Mapping Black Holes in (and out of) the Milky Way

Some years back, I reminisced in these pages about reading Poul Anderson’s World Without Stars, an intriguing tale first published in 1966 about a starship in intergalactic space that was studying a civilization for whom the word ‘isolation’ must have taken on utterly new meaning. Imagine a star system tens of thousands of light years away from the Milky Way, a place where an entire galaxy is but a rather dim feature in the night sky. Poul Anderson discussed this with Analog editor John Campbell:

One point came up which may interest you. Though the galaxy would be a huge object in the sky, covering some 20? of arc, it would not be bright. In fact, I make its luminosity, as far as this planet is concerned, somewhere between 1% and 0.1% of the total sky-glow (stars, zodiacal light, and permanent aurora) on a clear moonless Earth night. Sure, there are a lot of stars there — but they’re an awfully long ways off!

For more on galactic brightness, see The Milky Way from a Distance. The Anderson tale was originally serialized as The Ancient Gods in the June and July, 1966 issues of Campbell’s magazine. Long-time readers will remember its cover, which I ran back in 2012, along with a discussion of how artist Chesley Bonestell approached the cover art, which shows the distant galaxy as far brighter than it would actually appear. Bonestell brightened it even knowing this to make the cover interesting while still suggesting just how far away the vast ‘city of stars’ actually was in the story.

Where Black Holes Roam

Intergalactic space is, I would assume, about as empty a place as could be. Yet new work out of the University of Sydney delves into just what we might find if we could see what’s out there. And it turns out that it is quite a lot. The university’s David Sweeney is lead author on a paper in Monthly Notices of the Royal Astronomical Society. The researchers discuss what they describe as the ‘galactic underworld,’ which is comprised of the compact remnants of massive stars. In other words, stars that have collapsed onto themselves and produced neutron stars and black holes.

Remember that black holes and neutron stars form from stars more than eight times the size of our Sun. If less than about 25 times the mass of the Sun, the star forms a neutron star, its tiny sphere jammed with neutrons prevented from collapsing further by neutron degeneracy pressure. Sweeney and team say that thirty percent of the black holes and neutron stars out there have been completely ejected from the galaxy. Given the age of the galaxy, over 13 billion years, a vast number of such objects must have formed, the 30 percent ejected by the ‘kick’ induced by their creation in a supernova.

Image: A colour rendition of the visible Milky Way galaxy (top) compared with the range of the galactic underworld (bottom). Credit: Sydney University.

As you can see in the image, the galaxy’s underworld turns out to stretch well beyond the visible limits of the disk. Peter Tuthill (Sydney Institute for Astronomy) notes the challenges involved in creating this first chart of an unseen population:

“One of the problems for finding these ancient objects is that, until now, we had no idea where to look. The oldest neutron stars and black holes were created when the galaxy was younger and shaped differently, and then subjected to complex changes spanning billions of years. It has been a major task to model all of this to find them. Newly-formed neutron stars and black holes conform to today’s galaxy, so astronomers know where to look. It was like trying to find the mythical elephant’s graveyard”

The researchers used a stellar population synthesis computer code called GALAXIA, modifying it to include stars that have exhausted their nuclear fusion life cycle, leaving behind a remnant black hole or neutron star, and excluding stars below 8 solar masses. Additional custom code was then produced to capture velocity changes to the star caused by supernovae explosions (the so-called ‘natal kick’). The effects of the kick were added to each remnant’s velocity and transformed to galactocentric coordinates, with subsequent custom code showing evolution of the stars’ paths over time.

The distribution map that emerged depicts a galaxy, and thus its remnants, changing over time, so that the Milky Way’s present shape does not predict the distribution of neutron stars and black holes surrounding it. In fact, the relatively thin and flattened disk structure gives way to triple the scale height of the Milky Way we see.

Image: Point-cloud chart of the visible Milky Way galaxy (top) versus the galactic underworld. Credit: Sydney University.

As the paper notes:

The spatial distribution of compact remnants is different from that of visible stars. The remnants are more dispersed in the vertical direction with the scale height being about 3 times larger than that of the visible stars. This is mainly due to the significant velocity kicks received by the remnants at the time of their birth.

Also interesting are these two points:

The spatial distribution of BHs is more centrally concentrated as compared to the NSs due to the smaller velocity kick they receive.

For some remnants the kick is so large that their total velocity becomes greater than their escape velocity (40% of NS and 2% of BHs). We are able to estimate a Galactic mass loss in ejected compact remnants as 2.1×108M? or ?0.4% of the stellar mass of the Galaxy.

If 30 percent of the stellar remnants over the course of the galaxy’s evolution have been ejected into intergalactic space, that leaves 70 percent that still moves through the visible disk, so that neutron stars and black holes from the earliest days of the galaxy still move unattached to any nearby star through stellar neighborhoods like our own.

Black Holes and Their Neighbors in Space

In addition to these ‘free floating’ black holes, there are those in gravitational dance with nearby stars, leaving traces that are detectable. Making that point is the recent discovery of a black hole about 12 times the mass of the Sun at roughly 1650 light years from the Solar System, one that appears to be orbited by a visible star. This is “closer to the Sun than any black hole X-ray binaries with known distances…or any of the black holes identified through other techniques.”

The work, led by Sukanya Chakrabarti (University of Alabama, Huntsville), likewise highlights the role these remnants can play in the disk we see today. Says Chakrabarti:

“In some cases, like for supermassive black holes at the centers of galaxies, [black holes] can drive galaxy formation and evolution. It is not yet clear how these non-interacting black holes affect galactic dynamics in the Milky Way. If they are numerous, they may well affect the formation of our galaxy and its internal dynamics.”

Note the term ‘non-interacting,’ which the author uses to distinguish this kind of black hole from those that show an accretion disk of dust accumulating from another object. As you might imagine, interacting black holes – or the features they produce – are easier to detect at visible wavelengths.

Finding the black hole in this work involved analyzing data on almost 200,000 binary stars, as accumulated from the European Space Agency’s Gaia mission. The intent was to find objects that seemed to have a dark companion of large mass, looking for the gravitational effects of a black hole on a visible star. The most interesting sources were followed up by the Automated Planet Finder in California, Chile’s Giant Magellan Telescope and the W.M. Keck Observatory in Hawaii. Spectroscopic measurements confirmed that the binary system contains a visible star cataloged as Gaia DR3 4373465352415301632 orbiting a dark, massive object.

Image: The cross-hairs mark the location of the newly discovered black hole. Credit: Sloan Digital Sky Survey / S. Chakrabarti et al.

As to how this system of star and black hole originally formed, this interesting speculation:

Given the combination of the large mass of the dark companion and a semi-major axis of Gaia DR3 4373465352415301632 that is neither very large nor very small, the formation channel for this system is not immediately clear. However, the most natural scenario may be that the visible G star was originally the outer tertiary component orbiting a close inner binary with two massive stars.

So here we have a search for black holes bound to visible stars, with the authors estimating that perhaps a million such stars have black hole companions. That’s an early estimate for one population of black holes, but this object, in a 185-day orbit from the star, does not represent the class of black holes and neutron stars that may move through the galaxy untethered to any visible object, as found in the investigations of the Sydney team. Just how many black holes may be peppered through the several hundred billion stars of the Milky Way, and how widely spaced are they likely to be?

Finding untethered black holes, whether within or outside the galactic disk, is not work for the faint-hearted. Surely microlensing studies are our best way to proceed?

The paper is Sweeney et al., “The Galactic underworld: the spatial distribution of compact remnants,” Monthly Notices of the Royal Astronomical Society, Volume 516, Issue 4 (November 2022), pp. 4971–4979 (abstract / preprint). The black hole discovery paper is Chakrabarti et al., “A non-interacting Galactic black hole candidate in a binary system with a main-sequence star,” in process at the Astrophysical Journal (preprint).


Interstellar Deceleration: Can We Ride the ‘Bow Shock’?

Interesting things happen at the edge of the Solar System. Or perhaps I should say, at the boundary of the heliosphere, since the Solar System itself conceivably extends (in terms of possible planets) further out than the 100 or so AU that marks the heliosphere’s boundary at its closest. The fact that the heliosphere is pliable and reacts among other things to the solar cycle in turns means that the boundary is a moving target. It would be useful if we could get something like JHU/APL’s Interstellar Probe mission out well beyond the heliosphere to help us understand this morphology better.

But let’s think about the heliosphere’s boundaries from the standpoint of incoming spacecraft. Because deceleration at the destination system is a huge problem for starship mission planning. A future crew, human or robotic, could deploy a solar sail to slow down, but a magsail seems better, as its effects kick in earlier on the approach. Looking at the image below, however, suggests another possibility, one using the interactions between stars and the interstellar medium to assist the slowdown. And then the question arises: Does our own Sun produce a similar kind of bow shock?

Image: A multi-wavelength view of Zeta Ophiuchi. Credit: X-ray: NASA/CXC/Dublin Inst. Advanced Studies/S. Green et al.; Infrared: NASA/JPL/Spitzer.

Here we’re looking at a star, Zeta Ophiuchi, that is some 440 light years from Earth. It’s about 20 times as massive as the Sun, and evidently was once in a tight orbit around another star that became a supernova perhaps a million years ago. As a result, Zeta Ophiuchi was ejected from its binary orbit, and we have data from the Spitzer Space Telescope as well as the Chandra X-ray Observatory depicting the spectacular after-effects. The shock wave consists of matter blowing away from the star’s surface, slamming into gas. In the above image, the shock wave is in vivid red and green.

The latest work on Zeta Ophiuchi comes from a team led by Samuel Green (Dublin Institute for Advanced Studies, Ireland), with a paper laying out computer modeling of the shock wave and running the data against observational data obtained at X-ray, optical, infrared and radio wavelengths. Their results are interesting, as what can be found in data on the X-ray emissions shows that it is brighter than the modeling suggests. The bubble of X-ray emissions shows up in blue around the star in the image above. Its brightness indicates further modeling including turbulence and particle acceleration is needed.

I’ll send you to the paper for more on Zeta Ophiuchi, whose position – enveloped by the nebula Sh2-27 and pushing through dense dust clouds – makes it a natural for studying what happens when a shock wave develops. But let’s cut back to more mundane interactions, such as what happens when the Sun’s solar wind encounters the interstellar medium. Does a bow shock form here? Depending on the relative velocity of the heliosphere and the strength of the local interstellar magnetic field, such a phenomenon may or may not occur, as suggested by Voyager data as well as earlier findings from the Interstellar Boundary Explorer spacecraft (IBEX). A bow shock had been assumed, but we’re learning that these interactions are complicated.

While we investigate our heliosphere’s interactions with the interstellar medium, we can point to numerous bow shocks especially associated with more massive stars. In fact, a citizen science effort called The Milky Way Project is all about mapping bow shocks, building our catalog of these interesting astrophysical features. Learning more about how bow shocks form will clearly take us into the influence of interstellar magnetic fields as they roil the outflowing stellar winds they encounter. The density and pressures of the medium and the speed of the star’s astrosphere determine the result.

Image: Stars travel through the galaxy surrounded by a bubble of charged gas and magnetic fields, rounded at the front and trailing into a long tail behind. The bubble is called an astrosphere, or — in the case of the one around our Sun — a heliosphere. This image shows a few examples of astrospheres that are very strong and therefore visible.
Credit: NASA/Goddard Space Flight Center.

All of this has implications for our thinking about certain kinds of interstellar missions. If a star does form a bumper of plasma and higher density gas at the edge of its astrosphere, then as Gregory Benford has suggested (in correspondence some years back), we are looking at an obvious place to slow down an incoming starship. As Benford noted, the bow shock produces 3D structures, surfaces within which one can move while shedding speed, perhaps braking via a magsail. Each star would produce its own unique deceleration environment, allowing us to brake where possible along the bow shock, the astropause (cognate to the heliopause) and the termination shock.

We are talking about long, spiraling approaches to a destination system with continual magsail braking – decelerating from interstellar velocities is not going to be fast or easy. But it seems clear that one kind of precursor mission before we send missions that are more than flybys to other stars will be to visit our own shock environment at the edge of the Solar System, where we can learn more about using shock surfaces to slow down. I like the way Benford put it in an email: “As a starship approaches a star, sensing the shock structures will be like having a good eye for the tides, currents and reefs of a harbor.” For more, see 2012’s Starship Surfing: Ride the Bow Shock, where I assumed the existence of a solar bow shock.

All of this reminds us that the interstellar medium is anything but uniform. If the Sun is currently near the boundary of the Local Interstellar Cloud (and its exact position within it is unclear), the Alpha Centauri stars appear to be outside that cloud in the direction of the G cloud, another variation in the medium. So we have another kind of boundary crossing to consider. Different hydrogen densities play havoc with the Bussard ramjet concept, too. Robert Bussard assumed hydrogen densities in the range of 1 hydrogen atom per cubic centimeter, but move outside denser clouds and that figure should drop precipitously. If you’re flying an interstellar ramjet, pay attention to the clouds!

The Zeta Ophiuchi paper is Green et al., “Thermal emission from bow shocks. II. 3D magnetohydrodynamic models of zeta Ophiuchi,” in process at Astronomy & Astrophysics (abstract).


Two Close Stellar Passes

Interstellar objects are much in the news these days, as witness the flurry of research on ‘Oumuamua and 2I/Borisov. But we have to be cautious as we look at objects on hyperbolic orbits, avoiding the assumption that any of these are necessarily from another star. Spanish astronomers Carlos and Raúl de la Fuente Marcos dug several years ago into the question of objects on hyperbolic orbits, noting that some of these may well have origins much closer to home. Let me quote their 2018 paper on this:

There are mechanisms capable of generating hyperbolic objects other than interstellar interlopers. They include close encounters with the known planets or the Sun, for objects already traversing the Solar system inside the trans-Neptunian belt; but also secular perturbations induced by the Galactic disc or impulsive interactions with passing stars, for more distant bodies (see e.g. Fouchard et al. 2011, 2017; Królikowska & Dybczy?ski 2017). These last two processes have their sources beyond the Solar system and may routinely affect members of the Oort cloud (Oort 1950), driving them into inbound hyperbolic paths that may cross the inner Solar system, making them detectable from the Earth (see e.g. Stern 1987).

Scholz’s Star Leaves Its Mark

So much is going on in the outer reaches of the Solar System! In the 2018 paper, the two astronomers looked for patterns in how hyperbolic objects move, noting that anything approaching us from the far reaches of the Solar System seems to come from a well-defined location in the sky known as its radiant (also called its antapex). Given the mechanisms for producing objects on hyperbolic orbits, they identify distinctive coordinate and velocity signatures among these radiants.

Work like this relies on the past orbital evolution of hyperbolic objects using computer modeling and statistical analyses of the radiants, and I wouldn’t have dug quite so deeply into this arcane work except that it tells us something about objects that are coming under renewed scrutiny, the stars that occasionally pass close to the Solar System and may disrupt the Oort Cloud. Such passing stars are an intriguing subject in their own right and even factor into studies of galactic diffusion; i.e., how a civilization might begin to explore the galaxy by using close stellar passes as stepping stones.

But more about that in a moment, because I want to wrap up this 2018 paper before moving on to a later paper, likewise from the de la Fuente Marcos team, on close stellar passes and the intriguing Gliese 710. Its close pass is to happen in the distant future, but we have one well characterized pass that the 2018 paper addresses, that of Scholz’s Star, which is known to have made the most recent flyby of the Solar System when it moved through the Oort Cloud 70,000 years ago. In their work on minor objects with long orbital periods and extreme orbital eccentricity, the researchers find a “significant overdensity of high-speed radiants toward the constellation of Gemini” that may be the result of the passage of this star.

This is useful stuff, because as we untangle prior close passes, we learn more about the dynamics of objects in the outer Solar System, which in turn may help us uncover information about still undiscovered objects, including the hypothesized Planet 9, that may lurk in the outer regions and may have caused its own gravitational disruptions.

Before digging into the papers I write about today, I hadn’t realized just how many objects – presumably comets – are known to be on hyperbolic orbits. The astronomers work with the orbits of 339 of these, all with nominal heliocentric eccentricity > 1, using data from JPL’s Solar System Dynamics Group Small-Body Database and the Minor Planet Center Database. For a minor object moving with an inbound velocity of 1 kilometer per second, which is the Solar System escape velocity at about 2000 AU, the de la Fuente Marcos team runs calculations going back 100,000 years to examine the modeled object’s orbital evolution all the way out to 20,000 AU, which is in the outer Oort Cloud.

That overdensity of radiants toward Gemini that I mentioned above does seem to implicate the Scholz’s Star flyby. If so, then a close stellar pass that occurred 70,000 years ago may have left traces we can still see in the orbits of these minor Solar System bodies today. The uncertainties in the analysis of other stellar flybys relate to the fact that past encounters with other stars are not well determined, with Scholz’s Star being the prominent exception. Given the lack of evidence about other close passes, the de la Fuente Marcos team acknowledges the possibility of other perturbers.

Image: This is Figure 3 from the paper. Caption: Distribution of radiants of known hyperbolic minor bodies in the sky. The radiant of 1I/2017 U1 (‘Oumuamua) is represented by a pink star, those objects with radiant’s velocity > ?1?km?s?1 are plotted as blue filled circles, the ones in the interval (?1.5, ?1.0) km s?1 are shown as pink triangles, and those < ? 1.5?km?s?1 appear as goldenrod triangles. The current position of the binary star WISE J072003.20-084651.2, also known as Scholz’s star, is represented by a red star, the convergent brown arrows represent its motion and uncertainty as computed by Mamajek et al. (2015). The ecliptic is plotted in green. The Galactic disc, which is arbitrarily defined as the region confined between Galactic latitude ?5° and 5°, is outlined in black, the position of the Galactic Centre is represented by a filled black circle; the region enclosed between Galactic latitude ?30° and 30°? appears in grey. Data source: JPL’s SSDG SBDB. Credit: Carlos and Raúl de la Fuente Marcos.

The Coming of Gliese 710

Let’s now run the clock forward, looking at what we might expect to happen in our next close stellar passage. Gliese 710 is an interesting K7 dwarf in the constellation Serpens Cauda that occasionally pops up in our discussions because of its motion toward the Sun at about 24 kilometers per second. Right now it’s a little over 60 light years away, but give it time – in about 1.3 million years, the star should close to somewhere in the range of 10,000 AU, which is about 1/25th of the current distance between the Sun and Proxima Centauri. As we’re learning, wait long enough and the stars come to us.

Note that 10,000 AU; we’ll tighten it up further in a minute. But notice that it is actually inside the distance between the closest star, Proxima Centauri, and the Centauri A/B binary.

Image: Gleise 710 (center), destined to pass through the inner Oort Cloud in our distant future. Credit: SIMBAD / DSS

An encounter like this is interesting for a number of reasons. Interactions with the Oort Cloud should be significant, although well spread over time. Here I go back to a 1999 study by Joan García-Sánchez and colleagues that made the case that spread over human lifetimes, the effects of such a close passage would not be pronounced. Here’s a snippet from that paper:

For the future passage of Gl 710, the star with the closest approach in our sample, we predict that about 2.4 × 106 new comets will be thrown into Earth-crossing orbits, arriving over a period of about 2 × 106 yr. Many of these comets will return repeatedly to the planetary system, though about one-half will be ejected on the first passage. These comets represent an approximately 50% increase in the flux of long-period comets crossing Earth’s orbit.

As far as I know, the García-Sánchez paper was the first to identify Gliese 710’s flyby possibilities. The work was quickly confirmed in several independent studies before the first Gaia datasets were released, and the parameters of the encounter were then tightened using Gaia’s results, the most recent paper using Gaia’s third data release. Back to Carlos and Raúl de la Fuente Marcos, who tackle the subject in a new paper appearing in Research Notes of the American Astronomical Society.

The researchers have subjected the Gliese 710 flyby to N-body simulations using a suite of software tools that model perturbations from the star and factor in the four massive planets in our own system as well as the barycenter of the Pluto/Charon system. They assume a mass of 0.6 Solar masses for Gliese 710, consistent with previous estimates. In addition to the Gaia data, the authors include the latest ephemerides information for Solar System objects as provided by the Jet Propulsion Laboratory’s Horizons System.

Image: This is Figure 1 from the paper. Caption: Future perihelion passage of Gliese?710 as estimated from Gaia?DR3 input data and the N-body simulations discussed in the text. The distribution of times of perihelion passage is shown in the top-left panel and perihelion distances in the top-right one. The blue vertical lines mark the median values, the red ones show the 5th and 95th percentiles. The bottom panels show the times of perihelion passage (bottom-left) and the distance of closest approach (bottom–right) as a function of the observed values of the radial velocity of Gliese?710 and its distance (randomly generated using the mean values and standard deviations from Gaia?DR3), both as color coded scatter plots of the distribution in the associated top panel. Histograms have been produced using the Matplotlib library (Hunter 2007) with sets of bins computed using Numpy (Harris et al. 2020) by applying the Freedman and Diaconis rule; instead of considering frequency-based histograms, we used counts to form a probability density so the area under the histogram will sum to one. The colormap scatter plot has also been produced using Matplotlib. Credit: Carlos and Raúl de la Fuente Marcos.

The de la Fuente Marcos paper now finds that the close approach of Gliese 710 will take it to within 10635 AU plus or minus 500 AU, putting it inside the inner Oort Cloud in about 1.3 million years – both the distance of the approach and the time of perihelion passage are tightened from earlier estimates. And as we’ve seen, Scholz’s Star passed through part of the Oort Cloud at perhaps 52,000 AU some 70,000 years ago. We thus get a glimpse of the Solar System influenced by passing stars on a time frame that begins to take shape and clearly defines a factor in the evolution of the Solar System.

What Gaia Can Tell Us

We can now back out further again to a 2018 paper from Coryn Bailer-Jones (Max Planck Institute for Astronomy, Heidelberg), which examines not just two stars with direct implications for our Solar System, but Gaia data (using the Gaia DR2 dataset) on 7.2 million stars to look for further evidence for close stellar encounters. Here we begin to see the broader picture. Bailer-Jones and team find 26 stars that have or will approach within 1 parsec, 7 that will close to 0.5 parsecs, and 3 that will pass within 0.25 parsecs of the Sun. Interestingly, the closest encounter is with our friend Gliese 710.

How often can these encounters be expected to occur? The authors estimate about 20 encounters per million years within a range of one parsec. Greg Matloff has used these data to infer roughly 2.5 encounters within 0.5 parsecs per million years. Perhaps 400,000 to 500,000 years should separate close stellar encounters as found in the Gaia DR2 data. We should keep in mind here what Bailer-Jones and team say about the current state of this research, especially given subsequent results from Gaia: “There are no doubt many more close – and probably closer – encounters to be discovered in future Gaia data releases.” But at least we’re getting a feel for the time spans involved.

So given the distribution of stars in our neighborhood of the galaxy, our Sun should have a close encounter every half million years or so. Such encounters between stars dramatically reduce the distance for any would be travelers. In the case of Scholz’s Star, for instance, the distances involved cut the current distance to the nearest star by a factor of 5, while Gliese 710 is even more provocative, for as I mentioned, it will close to a distance not all that far off Proxima Centauri’s own distance from Centauri A/B.

A good time for interstellar migration? We’ve considered the possibilities in the past, but as new data accumulate, we have to keep asking how big a factor stellar passages like these may play in helping a technological civilization spread throughout the galaxy.

The earlier de la Fuente Marcos paper is “Where the Solar system meets the solar neighbourhood: patterns in the distribution of radiants of observed hyperbolic minor bodies,” Monthly Notices of the Royal Astronomical Society Letters Vol. 476, Issue 1 (May 2018) L1-L5 (abstract). The later de la Fuente Marcos paper is “An Update on the Future Flyby of Gliese 710 to the Solar System Using Gaia DR3: Flyby Parameters Reproduced, Uncertainties Reduced,” Research Notes of the AAS Vol. 6, No. 6 (June, 2022) 136 (full text). The García-Sánchez et al. paper is “Stellar Encounters with the Oort Cloud Based on Hipparcos Data,” Astronomical Journal 117 (February, 1999), 1042-1055 (full text). The Bailer-Jones paper is “New stellar encounters discovered in the second Gaia data release,” Astronomy & Astrophysics Vol. 616, A37 (13 August 2018). Abstract.


Unusual Transient: A New Kind of Magnetar?

Every time we look in a new place, which in astrophysics often means bringing new tools online, we find something unexpected. The news that an object has been detected that, for one minute in every 18, becomes one of the brightest radio sources in the sky, continues the series of surprises we’ve been racking up ever since first Galileo put eye to telescope. So what is this object, and why is it cause for such interest?

Here’s astronomer Natasha Hurley-Walker (Curtin University/International Centre for Radio Astronomy Research), who is lead author of the paper on the discovery:

“This object was appearing and disappearing over a few hours during our observations. That was completely unexpected. It was kind of spooky for an astronomer because there’s nothing known in the sky that does that. And it’s really quite close to us—about 4000 lightyears away. It’s in our galactic backyard.”

Image: A new view of the Milky Way from the Murchison Widefield Array in Western Australia, with the lowest frequencies in red, middle frequencies in green, and the highest frequencies in blue. The star icon shows the position of the mysterious repeating transient. Credit: Dr Natasha Hurley-Walker (ICRAR/Curtin) and the GLEAM Team.

Transients are nothing new to astronomers, but they usually don’t appear in this configuration. Pulsars can appear to flash on and off on cycles of milliseconds or even seconds, a fact that originally caused a flutter of SETI excitement when Jocelyn Bell (now Jocelyn Bell Burnell) and team went to work identifying their origin in November of 1967. The discovery of multiple sources of this nature soon made it clear that LGM1 (Bell’s joking reference to possible ‘little green men’) needed a less sensational name.

Slower transients are also common, the most obvious being supernovae, which can put on a celestial show before doing a slow fade that can take months. What distinguishes the new object is that it flashes on and off on a scale of minutes. It’s also emitting highly polarized radio waves, an indication that it possesses a strong magnetic field. Hurley-Walker suggests the possibility of an ‘ultra-long period magnetar,’ which would be a slowly spinning neutron star, and an unusually bright one at that.

Magnetars are neutron stars possessing a strong magnetic field. In an email exchange with Dr. Hurley-Walker, I learned that about 30 magnetars are currently known, most of them detected through X-ray observations. Most of these spin with periods between 1 and 10 seconds, and six of them have an unusual property: They flare suddenly at X-ray wavelengths and follow this with radio emission that can last for weeks, even months afterwards.

Several puzzling things leap out about the new discovery, as Hurley-Walker explained:

How is it producing radio waves, when it is spinning so slowly that it shouldn’t have enough energy? Are its magnetic fields getting twisted and it’s producing temporary emission like a radio magnetar?

How did it get to be so slow — if it aged “normally”, then its magnetic field should also be quite weak by now, in which case how is it producing radio emission?

How is it so bright, as bright as the youngest pulsars we know?

Image: An artist’s impression of what the object might look like if it’s a magnetar. Magnetars are incredibly magnetic neutron stars, some of which sometimes produce radio emission. Known magnetars rotate every few seconds, but theoretically, “ultra-long period magnetars” could rotate much more slowly. Credit: ICRAR.

The Murchison Widefield Array is unusually useful not only for studying this original find but also in looking for other objects that seem to be in the same category. Remember that the MWA is a precursor for the multinational Square Kilometre Array, which will connect radio telescopes in Western Australia and South Africa. The MWA is a low-frequency radio telescope operating between 80 and 300 MHz at the future site of the SKA, and its low frequency capabilities open up intriguing possibilities.

For when it comes to transients, we have a world of options in the high-frequency radio sky, from the above-mentioned supernovae to accretion events of various kinds, but beyond the pulsars we’ve already examined and studies of galactic nuclei, the low-frequency sky has been relatively tame. The paper on this work points out, however, that these wavelengths are sensitive to polarized radio emission processes like those at work here, fertile terrain for chasing down new transients.

This source, as an evidently slow rotator, stands out as a new category of magnetar, and one that could establish a possible population of slow-spinning magnetars that may in fact turn up in archival data at the MWA itself. Finding out whether this is the case or if we’re looking at an odd, one-off detection will take considerable digging. All of the data the MWA has produced for close to a decade is available at the Pawsey Research Supercomputing Centre in Perth, which houses MWA system data.

That’s a massive dataset, but future observations are in the cards as well. In her email, Hurley-Walker pointed to the way forward:

The MWA is currently down for maintenance but I’m planning to observe this source when it comes back up, as well as the rest of our galaxy, to try to find more of these objects. The source itself seems to be quiet at the moment, and (as far as we know) has been except in those few months in 2018.

Image: Composite image of the SKA-Low telescope in Western Australia. The image blends a real photo (on the left) of the SKA-Low prototype station AAVS2.0 which is already on-site, with an artist’s impression of the future SKA-Low stations as they will look when constructed. These dipole antennas, which will number in their hundreds of thousands, will survey the radio sky at frequencies as low as 50Mhz. Credit: ICRAR, SKAO.

An object that can convert magnetic energy to radio waves at this level stood out in the MWA observations, and we can assume that if other such objects exist, they will turn up in large numbers once the SKA comes online, offering a thousand times the sensitivity. Meanwhile, Dr. Hurley-Walker has modified her MWA search strategy to more readily spot other such transients should they occur.

The paper is Hurley-Walker et al., “A radio transient with unusually slow periodic emission,” Nature 601 (26 January 2022), 526-530. Abstract.


The Exoplanet Pipeline

Looking into Astro2020’s recommendations for ground-based astronomy, I was heartened with the emphasis on ELTs (Extremely Large Telescopes), as found within the US-ELT project to develop the Thirty Meter Telescope and the Giant Magellan Telescope, both now under construction. Such instruments represent our best chance for studying exoplanets from the ground, even rocky worlds that could hold life. An Astro2020 with different priorities could have spelled the end of both these ELT efforts in the US even as the European Extremely Large Telescope, with its 40-meter mirror, moves ahead, with first light at Cerro Armazones (Chile) projected for 2027.

So the ELTs persist in both US and European plans for the future, a context within which to consider how planet detection continues to evolve. So much of what we know about exoplanets has come from radial velocity methods. These in turn rely critically on spectrographs like HARPS (High Accuracy Radial Velocity Planet Searcher), which is installed at the European Southern Observatory’s 3.6m telescope at La Silla in Chile, and its successor ESPRESSO (Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations). We can add the NEID spectrometer on the WIYN 3.5m telescope at Kitt Peak to the mix, now operational and in the hunt for ever tinier Doppler shifts in the light of host stars.

We’re measuring the tug a planet puts on its star by looking radially — how is the star pulled toward us, then away, as the planet moves along its orbit? Given that the Earth produces a movement of a mere 9 centimeters per second on the Sun, it’s heartening to see that astronomers are closing on that range right now. NEID has demonstrated a precision of better than 25 centimeters per second in the tests that led up to its commissioning, giving us another tool for exoplanet detection and confirmation.

But this is a story that also reminds us of the vast amount of data being generated in such observations, and the methods needed to get this information distributed and analyzed. On an average night, NEID will collect about 150 gigabytes of data that is sent to Caltech, and from there via a data management network called Globus to the Texas Advanced Computing Center (TACC) for analysis and processing. TACC, in turn, extracts metadata and returns the data to Caltech for further analysis. The results are made available by the NASA Exoplanet Science Institute via its NEID Archive.

Image: The NEID instrument is shown mounted on the 3.5-meter WIYN telescope at the Kitt Peak National Observatory. Credit: NSF’s National Optical-Infrared Astronomy Research Laboratory/KPNO/NSF/AURA.

What a contrast with the now ancient image of the astronomer on a mountaintop coming away with photographic plates that would be analyzed with instruments like the blink comparator Clyde Tombaugh used to discover Pluto in 1930. The data now come in avalanche form, with breakthrough work occurring not only on mountaintops but in the building of data pipelines like these that can be generalized for analysis on supercomputers. The vast caches of data contain the seeds of future discovery.

Joe Stubbs leads the Cloud & Interactive Computing group at TACC:

“NEID is the first of hopefully many collaborations with the NASA Jet Propulsion Laboratory (JPL) and other institutions where automated data analysis pipelines run with no human-in-the-loop. Tapis Pipelines, a new project that has grown out of this collaboration, generalizes the concepts developed for NEID so that other projects can automate distributed data analysis on TACC’s supercomputers in a secure and reliable way with minimal human supervision.”

NEID also makes a unique contribution to exoplanet detection by being given over to the analysis of activity on our own star. Radial velocity is vulnerable to confusion over starspots — created by convection on the surface of exoplanet host stars and mistaken for planetary signatures. The plan is to use NEID during daylight hours with a smaller solar telescope developed for the purpose to track this activity. Eric Ford (Penn State) is an astrophysicist at the university where NEID was designed and built:

“Thanks to the NEID solar telescope, funded by the Heising-Simons Foundation, NEID won’t sit idle during the day. Instead, it will carry out a second mission, collecting a unique dataset that will enhance the ability of machine learning algorithms to recognize the signals of low-mass planets during the nighttime.”

Image: A new instrument called NEID is helping astronomers scan the skies for alien planets. TACC supports NEID with supercomputers and expertise to automate the data analysis of distant starlight, which holds evidence of new planets waiting to be discovered. WIYN telescope at the Kitt Peak National Observatory. Credit: Mark Hanna/NOAO/AURA/NSF.

Modern astronomy in a nutshell. We’re talking about data pipelines operational without human intervention, and machine-learning algorithms that are being tuned to pull exoplanet signals out of the noise of starlight. In such ways does a just commissioned spectrograph contribute to exoplanetary science through an ever-flowing data network now indispensable to such work. Supercomputing expertise is part of the package that will one day extract potential biosignatures from newly discovered rocky worlds. Bring on the ELTs.


Going After Sagittarius A*

Only time will tell whether humanity has a future beyond the Solar System, but if we do have prospects among the stars — and I fervently hope that we do — it’s interesting to speculate on what future historians will consider the beginning of the interstellar era. Teasing out origins is tricky. You could label the first crossing of the heliopause by a functioning probe (Voyager 1) as a beginning, but neither the Voyagers nor the Pioneers (nor, for that matter, New Horizons) were built as interstellar missions.

I’m going to play the ‘future history’ game by offering my own candidate. I think the image of the black hole in the galaxy M87 marks the beginning of an era, one in which our culture begins to look more and more at the universe beyond the Solar System. I say that not because of what we found at M87, remarkable as it was, but because of the instrument used. The creation of a telescope that, through interferometry, can create an aperture the size of our planet speaks volumes about what a small species can accomplish. An entire planet is looking into the cosmos.

So will some future historian look back on the M87 detection as the beginning of the ‘interstellar era’? No one can know, but from the standpoint of symbolism — and that’s what this defining of eras is all about — the creation of a telescope like this is a civilizational accomplishment. I think its cultural significance will only grow with time.

Image: Composite image showing how the M87 system looked, across the entire electromagnetic spectrum, during the Event Horizon Telescope’s April 2017 campaign to take the iconic first image of a black hole. Requiring 19 different facilities on the Earth and in space, this image reveals the enormous scales spanned by the black hole and its forward-pointing jet, launched just outside the event horizon and spanning the entire galaxy. Credit: the EHT Multi-Wavelength Science Working Group; the EHT Collaboration; ALMA (ESO/NAOJ/NRAO); the EVN; the EAVN Collaboration; VLBA (NRAO); the GMVA; the Hubble Space Telescope, the Neil Gehrels Swift Observatory; the Chandra X-ray Observatory; the Nuclear Spectroscopic Telescope Array; the Fermi-LAT Collaboration; the H.E.S.S. collaboration; the MAGIC collaboration; the VERITAS collaboration; NASA and ESA. Composition by J.C. Algaba.

Into the Milky Way’s Heart

The Event Horizon Telescope (EHT) is not a single physical installation but a collection of telescopes around the world that use Very Long Baseline Interferometry to produce a virtual observatory with, as mentioned above, an aperture the size of our planet. Heino Falcke’s book Light in the Darkness (HarperOne, 2021) tells this story from the inside, and it’s as exhilarating an account of scientific research as any I’ve read.

M87 seemed in some ways an ideal target, with a black hole thought to mass well over 6 billion times more than the Sun. In terms of sheer size, M87 dwarfed estimates of the Milky Way’s supermassive black hole (Sgr A*), which weighs in at 4.3 million solar masses, but it’s also 2,000 times farther away. Even so, it was the better target, for M87 was well off the galactic plane, whereas astronomers hoping to study the Milky Way’s black hole have to contend with shrouds of gas and dust and the fact that, while average quasars consume one sun per year, Sgr A* pulls in 106 times less.

But the investigation of Sgr A* continues as new technologies come into play, with the James Webb Space Telescope now awaiting launch in December and already on the scene in French Guiana. Early in JWST’s observing regime, Sgr A* is to be probed at infrared wavelengths, adding the new space-based observatory to the existing Event Horizon Telescope. Farhad Yusef-Zadeh, principal investigator on the Webb Sgr A* program, points out that JWST will allow data capture at two different wavelengths simultaneously and continuously, further enhancing the EHT’s powers.

Among other reasons, a compelling driver for looking hard at Sgr A* is the fact that it produces flares in the dust and gas surrounding it. Yusef-Zadeh (Northwestern University) notes that the Milky Way’s supermassive black hole is the only one yet observed with this kind of flare activity, which makes it more difficult to image the black hole but also adds considerably to the scientific interest of the investigation. The flares are thought to be the result of particles accelerating around the object, but details of the mechanism of light emission here are not well understood.

Image: An enormous swirling vortex of hot gas glows with infrared light, marking the approximate location of the supermassive black hole at the heart of our Milky Way galaxy. This multiwavelength composite image includes near-infrared light captured by NASA’s Hubble Space Telescope, and was the sharpest infrared image ever made of the galactic center region when it was released in 2009. While the black hole itself does not emit light and so cannot be detected by a telescope, the EHT team is working to capture it by getting a clear image of the hot glowing gas and dust directly surrounding it. Credit: NASA, ESA, SSC, CXC, STS.

Thus we combine radio data from the Event Horizon Telescope with JWST’s infrared data. How different wavelengths can tease out more information is evident in the image above. Here we have a composite showing Hubble near-infrared observations in yellow, and deeper infrared observations from the Spitzer Space Telescope in red, while light detected by the Chandra X-Ray Observatory appears in blue and violet. Flare detection and better imagery of the region as enabled by adding JWST to the EHT mix, which will include X-ray and other observatories, should make for the most detailed look at Sgr A* that has ever been attempted.

What light we detect associated with a black hole is from the accreting material surrounding it, with the event horizon being its inner edge — this is what we saw in the famous M87 image. The early JWST observations, expected in its first year of operation, are to be supplemented by further work to build up our knowledge of the flare activity and enhance our understanding of how Sgr A* differs from other supermassive black holes.

Image: Heated gas swirls around the region of the Milky Way galaxy’s supermassive black hole, illuminated in near-infrared light captured by NASA’s Hubble Space Telescope. Released in 2009 to celebrate the International Year of Astronomy, this was the sharpest infrared image ever made of the galactic center region. NASA’s upcoming James Webb Space Telescope, scheduled to launch in December 2021, will continue this research, pairing Hubble-strength resolution with even more infrared-detecting capability. Of particular interest for astronomers will be Webb’s observations of flares in the area, which have not been observed around any other supermassive black hole and the cause of which is unknown. The flares have complicated the Event Horizon Telescope (EHT) collaboration’s quest to capture an image of the area immediately surrounding the black hole, and Webb’s infrared data is expected to help greatly in producing a clean image. Credit: NASA, ESA, STScI, Q. Daniel Wang (UMass).

Whether we’re entering an interstellar era or not, we’re going to be learning a lot more about the heart of the Milky Way, assuming we can get JWST aloft. How many hopes and plans ride on that Ariane 5!