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First detection of a double caustic crossing in a microlensed quasar 4 Jul 2020
Einstein Cross
The Einstein Cross. The foreground galaxy is the object in the middle, the more distant quasar lies behind it. Lensed images of the quasar, labelled A-D, surround the foreground galaxy. This article concerns image "C". Image credit: 2020 Alexey Seregeyev (Einstein Cross image), J. Marchant (labels).

Some of the text in this article was adapted from an item in the "Gravitational Lenses" blog at the University of Cantabria website, and correspondence with the resulting paper's lead author Luis Goicoechea.

Astronomers have detected for the first time a double caustic-crossing in a microlensed quasar.

The collaborative project, between research teams in Russia, Spain, Ukraine and Uzbekistan, used the Liverpool Telescope and the 1.5m telescope at the Maidanak Observatory ("MT") in Uzbekistan to conduct a 14-year monitoring campaign (2006-2019) of the gravitationally-lensed quasar known as the "Einstein Cross" (see right).

The Einstein Cross is the name given to the quadruply-imaged gravitationally-lensed image of a single quasar that was discovered 35 years ago. It lies 8 billion light years away, directly behind a foreground spiral galaxy only 200 million light years away.

A quasar is an extremely luminous active galactic nucleus. At the centre of a galaxy sits a supermassive black hole weighing in at millions to billions of times the mass of the Sun, surrounded by an accretion disc of gas. As the gas falls in towards the black hole it emits huge quantities of electromagnetic radiation. Quasars have luminosities thousands of times that of a galaxy like our Milky Way.

Due to the fact that the Solar System, the foreground galaxy and the quasar are practically aligned, some light rays from the quasar that diverge slightly from that line and would otherwise miss the Earth are instead bent by the galaxy's gravity as they pass through its central bulge and reach the Earth after all.


Basic gravitational lensing. Light from a distant quasar is bent by the gravity of a foreground galaxy in the manner of a lens so that the light reaches Earth. From the Earth's viewpoint the incoming rays make it appear that there are multiple quasars surrounding the true position.
©2020 LT Group

The galaxy's gravity therefore acts like a "lens" bringing to Earth light that would have otherwise missed it. The tiny angular separation between the galaxy nucleus and the quasar, the galaxy's elliptical mass distribution, and the fact we're only looking at the tiny UV sources within the inner regions of the quasar's accretion disc, all combine to allow only four "bundles" of rays to make it to Earth. From here therefore we don't see extended arcs and rings but four distinct images of the same quasar surrounding the foreground galaxy (see diagram at left).

That's not the end of the story: these four bundles of rays travel through four different parts of the central bulge of the foreground galaxy, so they are therefore seen through densely populated stellar regions, and there is a high probability of detecting stellar gravitational effects, i.e. so-called microlensing effects. This is where the beams can be further affected by the gravitational fields of stellar populations in the galaxy itself as they move in and out of the beams. The gravity of individual stars in those regions can additionally bend and focus the quasar light even more, affecting the brightness of the image.

The microlenses (stars) affect the image of each source within the quasar's accretion disc to a different extent. More compact (hotter and bluer) sources suffer stronger effects. Additionally, due to the motions of the quasar, lens galaxy (and its stars) and observer, microlensing is a time-variable phenomenon. Therefore, a monitoring of the multi-wavelength microlensing-induced variability of the Einstein Cross can be used to probe the structure of its accretion disc.

Caustic map
Realistic microlensing magnification map for image C of the Einstein Cross. The trajectory of the quasar's accretion disc (yellow arrow) is only illustrative. An animated version is included in the new blog of the Glendama project at https://gravlens.unican.es/limaco/
©2020 Glendama team.

The areas of space where the foreground stars can affect the quasar's light in this way are called caustic regions. Within such a region the flux from the quasar is magnified so it appears brighter, while outside the caustic it is detectably fainter. The boundary between the two is called a caustic fold, and until now astronomers have only ever seen single-fold crossings, where the quasar images either enter or leave a caustic region. A complete traversal of a caustic region (example at right), with entry and exit marked by two fold crossings, had never been seen.

Fortunately, using a long-term multi-band photometric monitoring of the lens system with the LT and MT, the international research team reported the detection of the first double caustic-fold crossing (both the entry and exit of a caustic region) in image C of the Einstein Cross (see below). This confirms that UV continuum sources cross complex magnification maps as predicted by numerical simulations, and suggest that such sources belong to a standard gas disc model.

Caustic crossing graph
Double caustic crossing event in LT and Maidanak observatory data. Peaks mark caustic fold crossings. This is figure 3 in the A&A paper referenced in the text. ©2020 Glendama team

The project used over 4,000 frames taken by the LT and MT to monitor the double peaks (and higher plateau between them) of image C's brightness, characteristic of a caustic region crossing. The bright peaks are when the quasar image crosses the caustic fold surrounding the caustic region, while within the region the image is considerably brighter.

The monitoring campaign practically covered the full life of the LT so far (from 2006 to 2019). The associated paper has been recently published in the journal "Astronomy & Astrophysics" (A&A), and selected as an A&A Highlight in 2020.

Nearby stars imaged in 3D 24 Jun 2020

Images of Wolf 359 (arrowed) taken by the LT and New Horizons, 7 billion kilometers apart from each other. Dashed line and circle superimpose position of Wolf 359 from the LT's viewpoint into the New Horizons image. The resulting parallax is just under twenty arcseconds. © 2020 LT Group and JPL/NASA. (click for bigger version)

NASA recently released images of two of the nearest stars to the Sun, taken by its spacecraft "New Horizons" from its viewpoint in the outer reaches of the Solar System. From that position, 47 Astronomical Units (over 7 billion kilometres) from Earth, signals take over 6 hours to reach Earth, and the image data had to trickle across at speeds of less than 2kbps.

The two stars in question were Proxima Centauri, the nearest star to the Sun at 4.2 light years, and Wolf 359, 7.8 light years away. The former is only visible in the southern hemisphere, while Wolf 359 is more easily visible in the northern hemisphere.

This is the New Horizons Parallax Program, to compare the New Horizons images with similar ones taken at the same time from Earth. As these two stars are much closer than other stars in the background, there should be an obvious shift in apparent position, or parallax, of the foreground stars compared to the background stars.

Parallax

Parallax is easily demonstrated by holding an object at arm's length and looking at it, first with one eye, and then with the other. The object appears to shift position relative to the background which should be much further away than arm's length. If you could measure the apparent angular shift of the object, and knowing the distance between your eyes as the baseline between the two views, you could in theory calculate the length of your arm (though in practice it would be easier to just use a tape measure for this particular case).

Surveyors use the same technique (parallax, not a tape measure) to measure distances to remote landmarks, and astronomers have used this technique for nearly 200 years to also "survey" the nearby stars around the Sun. In this case however the baseline is the diameter of the Earth's orbit. By making one observation and then waiting half a year to make the other one, the Earth moves round to the other side of its orbit, creating a baseline of 2 Astronomical Units or 300 million km.

The parallax created in this way is tiny: the largest parallax, for Proxima Centauri, is just 0.8 arcseconds, and stars further out have correspondingly smaller parallaxes. Wolf 359's parallax is half that of Proxima Centauri's at just 0.4 arcseconds. This is about 1/5000 the diameter of the full Moon and absolutely not visible to the naked eye.

Proper motion

Astronomers around the world, professional and amateur alike, were encouraged by NASA to take images of the target stars, as close as possible in time (within a week) of the New Horizons images. Proxima Centauri is too far south to be observed by the LT, but Wolf 359 is an easy target. Our image above was taken on 26th March at 23:35 UT, some 88 hours after New Horizons'.

Why have a time constraint at all? The nearby stars especially actually move across the sky at small but measurable rates. All of the stars in the Milky Way (including the Sun and attendant planets) are orbiting Galactic centre, but all at slightly different velocities. The relative motion of nearby stars with respect to the Solar System, and with the Earth specifically, is called Proper Motion. Proxima Centauri is currently moving across the sky at the rate of 3.86 arcseconds a year, while Wolf 359 is slightly faster at 4.7 arcsec/year. At that rate, Wolf 359 could cover an angle the same size as the width of the full Moon in just under 400 years.

Therefore if the interval between the LT and New Horizons images was too large, the effect of proper motion would become apparent. Keeping the interval within a week kept the effect negligible. It's not negligible however when the interval is half a year, as in the case of measuring annual stellar parallax.

Measurement

The main picture at the top of the page shows the New Horizons and LT images of Wolf 359 (arrowed) and surrounding stars. Wolf 359's parallax is obvious as the star shifts between images.

The size of the parallax in the above images is measured approximately to be 15.6 arcseconds. The expected parallax is 15.9 arcseconds, based on the size of the Earth-New Horizons baseline, the distance from Earth to Wolf 359, and the angle between those two lines.

As big as the parallax appears in the image, it's still actually a tiny angle in human terms, and not visible to the naked eye. If the average interpupillary distance of the human adult represented the Earth-New Horizons distance, Wolf 359 would be 670 metres (2,200 feet) away. Experience shows it's not possible to observe an object that far away and see it in 3D against the background landscape.

Importance of the Parallax Program

This is the first ever obvious illustration of the shift in star positions as a natural consequence of interstellar flight. Not only that, it demonstrates the feasibility of using stars for autonomous navigation in interstellar space.

The parallax obtained in this way is also "pure" in the sense that, because the two images were taken simultaneously, the parallax is instantly right there in the image with no complicating effect of proper motion to take into account.

Finally, it's a nice demonstration of the sheer distance New Horizons has travelled since its launch in 2006. New Horizons is now over 47 AU from the Sun and exiting the Solar System at 13.9 km per second or 2.9 AU/yr. It's not going in the direction of Wolf 359, but if it were, at that speed it would get there in about 170,000 years.

New Python module for submitting observations via RTML 15 Apr 2020
Example Python code
The new ltrtml Python module allows users to create and submit RTML observing requests to the Liverpool Telescope in a purely pythonic way.

A new general purpose Python module for submitting observations to The Liverpool Telescope has been developed by Astrophysics Research Institute PhD student Kyle Medler.

With this module, users can now submit observations to the telescope from automated Python scripts. This is an alternative to using the fully featured PhaseII user interface, and is suitable for the most common observation modes. See the RTML page for more information on available instruments, features and how to apply for RTML access.

This module was created when Kyle Medler and Supervisor Professor Paolo Mazzali needed to automatically schedule transient follow-up observations from their pipeline. A couple of existing RTML methods to do this were investigated, but creating the new module enabled a simpler and more robust implementation without any Java dependecies or Python system calls.

Kyle was guided by Dr Doug Arnold, DevOps engineer for the Liverpool Telescope, in a short project to expand Kyle's software development skills with new concepts, whilst at the same time producing a usable library for LT users. Of the project Kyle said, "Being able to get into low level details of the Liverpool Telescope has been a great opportunity. I've developed my skills in Python language structure and features whilst creating this module and it's satisfying that it will be useful to astronomers using the Liverpool Telescope."

To obtain the module, along with instructions and example code, please see the ltrtml github repository.

Mercury Mission Flyby of Earth 10 May 2020
BepiColombo image
Artist's impression of BepiColombo flyby of Earth (click for full size version). Credit: ESA.

Last month the spacecraft BepiColombo swung by Earth on its way to the planet Mercury. BepiColombo is a joint mission between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA).

Launched in October 2018, the spacecraft spends seven years performing gravity-assist flybys past Earth, Venus and Mercury. These are to lose speed with respect to the Sun in order to go into orbit around Mercury itself in late 2025.

The first of the flybys occurred at 04:25 UTC on 10th April, when the spacecraft flew by Earth just 12,677 km above the South Atlantic. The flyby reduced BepiColombo's speed relative to the Sun by about 5 kilometres per second.

The LT was approached by the National Institute for Astrophysics (INAF), who provided several instruments for the mission, to observe BepiColombo while it was in the vicinity of the Earth. The LT would not observe the flyby on 10th April as it would only be 23° above the local horizon, just below the LT's 25° limit. It was decided to observe on subsequent nights when BepiColombo would be higher in the sky, albeit much further away and a lot fainter.

BepiColombo against starfield
LT images of BepiColombo (arrowed) on 15th and 19th April 2020.
©2020 LT Group

At right are LT images of BepiColombo taken on the nights of 15th and 19th April, when it was 2 million and 3.4 million km (5 and 9 Lunar distances) from Earth respectively.

BepiColombo is now 10 million km (26 Lunar distances) from Earth, en route to its next "braking" flyby manoeuvre at Venus in October. It's scheduled to arrive at Mercury in late 2025 where it will begin studying the planet's composition, geophysics, atmosphere and magnetosphere.

Links:

Equatorial outflows in the black hole transient Swift J1357.2-0933 16 Dec 2019
Artist's impression of the system.
© 2019 Gabriel Pérez, Servicio Multimedia (IAC)

Swift J1357.2-0933 is a black hole X-ray binary which shows transient behaviour, alternating long periods of quiescence with short (weeks long) and violent outbursts. These episodes are triggered by a sudden increase of mass accretion onto the black hole. The system was observed to go into outburst in 2017: the first such event since the outburst which led to its discovery in 2011. In a paper published recently in Monthly Notices of the Royal Astronomical Society, Jimenez-Ibarra et al. report high time resolution follow-up of the 2017 outburst.

In the paper, RISE light curves were combined with spectroscopy from the OSIRIS instrument on the 10.4m Gran Telescopio Canarias. The light curves show a series of dips up to 0.5 magnitudes deep and lasting around 2 minutes. The recurrence time of these dips gradually increases as the outburst evolves. Similar events were observed during the 2011 outburst. Spectra obtained during the dips show broad and blue shifted Balmer and He II absorption lines. The interpretation is that the dips are formed in a dense and clumpy outflow, produced near the disc equatorial plane and seen at high inclination.

The authors conclude that the detection of dips in Swift J1357.2-0933 may be favoured by its high orbital inclination (i.e. near edge-on geometry), and as such the clumpy equatorial wind might not be a peculiarity of this particular system but, rather a common phenomenon of accreting stellar-mass black holes in outburst. The study shows the power of combining simultaneous or quasi-simultaneous high time resolution photometric and spectroscopic observations, and is an example of the type of observing campaign which could be pursued entirely robotically when the 4-metre New Robotic Telescope joins the LT at the Observatorio del Roque de los Muchachos in the middle of the next decade.

The full paper can be found here: https://arxiv.org/pdf/1908.00356.pdf

New Exposure Time Calculators 28 Nov 2019
The new ETC for imaging with the LT. A similar ETC for spectroscopy is on the same page.

New Exposure Time Calculators (ETCs) for the LT have been installed on the website at the Exposure Time Calculator page.

Between the two ETCs (one for imaging, the other for spectroscopy), existing and prospective users can answer questions on what exposure times are necessary to achieve a required signal to noise ratio. Users can select any of the many instruments mounted on the LT and adjust their settings, as well as the effect of atmospheric turbulence ("seeing") and background sky brightness.

The original idea to upgrade the ETCs' usability with Google Charts was Dr Marco Lam's, who is Scientific Software Developer for the LT. The new ETCs were developed by Lam and Dr Doug Arnold, DevOps Engineer for the New Robotic Telescope.

"Interfaces are becoming more web based, and the new exposure time calculator, using Google Charts and extended JavaScript functionality, has really improved the usability of this tool" said Dr Arnold.

Feel free to use the ETCs for yourself and discover how the LT can obtain images and spectra of your target:

A Milestone Gamma Ray Burst Study: GRB190114C 26 Nov 2019

(adapted from LJMU press release)

Hubble Space Telescope image of gamma-ray burst afterglow (circled) in its host galaxy. Credit: NASA, ESA, V. Acciari et al 2019.

Liverpool John Moores University astrophysicists and the Liverpool Telescope contributed to a study published in Nature recently of a gamma-ray burst caused by the collapse of a massive star 5 billion light years away.

Analysis of the minutes immediately after the burst reveals emission of photons a trillion times more energetic than visible light.

“These are the highest energy photons ever seen from a gamma-ray burst,” stated Dr Daniel Perley, a senior lecturer at LJMU's Astrophysics Research Institute involved with the study.

Gamma-ray bursts are the most powerful explosions in the Universe, emitting more energy within seconds than the Sun provides in its entire life cycle. Most of this energy is in the form of gamma-rays, a form of electromagnetic radiation much more energetic than visible light or even the X-rays that are used in medicine.

On January 14 this year, an extremely bright and long gamma-ray burst, known as GRB 190114C, was detected by a suite of telescopes, including NASA’s Swift and Fermi satellites in space as well as the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes and the ARI-operated Liverpool Telescope on the ground.

It is believed the burst is produced when material is expelled from a collapsing star at virtually the speed of light. This material collides with gas that surrounds the star, causing a powerful shock wave. The new study attributes this shock wave as the source of the ultra-high-energy photons, which were detected by MAGIC for a period lasting almost an hour following the initial explosion of the burst. The LT simultaneously observed the source in optical light.

Scientists have been searching for this type of signal for many years, so this detection is considered a milestone in high-energy astrophysics.

Dr Perley added: “Gamma-ray bursts have been known about for decades but many aspects of them are still a mystery. How can a single explosion produce so much energy so fast? So, these new results help us understand what really happens under these extreme conditions.”

Dr Perley was accompanied in the publication by his PhD student Allister Cockeram as part of a team of more than 100 astronomers across the globe.

Andrew Levan of the Institute for Mathematics, Astrophysics & Particle Physics at Radboud University in the Netherlands, said: “The observations suggest that this particular burst was sitting in a very dense environment, right in the middle of a bright galaxy 5 billion light years away. This is really unusual, and suggests that might be why it produced this exceptionally powerful light.”

In addition to the Nature study, a further paper based on measurements from the event has also been submitted by the Liverpool Telescope Gamma-Ray Burst team in the Astrophysical Journal for publication soon. In this paper, a team of scientists currently or formerly based at Liverpool John Moores University and their students, including PhD student Jordana Mitjans at University of Bath and Prof Shiho Kobayashi at LJMU, used the Liverpool Telescope data from this burst to study the properties of the shock-wave in detail during the critical first minutes after the explosion.

A third paper, prepared by Antonio de Ugarto Postigo of the IAA-CSIC in Granada, Spain with contributions by Perley and others, studies the nature of the distant galaxy in which the burst exploded.