy nucleus), since the variability of some of its multiple images can be predicted in advance based on an optical follow-up of the lens system on a nightly basis. The double-image AGN 0957+561 (designation based on its position in the sky) is located at a luminosity distance from Earth of ∼33 thousand million light years (z = 1.41), and it was intensively monitored with the LT between 2005 and 2010. As intrinsic flux variations in the image 0957+561B lag those in the image 0957+561A by about 14 months, the LT detection of significant flux variations in 0957+561A between late 2008 and mid-2009 allowed the GLENDAMA team to organize a multiwavelength follow-up campaign during the first semester of 2010. This included observations with two NASA satellites: Chandra X-ray Observatory and Swift, supported by the LT in the Sloan "griz" passbands.

The X-ray, UV and optical brightness records of 0957+561B in 2010 confirmed its intrinsic origin and led to a reverberation mapping of the continuum sources in the distant AGN. For example, the UV-optical fluxes in the Ugr passbands were used to detect interband delays of several days, U leading and r trailing. These delays are consistent with the existence of a driving source very close to the central supermassive black hole, which illuminates a standard accretion disk and induces variations in disk rings at only a few light-days from the massive dark object, i.e., at sub-milliarcsecond angles as seen from Earth!

While the central driving source cannot be a standard corona that is emitting the observed X-rays with a power-law spectrum, a central extreme UV source is the best candidate to drive the variability of 0957+561. It would be sited just above the black hole and below the base of the jet perpendicular to the disk.

The GLENDAMA team plausibly interpret their data as evidence for a power-law X-ray source in the base of the jet at a typical height of ∼60 light-days. Although the rapid rotation of the 2.5 thousand million solar mass black hole presumably disrupts the corona and converts it into a jet, the jet itself would keep a hot base as a "residual corona". This is a benchmark study to understand the accretion flow in primeval AGNs displaying jets.

The data are available on-line, and the results are presented in the January 1, 2012 issue of The Astrophysical Journal (journal subscription necessary).

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Artist's impression of GRB101225A. The merging of the helium and neutron star produces a broad torus, plus two jets aligned with the rotation axis of the system. The jets interact with the previously ejected torus causing the observed black body spectrum. Image credit: A. Simonnet, NASA, E/PO, Sonoma State University.
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The Liverpool Telescope (LT) recently played an important role in the discovery of a new type of gamma-ray burst, as revealed in a new paper published in Nature this week (Thöne et al, Nature, 480, 72-74, 2011).

Gamma-Ray Bursts (GRBs) are brief and intense flashes of gamma-ray radiation that occur randomly from any direction of the sky. Their durations range from a few milliseconds up to over half an hour. They are so energetic that we can detect them even at distances of thousands of millions of light years. Since our atmosphere is opaque to gamma-ray photons, GRBs are detected by gamma-ray instruments on board spacecraft such as NASA's "Swift" satellite.

Swift localizes GRBs and distributes their coordinates (mainly through the internet) to astronomers all over the world, who can follow up these explosive events using ground-based telescopes. These observations have shown that GRBs are followed by fading optical, infrared and radio emissions called “afterglow”, which can be explained by synchrotron radiation emitted by charged particles moving in magnetic fields at ultra-relativistic speeds (velocities above 99% of the speed of light).

On Christmas Day 2010 a peculiar GRB occurred, designated GRB101225A (after the date of its discovery), also nicknamed “The Christmas Burst”. It lasted more than half an hour, much longer than most GRBs detected so far. Its low‑energy emission (i.e. all radiation measured below the gamma-ray regime) was dominated by a strong thermal component ‑ a classical black-body spectrum ‑ while all other GRBs were dominated by synchrotron radiation.

An international group of researchers, led by Dr. Thöne and Dr. de Ugarte Postigo from the Instituto de Astrofísica de Andalucía (IAA-CSIC, Granada, Spain), recently published an article on GRB 101225A in Nature. Based on a set of space- and ground-based observations, they propose a new scenario to explain this exotic explosive event.

At the moment there are two standard models to explain the two broad types of GRBs that have been observed:

• the "Compact Binary Merger" model, for short duration (<2 sec) GRBs
• the "Collapsar" model, for long duration (>2 sec) GRBs

However, according to Thöne et al, the peculiar properties of the Christmas Burst require a different model altogether to explain it.

They propose that the burst is the result of a neutron star merging with the helium core of an evolved giant star, at a distance from Earth of ~5.5 thousand million light-years (redshift z ~ 0.3). During the “inspiral” of the neutron star, the binary system underwent a common envelope phase where the giant star expelled most of its hydrogen envelope. The resulting explosion created a GRB-like jet, which became thermalised by its interaction with the dense common envelope, and thus gave rise to the observed black body spectrum. This ejected material was cooling down progressively from 43,000K 23 minutes after the burst, to 5,000K 40 days after the burst. Finally, ~10 days after the explosion a faint supernova component started to emerge, reaching its maximum 40 days after the GRB and dominating the fading black body radiation. This gradual transition from a smooth black body to a supernova spectrum has not been seen before^[*], and the Liverpool Telescope has been a key facility to monitor it.

Liverpool Telescope observation of the optical counterpart of GRB101225A (arrowed). The image was taken ~10 days after the GRB at the onset of the supernova.
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Observations carried out with the Liverpool Telescope 10 days after the Christmas Burst revealed a reddish re-brightening of the optical emission which could not be explained by a mere black body evolution. The high quality data of the Liverpool Telescope revealed the optical counterpart with a reasonable error in spite of its faintness (i_AB=24.01±0.13, see figure at right). This showed that the optical emission was ~0.3 magnitudes brighter than the predictions made by a simple cooling black body model. In order to study the nature of this emission excess, a monitoring program was planned based on larger aperture telescopes.

Subsequent observations by the Gran Telescopio CANARIAS and Gemini telescopes confirmed the existence of a faint supernova component in both the optical lightcurve and its corresponding spectral energy distribution (SED). The best fit to this supernova component consisted in a faint Ic-type like supernova at a redshift of z~0.3 (which corresponds to a distance of 5.5 thousand million light-years). The proposed helium-neutron star scenario predicts the production of a modest amount of radioactive nickel that would lead to a weak supernova component, consistent with the observations. The resulting remnant could be a magnetar whose prolonged activity would explain the unusually long duration of the gamma-ray emission.

Even after many years of research, GRBs can still surprise us. In the same way that the classification of supernovae has become more diverse with time, that of GRBs might have to be revisited as well. It seems stars find many different ways to die.

[*] Although previously two GRBs seemed to show some evidence of this black-body-to-supernova transition.

	Download the movie of the simulated burst (43MB). Credit: S. Wiessinger, NASA.
Link to article in Nature

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Three non-consecutive frames from the ten-frame sequence covering the evolution of the ionised meteor train after the initial fireball. Frame (c) shows an apparent second train (top left) adopting the same shape as the first. Image sizes are 12.3°x8.3°.© 2011 Liverpool John Moores University (LJMU)

Geert Barentsen of Armagh Observatory recently used the LT to monitor the radiant of the Draconid meteor shower with RISE as the Earth flew through a particularly dense part of the meteor stream. While a sharp peak in the number of meteors was seen for a couple of hours by observers in Europe and Asia, the LT's 20°-field-of-view SkycamT also recorded many meteors, most notably a bright fireball resulting in a spectacular ionisation train which persisted for at least nine minutes.

This meteor burst and train on 8th October were recorded over ten frames, three of which are pictured at right. Frame (a) shows the meteor event itself, while frame (b), the third in the sequence, shows the debris cloud as 2-3 bright knots in the centre of the frame, drifting to the south west (arrowed).

Five minutes later in frame c, the train has elongated into a long n-shaped form (lower right). Interestingly, what might be the fainter train from another meteor burst two frames earlier has also evolved into the same shape (upper left arrow). The beginning of this second event is not shown in this sequence, but is visible in the composite picture below and in the animation (3.1 MB).

Composite image of all ten frames in the meteor train sequence captured by SkycamT. Note the smaller train which appeared halfway through the sequence, arrowed top left. Click image for bigger 61KB version.

A fullsize animated GIF of the sequence is available here (3.1 MB).

The 180° SkycamA, 20° SkycamT and 1° SkycamZ cameras are running continuously at 1 frame per minute on the telescope. SkycamZ data remains proprietary, but Skycams A and T provide valuable wide field and contextual data for any LT observers who want to make use of them. Though the data are available freely on-line we would encourage anyone accessing them to contact us so that we can better understand their usefulness to the community.

Further information:

• Skycam on the LT
• These fireball images featured in Sky and Telescope magazine
• Draconids 2011 Results
• International Meteor Organization

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