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Tracking the JWST 21 Jan 2022

Artist's impression of JWST.

It's well-known that the LT is designed to observe transient natural sidereal phenomena (e.g. supernovae, gamma-ray bursts, optical counterparts of gravity wave events) and non-sidereal targets (near-Earth asteroids and comets). However, it can also track some artificial satellites and spacecraft too, one example being the Gaia space observatory. Since its launch in 2013, Gaia has been tracked by the LT to monitor its position accurately in its halo orbit about a special position in space known as the second Sun-Earth Lagrange point (L2).

This position is one of five Lagrange points in the Sun-Earth co-rotating orbital plane where the gravitational forces of both bodies balance each other. A spacecraft placed at any of them needs very little orbit correction fuel to maintain its position relative to Earth. L2 lies ~1.5 million kilometres "behind" the Earth as seen from the Sun, about four times further away than the Moon. That area is becoming an increasingly-preferred location for space observatories. From L2 (or a halo orbit around it) both Earth and Sun appear close together, so it's much easier to thermally shield temperature-sensitive telescopes from both bodies.

A few weeks ago one of the most exciting telescope-related events of 2021 was the launch of the James Webb Space Telescope (JWST). It began its journey with an accurate launch into L2 transfer orbit at 12:20UT on 25th December 2021, and was observed by the LT that night. By then JWST was 139,000km above the Atlantic Ocean, and still moving at over 2.1 km/s relative to the Earth's centre.

Top: The JWST is the streak near the centre of this 60-second LT image taken early 26th December 2021. Bottom: Animation of ten 10-second LT images of JWST, taken on 11th January 2022. See text for more details of both images. Credit: LJMU/LT/J.Marchant

The LT imaged JWST again on 11th January 2022 in a sequence of ten 10-second images. By then it was 1.14 million kilometres away and considerably slower, moving at just ~300 metres/second. Both image sets featured here were taken using the LT's IO:O instrument, the facility's main workhorse imager.

JWST is a revolutionary telescope, and astronomers and engineers across the world have waited patiently for its completion and launch for many years. Because they're located outside Earth's atmosphere, space observatories like the Hubble Space Telescope (HST) and JWST avoid the filtering and turbulent effects of the atmosphere and obtain the best view of the Universe for a telescope of that size.

The HST with its 2.4-metre primary mirror has provided astronomers with incredibly deep images of distant galaxies, but JWST will have a 6.5 metre mirror, capable of catching more photons and therefore probing even further into the depths of our Universe.

The JWST will look back over 13.5 billion years at our Universe in optical and infrared wavelengths and explore the formation of the first stars and galaxies. As JWST is designed to allow for measurements at infrared wavelengths, it is perfectly primed to see through the dust enshrouding many objects in the Universe such as stars and planetary systems. JWST will also explore distant exoplanet systems and objects within our own Solar System.

At time of writing, JWST has successfully deployed its sunshield and primary mirror, and is on course for starting its manoeuvre on 23rd January to enter L2 halo orbit. This rocket burn corrects any residual trajectory errors and adjusts the final L2 orbit. Due mainly to the accuracy of the launch however, less propellant will be needed for midcourse corrections than expected. The extra fuel remaining can therefore be used for stationkeeping and momentum management/attitude control instead, effectively extending JWST's stay at L2 from its original estimate of maybe 10 years to perhaps as much as 20 years.

Liverpool Telescope Unveils a New Type of Cosmic Explosion, Possibly Linked to Black Hole Formation 12 Jan 2022

Hubble Space Telescope image of the star WR 124, surrounded by hot clumps of its own outer atmosphere ejected into space by fierce stellar winds over the last ten thousand years. This star is similar to those which created the supernovae SN 2021csp and SN 2019hgp mentioned in the text. Credits: original image ©1998 NASA/ESA, reprocessed version ©2015 Judy Schmidt.

Liverpool Telescope observations have helped to unveil a previously unknown class of cosmic explosion.

Exactly what happens to the most massive stars at the end of their lives has long been mysterious. From observations of stellar populations within our own Milky Way galaxy and its neighbours, we know that many such stars lose their outer hydrogen layers to become Wolf-Rayet stars — very hot and luminous stars that are rapidly shedding material into space in high-velocity winds. Some models predict that such stars should eventually collapse to form black holes, but observational evidence of this has so far been lacking.

In work recently published in Nature and soon to be published in the Astrophysical Journal [preprint], a team of astronomers used the Liverpool Telescope (LT) alongside other facilities worldwide to identify a new class of supernova — dubbed type "Icn" — that may reveal this transition.

The first such event (known as SN 2019hgp) was discovered in 2019 by the Zwicky Transient Facility (ZTF), a survey telescope in California. LT astronomers were able to obtain follow-up imaging observations (with IO:O) and spectroscopy (with SPRAT) within a day of discovery, giving crucial insight into the early phases of the explosion. The spectrum of the supernova was dominated by narrow lines of highly-excited carbon, oxygen, and neon — a combination never seen before in any cosmic transient. The properties of these lines suggested that material ejected at high velocities by a dying star had slammed into a dense sphere of carbon and oxygen rich material, much as one would expect from the explosion of a Wolf-Rayet star. The study of this event has recently been published in Nature by a team led by Avishay Gal-Yam, a scientist at the Weizmann Institute for Science in Israel.

The second event (known as SN 2021csp) was discovered in February 2021, also by ZTF. The Liverpool Telescope was the first facility on the scene after discovery, again acquiring SPRAT and IO:O observations that showed a fast and luminous supernova with strong, narrow carbon and oxygen lines — much like in SN 2019hgp. This provided the impetus for an LJMU-led team to quickly obtain crucial ultraviolet observations from the Hubble Space Telescope before the source faded away, providing even stronger evidence in favor of a Wolf-Rayet like progenitor. Additionally, continued observations from the Nordic Optical Telescope (one of the Liverpool Telescope's "neighbours" on the island of La Palma) showed the explosion to fade away almost to nothing within just two months — quite unlike normal supernovae, which take years to fade. The team interpreted this as evidence that most of the star's mass had collapsed into a black hole, rather than being ejected into space. This study was led by Daniel Perley, a staff member at LJMU's Astrophysics Research Institute, and has recently been accepted by the Astrophysical Journal [preprint].

Together, these two events suggest a new scenario for the fates of the most massive stars: they may produce a special kind of fast and fleeting supernova dominated by interaction between a small amount of material ejected outward early in the collapse and the pre-existing Wolf-Rayet stellar wind.

Type Icn supernovae are rare events and cannot on their own represent the deaths of all Wolf-Rayet stars. However, luminous events like SN 2019hgp and SN 2021csp may be just the tip of the iceberg: "If a similar explosion occurred, but the material was expelled more slowly or if the Wolf-Rayet wind was weaker, we would never know it even happened," points out Dr Perley.

Future, more sensitive surveys — such as with the upcoming Vera Rubin Observatory coupled to follow-up facilities like the New Robotic Telescope — will have a chance of unveiling such "hidden" transients in the coming years.

Monitoring Maintenance in Geostationary Orbit 3 Nov 2021

Artist's impression of MEV-1 (silver satellite at right) docked with Intelsat-901 (gold satellite). Credit: Northrop Grumman / Space Logistics.

For the first time, remote-controlled spacecraft have begun servicing communications and Earth observation satellites in geostationary orbit to extend the amount of time they can remain in service.

Geostationary Earth Orbit (GEO), a circular equatorial orbit at an altitude of 35,786km, allows a satellite placed there to remain apparently fixed over a point on the Earth's surface. Being very useful for communications, navigation and Earth observation, this orbit has become home to hundreds of satellites from many nations over the decades. These "GEOsats" use onboard propellant for attitude control and to maintain their exact position in orbit ("stationkeeping"). Over the years this fuel runs out, and so using the last dregs, the otherwise still-functional satellite is consigned to a "graveyard orbit" some 300km higher than GEO, where it is switched off.

In recent years however it has become feasible to start sending servicing spacecraft to rendezvous with near-empty GEOsats, and perform on-orbit servicing and mission extension activities, such as taking over stationkeeping duties. Many companies are now beginning to offer these services.

The first of these companies to launch a servicing mission was SpaceLogistics, a wholly-owned subsidiary of Northrop Grumman. It began operating its Mission Extension Vehicle (MEV) fleet of commercial on-orbit servicing spacecraft in 2019-2020.

Once launched into an initial elliptical orbit, a MEV would spend months using its electric propulsion system near-continuously to raise and circularise its orbit to match that of its target client satellite. After some time in rendezvous and proximity operations (RPO), a MEV would use telerobotics to dock with the satellite. It would then use its own propulsion system to take over manoeuvering and attitude control duties, extending the satellite's functional lifetime for however long the client wished. The MEVs have enough fuel to service several satellites in this way, and can move from one satellite to another as and when client contracts dictate.

The first two missions were monitored by a coalition of optical and radio ground-based facilities. Tasks included determining MEV/client detection and tracking capabilities, and maintaining continuous coverage by autonomously networking time-critical information between observing stations.

IO:O image of MEV-2 and Intelsat 10-02, 1.5km apart, during approach and docking.
Credit: LJMU, UoL.

The first MEV mission in 2019-2020 saw "MEV-1" return Intelsat 901 back to active service. This satellite had been dormant for some time in graveyard orbit. MEV-1 repositioned Intelsat 901 back into geostationary orbit, where it will remain in service for several years until completion of contract. MEV-1 will then put it back where it came from and move on to the next client. Lessons learned during observations of this mission were applied to those of the "MEV-2" mission in 2020-2021.

The MEV-2 mission saw the first servicing of an active satellite in GEO, Intelsat 10-02. MEV-2's orbit-raising manoeuvres to GEO were followed closely by the coalition, which for this mission included the LT using its IO:O hi-res camera. The LT became involved through a request from the University of Liverpool (UoL) who had been involved with the coalition for some time. IO:O also provided imagery for the RPO phase of the mission, where MEV-2 circled Intelsat 10-02 at close quarters and then docked. Observations by the LT and others in the coalition also proved that during RPO the two satellites could still be separated in the data using spectroscopy and polarimetry.

These observations are described in detail in a recent paper by George, S., et al: "PHANTOM ECHOES 2: A Five-Eyes SDA Experiment on GEO Proximity Operations", Proceedings of Advanced Maui Optical and Space Surveillance Technologies Conference (AMOS) Conference 2021 [download paper].

At the same time, a separate team used the LT's MOPTOP polarimeter to also observe the spacecraft in polarised light, both before and after docking. The team, led by Klaas Wiersema and Paul Chote of University of Warwick Astronomy and Astrophysics Group found the two spacecraft could be well separated in the data. Observations continued for several weeks after docking so that the polarimetric signature of the "combined stack" could be compared to those of the separate spacecraft previously.

Their data shows the polarisation of MEV-2 and Intelsat 10-02 does indeed differ after docking. The team will now compare the measured polarisation values as a function of viewing angle, with detailed numerical models of the reflection off the flat solar panels and main body. Comparing them to the known orientation of the satellites will determine if polarisation observations can indeed provide independent measurements of spacecraft orientations.

David Carter 24 May 2021
Dave Carter at the LT construction site around 2001.

We are sad to report the death over the weekend of our friend and colleague Professor David Carter. Dave obtained his PhD from the Institute of Astronomy, Cambridge in 1977 working on the surface brightness profiles of galaxies. He subsequently worked at the University of Oxford and the Anglo-Australian, Mount Stromlo and Siding Spring Observatories, before moving on to the Isaac Newton Group on La Palma and at RGO Cambridge where he combined his research with involvement in cutting edge instrumentation.

Dave joined LJMU in 1996 as Project Scientist for the Liverpool Telescope and his determined efforts played a large part in keeping the project on track during a difficult construction phase. He was an outstanding scientist who always took a constructively sceptical approach to the prevailing consensus, and an excellent mentor to younger colleagues and students. He was also a keen cricketer!

Dave took early retirement from LJMU in 2012 but continued to work on the major HST/ACS Coma Cluster survey he established as well as being a regular participant in group seminars and journal clubs. This also gave him the opportunity to step up his work at the Neston Methodist Church and Community Centre and become a local parish councillor. Dave was a great family man, and we send our condolences to his wife and three sons at this difficult time. He will be sadly missed by all.

Walk around the LT site 25 Mar 2021

A scene from the site tour, featuring navigation icons, the site annexe/workshop on the left, and the telescope enclosure on the right. Click the image to go to the Site Tour page.

A virtual tour of the Liverpool Telescope site in La Palma is on this website at the Site Tour page. You can walk around the site, even into the telescope enclosure itself, and switch between day and night views.

Liverpool John Moores University's Press Office approached the LT group with a 360° camera and the suggestion we could borrow it to promote the telescope in any way we liked. We decided to use the camera to make a "Google Streetview" style tour around the site.

The images were made with a RICOH Theta S camera during a site maintenance trip in July 2019. They were processed into a tour using the Holobuilder interface (an adapted version of which also hosts the tour) and released on the website in October 2020. It's planned to integrate this tour into Google Streetview in the future.

Two new nova shells discovered 5 Mar 2021

Artist's impression of classical nova. Hydrogen spiralling onto the white dwarf's surface from the red giant has just ignited. See text for details.
Image credit: David A. Hardy www.astroart.org

The expanding debris shells from two separate novae that were seen to erupt decades ago, have been discovered and characterised in the recent paper "Two new nova shells associated with V4362 Sagittarii and DO Aquilae", Harvey et al, Monthly Notices of the Royal Astronomical Society (MNRAS), Vol 499, Issue 2, December 2020 (doi: 10.1093/mnras/staa2896).

Because these nova shells are big enough to make out their structure, they are rare objects and of great interest to astronomers. That's because analysis of the debris shells' shapes can reveal insights into the mechanics of the nova eruptions that created them, and also shed light on conditions leading up to the event.

##### Classical novae

Classical novae occur in close stellar binary systems, with one star a white dwarf and a main sequence star as its companion. The stars are close enough that hydrogen from the companion's outer layers can transfer over to the white dwarf and spiral down to its surface, forming an accretion disk as it does so (see illustration at right).

The infalling hydrogen forms a shallow hydrogen atmosphere at tremendous pressure and temperature. Over time as more hydrogen builds up, pressures and temperatures in the lower layers build up to the point where a runaway thermonuclear reaction starts. The bottom of the white dwarf's atmosphere ignites; heat released from the hydrogen fusing to helium raises the temperature further, increasing the fusion rate and driving the reaction more. This is the third most energetic stellar explosion in nature, and the energy release blows out the remaining atmosphere at speeds of 1000 km/s or more.

This debris of unburnt hydrogen and thermonuclear reaction products flies away from the binary system as an expanding shell. However, rather than being a perfect sphere, the shells that can be made out from Earth take the form of an axisymmetric set of zones. Usually the pattern is one of an equatorial belt and polar cones, but often more complex shapes are formed.

##### Rare specimens

Classical novae are actually non-destructive — both stars survive the event. This leaves them in place for the accretion process to begin again, potentially leading to another nova many years later. It's thought about 50 novae occur every year in the Milky Way galaxy alone, though fewer are actually observed due to intervening dust and gas obscuring the view.

Novae have also been observed in other galaxies, identified as such by their lightcurves and spectra, but none are close enough to make out any shells they might have. Of the few hundred novae in the Milky Way observed over the years, of order only 50 have discernible shells, so only that many sources of extra clues to the inner workings of novae have been observed and characterised. Any addition to this rare subgroup is therefore noteworthy, and the two new shells discussed in the paper by Harvey at al certainly qualify.

##### Observations

The team selected 12 reasonably bright nova systems that erupted more than 15 years before the start of the survey in 2015. Those regions had previously been scanned by the Wide-Field Infrared Survey Explorer (WISE) space telescope as part of its all-sky survey, and showed plausible hints of nebulosity. They then performed deeper narrowband imaging using the 3.2m Aristarchos telescope in H-alpha plus NII (6578A) and OIII (5011A) bands. Shells were found around two systems, V4362 Sagittarii and DO Aquilae.

The Liverpool Telescope's SPRAT spectrograph was used to apply velocity constraints and first-pass nebular analysis of the shells. Hi-resolution spectra were obtained of the V4362 Sgr nova shell using the second version of the Manchester Echelle Spectrograph, installed on the San Pedro Mártir Observatory 2.1m telescope.

DO Aquilae

Narrowband images of DO Aquilae and V4362 Sagittarii taken in 2016 & 2017 by Harvey et al using the Aristarchos 3.2m telescope. This image adapted from Figures 3 & 4 in the paper. Click image for bigger version.

DO Aquilae (DO Aql) was observed to go nova in 1925. The observations in this study, made in 2015 & 2017, reveal a previously undiscovered nova shell, expanding at a rate of about 0.07 arcseconds/year. The team estimate this system to be 6.7±3.5 kiloparsecs (22±11 thousand light years) distant.

V4362 Sagittarii

V4362 Sagittarii (V4362 Sgr) was discovered in 1994. Observations made two months later of polarised light emitted by the shell suggested it was axisymmetric, and possibly consisting of a circular equatorial ring and narrow conical polar caps.

Harvey et al processed data from the 1.3m Skinakas Telescope in Crete made in 2006, and found the nova shell back then to be 2.5x3.1 arcseconds in size, in Halpha+NII narrowband imagery. The 2016 data obtained with the Aristarchos telescope gave dimensions as 5.2x5.6" in Hα+NII and 5.2x5.5" in OIII, implying an expansion rate of 0.32 arcseconds/year.

The probable inclination of the system is 70-80°, based on lightcurves taken in 2018 with the RISE2 instrument on the Aristarchos telescope that suggest the system is eclipsing. If eclipses are observed then the system is probably being viewed nearly edge-on to the plane of its orbit. However, eclipses don't always mean edge-on viewing, so more detailed observations of this system are needed to make sure.

Spectral measurements show the shell velocity is relatively slow at just 350 km/s, rather than the average 1000 km/s. Coupling that with the observed increase in angular diameter over a known period of time and taking due notice of the shell's non-sphericity (the expansion parallax method), the distance to the system is found to be just $$0.5_{-0.2}^{+1.4}$$ kiloparsecs (between 1-6 thousand light years), making it one of the closest and brightest nova shells known.

Detail adapted from Figure 12 of the paper, showing a model of the shell's emission in the NII and OIII lines, see text for more details. The full figure shows the shell in other wavelengths and viewing angles. Click image for bigger version.

The shell of V4362 Sgr is poorly resolved in the Aristarchos imagery, so the team tried to visualise its shape and dynamics. They settled on a morphology of an equatorial belt, tropical rings and polar cones. The image at right shows a pseudo-3D photoionisation model of the shell's structure seen from an angle 10° above the orbit plane. There are two views, showing the shell in the light of singly-ionised nitrogen ("NII") at 6584Å, and doubly-ionised oxygen ("OIII") at 5007Å. The model replicates the ratio of NII to OIII emission observed. Note the NII emission structure is larger than the OIII, which is often seen in nova shells. The NII structure's more extended polar caps in this model may be why the real thing is so bright at these wavelengths.

##### Summing up

This paper shows that new nova shells can be found from archive data and new limited multi-epoch followup data from small to medium-sized research telescopes — two previously unknown nova shells were discovered and characterised in this way.

There are potentially very many nova shells remaining to be discovered, because (a) so far only 10% of the observed nova system population of the Milky Way galaxy are known to have shells, and (b) it should be possible to detect and determine the structure of more shells if larger aperture and/or space-based telescopes are used.

Because the processes at play during nova events leave their fingerprints in the shell's structure, and nova systems previously thought to be shell-less might actually harbour shells after all, the authors would like there to be more deep followup observations of historical novae. Untangling the geometry of new shell structures would lead not only to a better understanding of the geometry, ionisation conditions and abundances of chemicals in the nova systems, but also reveal information on the chemical enrichment of the interstellar medium through elements created during the thermonuclear process in the nova events themselves.

Broad Insterstellar Absorption Band Discovered in Optical Region 17th November 2020

One of the many objects featured in the analysis that discovered the new interstellar absorption band: NGC 2024-1 (arrowed) is a young stellar object in the Flame Nebula. It lies on the boundary between the bright ionised hydrogen region and the dark dust lane that completely blocks its optical emission towards the East. Image from Figure 7 in the paper.

A broad absorption band in the interstellar medium has been discovered recently in the optical range. This is the first time a band of this type has been found in the visible part of the spectrum.

The broad interstellar band (BIB) is centered close to 7700Å with a full width at half maximum of 177Å. It is significantly wider than the numerous diffuse interstellar bands (DIBs) in the optical range that have widths of a few tens of angstroms at most.

Full details are in the paper "Galactic extinction laws: II. Hidden in plain sight, a new interstellar absorption band at 7700 Å broader than any known DIB" by Jesus Maíz Apellániz et al, in Monthly Notices of the Royal Astronomical Society, 25 August 2020. A press release at the Spanish National Research Council (CSIC) Centro de Astrobiologica can be found here.

The BIB remained undiscovered until now because it lay in the same wavelength region as a strong "telluric" (Earth-generated) absorption feature caused by oxygen molecules in the Earth's atmosphere.

It was discovered during the creation of a library of spectra obtained by the Hubble Space Telescope (HST). Being outside the atmosphere, the HST spectra were unaffected by telluric effects and the new band was therefore more readily detected.

Further detections soon followed. The authors looked at data previously taken by ground-based telescopes because, as Maíz Apellániz himself noted, "once we knew where to look it was not difficult to find the absorption band in ground-based data". They also took new data with the HST and the Liverpool Telescope's FRODOspec spectrograph.

They found that the absorbing material (the "carrier") causing the BIB is generally ubiquitous in the low/intermediate density regions of the interstellar medium. However, it appears to be strongly depleted in the clouds containing a lot of molecular carbon, in young star-forming regions such as those where O and B-type stars are usually located.

Most of the molecular carbon in these star-forming regions has been destroyed by the intense ultraviolet (UV) from the host stars. The clouds however are shielded from the UV and contain a lot of molecular carbon. The BIB band is not present in sightlines passing through these clouds, but is present in lines passing through the UV-cleared regions.

Future work in this field will involve looking at a large sample of interstellar absorption lines to produce a better understanding of how different regions of the interstellar medium filter and absorb the starlight passing through it.

First detection of a double caustic crossing in a microlensed quasar 4 Jul 2020

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.

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.

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/

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.

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

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

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.

LT images of BepiColombo (arrowed) on 15th and 19th April 2020.

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.

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

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.