Topic: Late September Astronomy Bulletin

[Clive]

METEOR IMPACT FLASH AT JUPITER
RAS

Analysis of a bright flash in Jupiter's atmosphere observed by an amateur astronomer in August 2019 has revealed that the likely cause was a small asteroid with a density typical of stony-iron meteors. The impact is estimated to have released energy equivalent to an explosion of 240 kilotons of TNT -- around half the energy released in the 2013 Chelyabinsk event on Earth. Ethan Chappel from Cibolo Texas captured a short flash of light at 04:07 UTC on 7 August in video observations of Jupiter using a small telescope in his back yard. The flash lasted for about 1.5 seconds and, at its peak, appeared as bright as Jupiter's moon Io. Chappel continued his observations for the next half hour without knowing he had been the only witness of a planetary collision. Once indoors, Chappel analysed the video data using DeTeCt, an open-source software specially designed to identify impacts in Jupiter. On finding a clear image of a flash in one of the videos, he quickly got in touch with the developers of the DeTeCt project who in turn contacted their large network of amateurs to see if any other
detections had been made. These detections are extremely rare because the impact flashes are faint, short and can be easily missed while observing the planets for hours. However, once a flash is found in a video recording it can be analysed to quantify the energy required to make it visible at a distance of 700 million kilometres.

It is estimated from the energy released by the flash that the impactor could have been an object around 12-16 metres in diameter and with a mass of about 450 tons, that disintegrated in the upper atmosphere at an altitude of about 80 kilometres above Jupiter's clouds. Models of the light-curve for the flash suggest the impactor had a density typical of stony-iron meteors, indicating that it was a small asteroid rather than a comet. With six impact flashes observed in ten years since the first flash was discovered in 2010, scientists are becoming more confident in their estimates of the impact rate of these objects in Jupiter. Most of those objects hit Jupiter without being seen by observers on Earth. However, it is now estimated that 20-60 similar objects impact with Jupiter each year. Because of Jupiter's  large size and gravitational field that impact rate is ten thousand times larger than the impact rate of similar objects on Earth.

 
COMET IS PROBABLY INTERSTELLAR VISITOR
NASA

A newly discovered comet has excited the astronomical community because it appears to have originated from outside the Solar System. The object - designated C/2019 Q4 (Borisov) - was discovered on 2019 Aug 30 by Gennady Borisov at the MARGO observatory in Nauchnij, Crimea. The official confirmation that comet C/2019 Q4 is an interstellar comet has not yet been made, but if it is interstellar, it would be only the second such object detected. The first, 'Oumuamua, was observed and confirmed in October 2017.  The new comet, C/2019 Q4, is still inbound toward the Sun, but it will remain farther than the orbit of Mars and will approach no closer to Earth than about 300 million kilometres. After the initial detections of the comet, Scout system, which is located at NASA's Jet Propulsion Laboratory in Pasadena, California, automatically flagged the object as possibly being interstellar. The comet is currently 420 million kilometres from the Sun and will reach its closest point, or perihelion, on 2019 Dec. 8, at a distance of about 300 million kilometres. The comet's current velocity is high, about 150,000 kph, which is well above the typical velocities of objects orbiting the Sun at that distance. The high velocity indicates not only that the object probably originated from outside our solar system, but also that it will leave and head back to interstellar space.

Currently on an inbound trajectory, comet C/2019 Q4 is heading toward the inner solar system and will enter it on Oct. 26 from above at roughly a 40-degree angle relative to the ecliptic plane. That's the plane in which the Earth and planets orbit the Sun. C/2019 Q4 was established as being cometary by its fuzzy appearance, which indicates that the object has a central icy body that is producing a surrounding cloud of dust and particles as it approaches the Sun and heats up. Its location in the sky (as seen from Earth) places it near the Sun -- an area of sky not usually scanned by the large ground-based asteroid surveys or NASA's asteroid- hunting NEOWISE spacecraft. C/2019 Q4 can be seen with professional telescopes for months to come. The object will peak in brightness in mid-December and continue to be observable with moderate-size telescopes until April 2020. After that, it will only be observable with larger professional telescopes until October 2020. Observations by the University of Hawaii indicate the comet nucleus is somewhere between 2 and 16 kilometres in diameter. Astronomers will continue to collect observations to characterize further the comet's physical properties (size, rotation, etc.) and also continue to improve knowledge of its trajectory.


AGE-OLD DEBATE ON SATURN'S RINGS REIGNITED
RAS

A team of researchers has reignited the debate about the age of Saturn's rings with a study that dates the rings as most likely to have formed early in the Solar System. The researchers suggest that processes that preferentially eject dusty and organic material out of Saturn's rings could make the rings look much younger than they actually are. Cassini's dive through the rings during the mission's "Grand Finale" in 2017 provided data that were interpreted as evidence that Saturn's rings formed just a few tens of millions of years ago. Gravity measurements taken during the dive gave a more accurate estimate of the mass of the rings, which are made up of more than 95% water ice and less than 5% rocks, organic materials and metals. The mass estimate was then used to work out how long the pristine ice of the rings would need to be exposed to dust and micrometeorites to reach the level of other "pollutants" that we see today. For many, this resolved the mystery of the age of the rings. The rings are made of particles and blocks ranging in size from metres down to micrometres. Viscous interactions between the blocks cause the rings to spread out and carry material away like a conveyor-belt. That leads to mass loss from the innermost edge, where particles fall into the planet, and from the outer edge, where material crosses the outer boundary into a region where moonlets and satellites start to form. More massive rings spread more rapidly and lose mass faster. The models show that whatever the initial mass of the rings, there is a tendency for the rings to converge on a mass measured by Cassini after around 4 billion years, matching the time-scale of the formation of the Solar System.  From our present understanding of the viscosity of the rings, the mass measured during the Cassini Grand Finale would be the natural product of several billion years of evolution, which is appealing.

Admittedly, nothing forbids the rings from having been formed very recently with this precise
mass and having barely evolved since. However, that would be quite a coincidence. The team announced the results in October 2018 from Cassini's Cosmic Dust Analyzer, which showed 600 kilogrammes of silicate grains fall on Saturn from the rings every second. Other studies using data from the Cassini Ion and Neutral Mass Spectrometer have shown the presence of organic molecules in Saturn's upper atmosphere that are thought to derive from the rings. These results suggest that the rings are cleaning themselves of pollutants. The nature of this potential ring-cleaning process is still mysterious. However, the study shows that the exposure age is not necessarily linked to the formation age, thus the rings may appear artificially young.


RARE MOLECULE IN DUST ORBITING YOUNG STAR
University of Leeds

Astronomers using one of the most advanced radio telescopes have discovered a rare molecule in the dust and gas disc around a young star -- and it may provide an answer to one of the conundra facing astronomers. The star, named HD 163296, is located 330 light years from Earth and formed over the last six million years. It is surrounded by a disc of dust and gas -- a so-called protoplanetary disc. It is within such discs that young planets are born. Using a radio telescope in the Atacama Desert in Chile, researchers were able to detect an extremely faint signal showing the existence of a rare form of carbon monoxide -- known as an isotopologue (13C17O). The detection has allowed an international collaboration of scientists to measure the mass of the gas in the disc more accurately than ever before. The results show that disc is much more 'massive' than previously thought. The scientists' conclusions are well timed. Recent observations of protoplanetary discs have perplexed astronomers because they did not seem to contain enough gas and dust to create the planets observed.  The disc-exoplanet mass discrepancy raises serious questions about how and when planets are formed. However, if other discs are hiding similar amounts of mass as HD 163296, then we may just have underestimated their masses until now. We can measure disc masses by looking at how much light is given off by molecules like carbon monoxide. If the discs are sufficiently dense, then they can block the light given off by more common forms of carbon monoxide -- and that could result in scientists underestimating the mass of the gas present. This study has used a technique to observe the much rarer 13C17O molecule -- and that has allowed us to peer deep inside the disc and find a previously hidden reservoir of gas.


MOST MASSIVE NEUTRON STAR DETECTED
Green Bank Observatory

Neutron stars -- the compressed remains of massive stars gone supernova -- are the densest "normal" objects in the known Universe. (Black holes are technically denser, but far from normal.) Just a single sugar-cube worth of neutron-star material would weigh 100 million tons here on Earth. Though astronomers and physicists have studied and marvelled at these objects for decades, many mysteries remain about the nature of their interiors: Do crushed neutrons become "superfluid" and flow freely? Do they breakdown into a soup of subatomic quarks or other exotic particles? What is the tipping point when gravity wins out over matter and forms a black hole? Researchers, members of the NANOGrav Physics Frontiers Center, discovered that a rapidly rotating millisecond pulsar, called J0740+6620, is the most massive neutron star ever measured, packing 2.17 times the mass of our Sun into a sphere only 30 kilometers across. That measurement approaches the limits of how massive and compact a single object can become without crushing itself down into a black hole. Recent work involving gravitational waves observed from colliding neutron stars by LIGO suggests that 2.17 solar masses might be very near that limit. Neutron stars are so massive that their interiors take on weird properties. Finding the maximum mass that physics and nature will allow can teach us a great deal about this otherwise inaccessible realm in astrophysics. Pulsars get their name because of the twin beams of radio waves they emit from their magnetic poles. These beams sweep across space in a lighthouse-like fashion. Some rotate hundreds of
times each second. Since pulsars spin with such phenomenal speed and regularity, astronomers can use them as the cosmic equivalent of atomic clocks. Such precise timekeeping helps astronomers study the nature of spacetime, measure the masses of stellar objects, and improve their understanding of general relativity.

In the case of this binary system, which is nearly edge-on in relation to  Earth, this cosmic precision provided a pathway for astronomers to calculate the mass of the two stars. As the ticking pulsar passes behind its white dwarf companion, there is a subtle (on the order of 10 millionths of a second) delay in the arrival time of the signals. This phenomenon is known as "Shapiro Delay". In essence, gravity from the white dwarf star slightly warps the space surrounding it, in accordance with Einstein's general theory of relativity. That warping means the pulses from the rotating neutron star have to travel just a little bit farther as they wend their way around the distortions of spacetime caused by the white dwarf. Astronomers can use the amount of that delay to calculate the mass of the white dwarf. Once the mass of one of the co-orbiting bodies is known, it is a relatively straightforward process to determine accurately the mass of the other.

 
LARGE FEATURES NEAR MILKY WAY CENTRE
University of Oxford

Using the South African Radio Astronomy Observatory (SARAO) MeerKAT telescope, astronomers have mapped out broad regions in the centre of the galaxy, conducting observations at wavelengths near 23 centimetres. Radio emission of that kind is generated in a process known as synchrotron radiation, in which free-floating electrons are accelerated as they interact with powerful magnetic fields. This produces a characteristic radio signal that can be used to trace energetic regions in space. The radio light seen by MeerKAT penetrates the dense clouds of dust that block visible light from the centre of the galaxy. The centre of our galaxy is relatively calm when compared to other galaxies with very active central black holes. Even so, the Milky Way's central black hole can become uncharacteristically active, flaring up as it periodically devours massive clumps of dust and gas. It's possible that one such feeding frenzy triggered powerful outbursts that inflated this previously unseen feature. By examining the nearly identical extent and morphology of the twin bubbles, the scientists believe they have found convincing evidence that these features were formed from a violent eruption that over a short period of time punched through the interstellar medium in opposite directions. MeerKAT has unprecedented sensitivity and imaging capabilities which, coupled with its geographic vantage point for observing the Galactic centre, has resulted in the clearest-ever image of the radio waves emanating from the centre of the Milky Way, a part of the sky that is notoriously difficult to image at such wavelengths.

These new observational capabilities are unlocking a "fossil record" which allows scientists to piece together the history of the Galactic centre and the supermassive black hole that lurks there. Although the structure is likely to be a few million years old it is still possible to observe it, and from there scientists can infer from where it came. The eruption was possibly triggered by vast amounts of interstellar gas falling in on the black hole, or a massive burst of star formation which sent shock waves careening through the Galactic centre. In effect, this inflated energetic bubbles in the hot, ionized gas near the Galactic centre, energizing it and generating radio waves which we could eventually detect here on Earth. The event could also be the origin of the population of electrons that are required to power the radio emission from mysterious magnetised filaments.  These thread-like structures have been seen nowhere but in the Galactic centre, and there has been no definitive explanation for their origin since they were first discovered 35 years ago. Almost all of the more than one hundred filaments are confined by the radio bubbles. Those enormous bubbles have until now been hidden by the glare of extremely bright radio emission from the centre of the galaxy.

 
BLACK HOLE IN GALACTIC CENTRE
University of California, Los Angeles

The enormous black hole at the centre of our galaxy is having an unusually large meal of interstellar gas and dust, and researchers don't yet understand why. Researchers analyzed more than 13,000 observations of the black hole from 133 nights since 2003. The images were gathered by the W.M. Keck Observatory in Hawaii and the European Southern Observatory's Very Large Telescope in Chile. The team found that on May 13, the area just outside the black hole's "point of no return" (so called because once matter enters, it can never escape) was twice as bright as the next-brightest observation. They also observed large changes on two other nights this year; all three of those changes were "unprecedented". The brightness the scientists observed is caused by radiation from gas and dust falling into the black hole; the findings prompted them to ask whether this was an extraordinary singular event or a precursor to significantly increased activity. The big question is whether the black hole is entering a new phase -- for example if the spigot has been turned up and the rate of gas falling down the black hole 'drain' has increased for an extended period -- or whether we have just seen the fireworks from a few unusual blobs of gas falling in. The team has continued to observe the area and will try to settle that question on the basis of what they see from new images. The new findings are based on observations of the black hole which is called Sagittarius A*, or Sgr A* -- during four nights in April and May at the Keck Observatory. The brightness surrounding the black hole always varies somewhat, but the scientists were stunned by the extreme variations in brightness during that time frame, including their observations on May 13.

In the first image seen that night, the black hole was so bright it was initially mistaken for the star S0-2, because Sagittarius A* had never been seen that bright. But it quickly became clear the source had to be the black hole. One hypothesis about the increased activity is that when the star S0-2 made its closest approach to the black hole during the summer 2018, it launched a large quantity of gas that reached the black hole this year. Another possibility involves a bizarre object known as G2, which is most likely a pair of binary stars, which made its closest approach to the black hole in 2014. It's possible the black hole could have stripped off the
outer layer of G2, which could help explain the increased brightness just outside the black hole. Another possibility is that the brightening corresponds to the demise of large asteroids that have been drawn in to the black hole. The black hole is some 26,000 light-years away and poses no danger to our planet. The radiation would have to be 10 billion times as bright as what the astronomers detected to affect life on Earth.


REPEATING FAST RADIO BURST DETECTED
Science Alert

China's Five-hundred-metre Aperture Spherical Radio Telescope (FAST) has picked up a mysterious space signal known as a fast radio burst. Fast radio bursts or FRBs are brief but powerful pulses of energy from distant parts of the cosmos. The first one was detected in 2007, and we're finding more of them all the time. While astronomers have recently made some exciting progress in tracing FRBs, we just don't know exactly what those signals are, or how they originate. They might be caused by black holes or neutron stars called magnetars, perhaps. What's exciting about the detection by FAST is that this fast radio burst is a repeater. The burst is officially known as FRB 121102: first picked up in 2012 at the Arecibo Observatory in Puerto Rico, it's appeared several times since. Researchers note that the signal has travelled around 3 billion light-years across the Universe to reach us.  FAST latched on to FRB 121102 on August 30, before recording dozens of later pulses (on one particular day, September 3, more than 20 pulses were detected). So this looks like a particularly persistent FRB.

The 19-beam receiver on FAST is especially sensitive to radio signals, covering the 1.05-1.45 GHz frequency range, and that makes it perfect for keeping an eye on FRB 121102. The more observations we can make of these FRBs, the better our chances of being able to work out exactly what they are. One idea is that FRBs are produced upon disintegration of the crusts of certain types of neutron stars. Another hypothesis posits that different FRBs actually have different causes, which may explain why FRB 121102 repeats and others don't appear to do so. We are at least getting better at pinpointing where these mysterious bursts of electromagnetic radiation come from. Now we can add the data gathered by FAST to our growing database of knowledge on these most intriguing of space phenomena. The team at the telescope has already been able to eliminate aircraft and satellite interference from their measurements.