A New Look at Gamma Rays from Our Galaxy’s Next-Door Neighbour

An ultraviolet image of the Andromeda Galaxy from NASA's Galaxy Evolution Explorer.
Credit: NASA/JPL-Caltech

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Title: On the Gamma-Ray Emission of the Andromeda Galaxy M31
Authors: Yi Xing et al.
First Author’s Institution: Shanghai Astronomical Observatory, Chinese Academy of Sciences
Status: Published in ApJL

Gamma rays are the highest-energy photons in our universe. Naturally, they come from some of the most extreme environments in the universe, such as pulsars, active galactic nuclei, supernovae, and potentially even dark matter. Though many gamma-ray sources have been detected both in the Milky Way and extragalactically, the nature of gamma-ray emission from our closest neighbouring galaxy, Andromeda (or Messier 31), remains somewhat of a mystery.

The Fermi Large Area Telescope (Fermi-LAT) is an instrument on the Fermi Gamma-ray Space Telescope that has been surveying the sky for high-energy gamma rays since 2008, with ample data taken on Andromeda throughout its flight. Many groups have analyzed these data, with more data giving more insight into what’s making these gamma rays.

Figure 1: Significance maps of Andromeda at energies from 0.1 to 500 gigaelectronvolts (left) and 2 to 500 gigaelectronvolts (right). The region of optical emission is represented by the white contour. The colorbar corresponds to test statistic, which is similar to significance. A test statistic of 25 corresponds to a detection. Green markers correspond to nearby sources found in the SIMBAD database. The left figure shows a hint of additional structure in the southeast region of Andromeda, but both point sources emerge out of the seemingly extended region only with the lowest energies cut out. Credit: Xing et al. 2023

To Extend or Not to Extend?

Up until today’s article, it looked like gamma rays from Andromeda were coming from a blob-like shape (i.e., extended emission) surrounding the centre of the galaxy (similar to Figure 1, left). This was particularly exciting, since extended structure in gamma-ray emission often suggests either a distribution of cosmic rays or the presence of a massive dark matter halo.

Cosmic rays are charged particles that travel at relativistic speeds through the universe but get easily diverted by magnetic fields, making it very difficult to trace their origin from Earth. Luckily, since there are processes that produce gamma rays from charged particles (hadronic processes), identifying regions of extended gamma rays can trace regions where populations of cosmic rays are interacting with their environments. On the other hand, clumps of massive dark matter located in the centre of Andromeda could decay or annihilate, producing gamma rays in the process.

Figure 2: A spectral energy distribution showing flux (quantity of gamma rays received) plotted against energy of Andromeda’s centre (black) and southeast (red) emission regions, along with the Milky Way’s galactic centre (blue). It is apparent that both sources are not only similar in brightness but are also producing significantly more gamma rays than our galactic centre. Click to enlarge. Credit: Xing et al. 2023

 
Where are the Gamma Rays Coming From?

A reanalysis of 14 years of Fermi-LAT data by the authors reveals that the emission of gamma rays isn’t extended after all. In fact, it seems that it’s constrained to two point sources: one located right at the centre of the galaxy and another ~20,000 light-years to the southeast (see Figure 1). This only became apparent when the authors cut out the lowest-energy gamma rays, which still make the data appear more or less extended when they’re included. Even more curiously, the authors found that both of these regions are significantly brighter than expected when compared to the gamma-ray emission of our own galactic centre (see Figure 2).

This new picture of Andromeda’s gamma rays changes a lot about our understanding of the galaxy. It’s no longer likely that Andromeda’s central gamma-ray hotspot is coming from a dark matter halo or cosmic ray distribution, so the authors looked to the Milky Way’s galactic centre to figure out what sorts of objects could be responsible for the gamma rays. One of the leading theories for our own galactic centre gamma rays is a population of old, unresolved objects, such as millisecond pulsars. However, in the case of Andromeda, at least 15,000 millisecond pulsars would be needed to account for the especially bright gamma-ray emission. While it’s still uncertain whether or not the centre of Andromeda can host this huge number of pulsars, we’ve only detected around 200 in the Milky Way’s centre, so this explanation seems unlikely.

The authors also investigate the southeast source that appeared in their new analysis. Since galaxies are pretty far apart from one another, the chance of finding two or more galaxies by coincidence in a circle drawn around both the central and southeast sources is only ~0.4%. This means that the emission is most likely coming from within Andromeda. As seen in Figure 2, the off-centre source is almost exactly the same brightness as Andromeda’s centre source (which is peculiar in its own right!), leading to the same problem of identifying sources capable of emitting such bright emission. After looking through X-ray and optical surveys, the authors determined that there weren’t any good counterparts for this region in other wavelengths either. Even considering the low probability of this being an extragalactic source behind Andromeda, there aren’t any known counterparts in the region of the sky where this hotspot is located.

The results are certainly unexpected and open up a whole new can of worms when it comes to figuring out the origin of the gamma rays in our neighbouring galaxy. Even though there are still a lot of unknowns, future observations and analyses of these newly constrained regions will help us understand how bright gamma rays are produced near the centres of galaxies and may even help us better understand our own galactic centre.

Original astrobite edited by Ivey Davis and Katya Gozman.

Source: American Astronomical Society - AAS Nova

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Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

 

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About the author, Samantha Wong:

I’m a graduate student at McGill University, where I study high-energy astrophysics. This includes studying all sorts of extreme environments in the universe like active galactic nuclei, pulsars, and supernova remnants with the VERITAS gamma-ray telescope.

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XMM-Newton's view of pulsar J1826-1256

XMM-Newton's view of pulsar J1826-1256

Credit: ESA/XMM-Newton/J. Li, DESY, Germany

Hi-res image

Based on a new theoretical model, a team of scientists explored the rich data archive of ESA's XMM-Newton and NASA's Chandra space observatories to find pulsating X-ray emission from three sources. The discovery, relying on previous gamma-ray observations of the pulsars, provides a novel tool to investigate the mysterious mechanisms of pulsar emission, which will be important to understand these fascinating objects and use them for space navigation in the future. 

Lighthouses of the Universe, pulsars are fast-rotating neutron stars that emit beams of radiation. As pulsars rotate and the beams alternatively point towards and away from Earth, the source oscillates between brighter and dimmer states, resulting in a signal that appears to 'pulse' every few milliseconds to seconds, with a regularity rivalling even atomic clocks.

Pulsars are the incredibly dense, extremely magnetic, relics of massive stars, and are amongst the most extreme objects in the Universe. Understanding how particles behave in such a strong magnetic field is fundamental to understanding how matter and magnetic fields interact more generally.

Originally detected through their radio emission, pulsars are now known to also emit other types of radiation, though typically in smaller amounts. Some of this emission is standard thermal radiation – the type that everything with a temperature above absolute zero emits. Pulsars release thermal radiation when they accrete matter, for example from another star.

But pulsars also emit non-thermal radiation, as is often produced in the most extreme cosmic environments. In pulsars, non-thermal radiation can be created via two processes: synchrotron emission and curvature emission. Both processes involve charged particles being accelerated along magnetic field lines, causing them to radiate light that can vary in wavelength from radio waves to gamma-rays.

Non-thermal X-rays result mostly from synchrotron emission, while gamma-rays may come from so-called synchro-curvature emission – a combination of the two mechanisms. It is relatively easy to find pulsars that radiate gamma-rays – NASA's Fermi Gamma-Ray Space Telescope has detected more than 200 of them over the past decade, thanks to its ability to scan the whole sky. But only around 20 have been found to pulse in non-thermal X-rays.

"Unlike gamma-ray detecting survey instruments, X-ray telescopes must be told exactly where to point, so we need to provide them with some sort of guidance," says Diego Torres, from the Institute of Space Sciences in Barcelona, Spain.

Aware that there should be many pulsars emitting previously undetected non-thermal X-rays, Torres developed a model that combined synchrotron and curvature radiation to predict whether pulsars detected in gamma-rays could also be expected to appear in X-rays.

"Scientific models describe phenomena that can't be experienced directly," explains Torres.

"This model in particular helps explain the emission processes in pulsars and can be used to predict the X-ray emission that we should observe, based on the known gamma-ray emission.

"

The model describes the gamma-ray emission of pulsars detected by Fermi – specifically, the brightness observed at different wavelengths – and combines this information with three parameters that determine the pulsar emission. This allows a prediction of their brightness at other wavelengths, for instance in X-rays.

Torres partnered with a team of scientists, led by Jian Li from the Deutsches Elektronen Synchrotron in Zeuthen near Berlin, Germany, to select three known gamma-ray emitting pulsars that they expected, based on the model, to also shine brightly in X-rays. They dug into the data archives of ESA's XMM-Newton and NASA's Chandra X-ray observatories to search for evidence of non-thermal X-ray emission from each of them.

"Not only did we detect X-ray pulsations from all three of the pulsars, but we also found that the spectrum of X-rays was almost the same as predicted by the model," explains Li.

"This means that the model very accurately describes the emission processes within a pulsar."

Non-thermal X-ray emission from three pulsars

Credit: Adapted from J. Li et al. (2018)

Hi-res images

In particular, XMM-Newton data showed clear X-ray emission from PSR J1826-1256 – a radio quiet gamma-ray pulsar with a period of 110.2 milliseconds. The spectrum of light received from this pulsar was very close to that predicted by the model. X-ray emission from the other two pulsars, which both rotate slightly more quickly, was revealed using Chandra data.

This discovery already represents a significant increase in the total number of pulsars known to emit non-thermal X-rays. The team expects that many more will be discovered over the next few years as the model can be used to work out where exactly to look for them.

Finding more X-ray pulsars is important for revealing their global properties, including population characteristics. A better understanding of pulsars is also essential for potentially taking advantage of their accurate timing signals for future space navigation endeavours.

The result is a step towards understanding the relationships between the emission by pulsars in different parts of the electromagnetic spectrum, enabling a robust way to predict the brightness of a pulsar at any given wavelength. This will help us better comprehend the interaction between particles and magnetic fields in pulsars and beyond.

"This model can make accurate predictions of pulsar X-ray emission, and it can also predict the emission at other wavelengths, for example visible and ultraviolet," Torres continues.

"In the future, we hope to find new pulsars leading to a better understanding of their global properties."

The study highlights the benefits of XMM-Newton's vast data archive to make new discoveries and showcases the impressive abilities of the mission to detect relatively dim sources. The team is also looking forward to using the next generation of X-ray space telescopes, including ESA's future Athena mission, to find even more pulsars emitting non-thermal X-rays.

"As the flagship of European X-ray astronomy, XMM-Newton is detecting more X-ray sources than any previous satellite. It is amazing to see that it is helping to solve so many cosmic mysteries," concludes Norbert Schartel, XMM-Newton Project Scientist at ESA.

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Notes for Editors

"Theoretically motivated search and detection of non-thermal pulsations from PSRs J1747-2958, J2021+3651, and J1826-1256" by Li et al. is published in Astrophysical Journal Letters.

DOI: 10.3847/2041-8213/aae92b

The prepint is available on the arXiv/astro-ph server (arXiv:1811.08339).


For more information, please contact:

Jian Li
Deutsches Elektronen Synchrotron DESY
Zeuthen, Germany
Email: jian.li@desy.de

Diego Torres
Institute of Space Sciences (ICE, CSIC)
Institut d'Estudis Espacials de Catalunya (IEEC)
Institució Catalana de Recerca i Estudis Avanc¸ats (ICREA)
Barcelona, Spain
Email: dtorres@ice.csic.es

Norbert Schartel
XMM-Newton Project Scientist
European Space Agency
Email: norbert.schartel@esa.int

Source: ESA/XMM-Newton

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NASA's Fermi Finds Possible Dark Matter Ties in Andromeda Galaxy

NASA’s Fermi telescope has detected a gamma-ray excess at the center of the Andromeda galaxy that's similar to a signature Fermi previously detected at the center of our own Milky Way. Watch to learn more. Credits: NASA’s Goddard Space Flight Center/Scott Wiessinger, producer 

The gamma-ray excess (shown in yellow-white) at the heart of M31 hints at unexpected goings-on in the galaxy's central region. Scientists think the signal could be produced by a variety of processes, including a population of pulsars or even dark matter.  Credits: NASA/DOE/Fermi LAT Collaboration and Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF

NASA’s Fermi Gamma-ray Space Telescope has found a signal at the center of the neighboring Andromeda galaxy that could indicate the presence of the mysterious stuff known as dark matter. The gamma-ray signal is similar to one seen by Fermi at the center of our own Milky Way galaxy.

Gamma rays are the highest-energy form of light, produced by the universe’s most energetic phenomena. They’re common in galaxies like the Milky Way because cosmic rays, particles moving near the speed of light, produce gamma rays when they interact with interstellar gas clouds and starlight.

Surprisingly, the latest Fermi data shows the gamma rays in Andromeda — also known as M31 — are confined to the galaxy’s center instead of spread throughout. To explain this unusual distribution, scientists are proposing that the emission may come from several undetermined sources. One of them could be dark matter, an unknown substance that makes up most of the universe.

“We expect dark matter to accumulate in the innermost regions of the Milky Way and other galaxies, which is why finding such a compact signal is very exciting,” said lead scientist Pierrick Martin, an astrophysicist at the National Center for Scientific Research and the Research Institute in Astrophysics and Planetology in Toulouse, France. “M31 will be a key to understanding what this means for both Andromeda and the Milky Way.”

A paper describing the results will appear in an upcoming issue of The Astrophysical Journal.

Another possible source for this emission could be a rich concentration of pulsars in M31’s center. These spinning neutron stars weigh as much as twice the mass of the sun and are among the densest objects in the universe. One teaspoon of neutron star matter would weigh a billion tons on Earth. Some pulsars emit most of their energy in gamma rays. Because M31 is 2.5 million light-years away, it’s difficult to find individual pulsars. To test whether the gamma rays are coming from these objects, scientists can apply what they know about pulsars from observations in the Milky Way to new X-ray and radio observations of Andromeda.

Now that Fermi has detected a similar gamma-ray signature in both M31 and the Milky Way, scientists can use this information to solve mysteries within both galaxies. For example, M31 emits few gamma rays from its large disk, where most stars form, indicating fewer cosmic rays roaming there. Because cosmic rays are usually thought to be related to star formation, the absence of gamma rays in the outer parts of M31 suggests either that the galaxy produces cosmic rays differently, or that they can escape the galaxy more rapidly. Studying Andromeda may help scientists understand the life cycle of cosmic rays and how it is connected to star formation.

“We don’t fully understand the roles cosmic rays play in galaxies, or how they travel through them,” said Xian Hou, an astrophysicist at Yunnan Observatories, Chinese Academy of Sciences in Kunming, China, also a lead scientist in this work. “M31 lets us see how cosmic rays behave under conditions different from those in our own galaxy.”

The similar discovery in both the Milky Way and M31 means scientists can use the galaxies as models for each other when making difficult observations. While Fermi can make more sensitive and detailed observations of the Milky Way’s center, its view is partially obscured by emission from the galaxy’s disk. But telescopes view Andromeda from an outside vantage point impossible to attain in the Milky Way.

“Our galaxy is so similar to Andromeda, it really helps us to be able to study it, because we can learn more about our galaxy and its formation,” said co-author Regina Caputo, a research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “It’s like living in a world where there’s no mirrors but you have a twin, and you can see everything physical about the twin.” 

While more observations are necessary to determine the source of the gamma-ray excess, the discovery provides an exciting starting point to learn more about both galaxies, and perhaps about the still elusive nature of dark matter.

“We still have a lot to learn about the gamma-ray sky,” Caputo said. “The more information we have, the more information we can put into models of our own galaxy.”

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

For more information on Fermi, visit:  http://www.nasa.gov/fermi

By Claire Saravia
NASA's Goddard Space Flight Center, Greenbelt, Md.

Editor: Rob Garner

Source: NASA/Fermi Space Telescope

NASA's Fermi Mission Expands its Search for Dark Matter

Top: Gamma rays (magenta lines) coming from a bright source like NGC 1275 in the Perseus galaxy cluster should form a particular type of spectrum (right). Bottom: Gamma rays convert into hypothetical axion-like particles (green dashes) and back again when they encounter magnetic fields (gray curves). The resulting gamma-ray spectrum ((lower curve at right) would show unusual steps and gaps not seen in Fermi data, which means a range of these particles cannot make up a portion of dark matter. Credits: SLAC National Accelerator Laboratory/Chris Smith. Download additional visuals at NASA's Scientific Visualization Studio

Dark matter, the mysterious substance that constitutes most of the material universe, remains as elusive as ever. Although experiments on the ground and in space have yet to find a trace of dark matter, the results are helping scientists rule out some of the many theoretical possibilities. Three studies published earlier this year, using six or more years of data from NASA's Fermi Gamma-ray Space Telescope, have broadened the mission's dark matter hunt using some novel approaches.

“We've looked for the usual suspects in the usual places and found no solid signals, so we've started searching in some creative new ways," said Julie McEnery, Fermi project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "With these results, Fermi has excluded more candidates, has shown that dark matter can contribute to only a small part of the gamma-ray background beyond our galaxy, the Milky Way, and has produced strong limits for dark matter particles in the second-largest galaxy orbiting it."

Dark matter neither emits nor absorbs light, primarily interacts with the rest of the universe through gravity, yet accounts for about 80 percent of the matter in the universe. Astronomers see its effects throughout the cosmos -- in the rotation of galaxies, in the distortion of light passing through galaxy clusters, and in simulations of the early universe, which require the presence of dark matter to form galaxies at all.

The leading candidates for dark matter are different classes of hypothetical particles. Scientists think gamma rays, the highest-energy form of light, can help reveal the presence of some of types of proposed dark matter particles. Previously, Fermi has searched for tell-tale gamma-ray signals associated with dark matter in the center of our galaxy and in small dwarf galaxies orbiting our own.

Although no convincing signals were found, these results eliminated candidates within a specific range of masses and interaction rates, further limiting the possible characteristics of dark matter particles.

Among the new studies, the most exotic scenario investigated was the possibility that dark matter might consist of hypothetical particles called axions or other particles with similar properties. An intriguing aspect of axion-like particles is their ability to convert into gamma rays and back again when they interact with strong magnetic fields. These conversions would leave behind characteristic traces, like gaps or steps, in the spectrum of a bright gamma-ray source.

Manuel Meyer at Stockholm University led a study to search for these effects in the gamma rays from NGC 1275, the central galaxy of the Perseus galaxy cluster, located about 240 million light-years away. High-energy emissions from NGC 1275 are thought to be associated with a supermassive black hole at its center. Like all galaxy clusters, the Perseus cluster is filled with hot gas threaded with magnetic fields, which would enable the switch between gamma rays and axion-like particles. This means some of the gamma rays coming from NGC 1275 could convert into axions -- and potentially back again -- as they make their way to us.

"While we don't yet know what dark matter is, our results show we can probe axion-like models and provide the strongest constraints to date for certain masses," Meyer said. "Remarkably, we reached a sensitivity we thought would only be possible in a dedicated laboratory experiment, which is quite a testament to Fermi."

Another broad class of dark matter candidates are called Weakly Interacting Massive Particles (WIMPs). In some versions, colliding WIMPs either mutually annihilate or produce an intermediate, quickly decaying particle. Both scenarios result in gamma rays that can be detected by the LAT.

Regina Caputo at the University of California, Santa Cruz, sought these signals from the Small Magellanic Cloud (SMC), which is located about 200,000 light-years away and is the second-largest of the small satellite galaxies orbiting the Milky Way. Part of the SMC's appeal for a dark matter search is that it lies comparatively close to us and its gamma-ray emission from conventional sources, like star formation and pulsars, is well understood. Most importantly, astronomers have high-precision measurements of the SMC's rotation curve, which shows how its rotational speed changes with distance from its center and indicates how much dark matter is present. In a paper published in Physical Review D on March 22, Caputo and her colleagues modeled the dark matter content of the SMC, showing it possessed enough to produce detectable signals for two WIMP types.

The Small Magellanic Cloud (SMC), at center, is the second-largest satellite galaxy orbiting our own. This image superimposes a photograph of the SMC with one half of a model of its dark matter (right of center). Lighter colors indicate greater density and show a strong concentration toward the galaxy's center. Ninety-five percent of the dark matter is contained within a circle tracing the outer edge of the model shown. In six years of data, Fermi finds no indication of gamma rays from the SMC's dark matter. Credits: Dark matter, R. Caputo et al. 2016; background, Axel Mellinger, Central Michigan University

"The LAT definitely sees gamma rays from the SMC, but we can explain them all through conventional sources," Caputo said. "No signal from dark matter annihilation was found to be statistically significant."

In the third study, researchers led by Marco Ajello at Clemson University in South Carolina and Mattia Di Mauro at SLAC National Accelerator Laboratory in California took the search in a different direction. Instead of looking at specific astronomical targets, the team used more than 6.5 years of LAT data to analyze the background glow of gamma rays seen all over the sky.

The nature of this light, called the extragalactic gamma-ray background (EGB) has been debated since it was first measured by NASA's Small Astronomy Satellite 2 in the early 1970s. Fermi has shown that much of this light arises from unresolved gamma-ray sources, particularly galaxies called blazars, which are powered by material falling toward gigantic black holes. Blazars constitute more than half of the total gamma-ray sources seen by Fermi, and they make up an even greater share in a new LAT catalog of the highest-energy gamma rays.

This animation switches between two images of the gamma-ray sky as seen by Fermi's Large Area Telescope (LAT), one using the first three months of LAT data, the other showing a cumulative exposure of seven years. The blue color, representing the fewest gamma rays, includes the extragalactic gamma-ray background. Blazars make up most of the bright sources shown (colored red to white). With increasing exposure, Fermi reveals more of them. A new study shows blazars are almost completely responsible for the background glow.Credits: NASA/DOE/Fermi LAT Collaboration

Some models predict that EGB gamma rays could arise from distant interactions of dark matter particles, such as the annihilation or decay of WIMPs. In a detailed analysis of high-energy EGB gamma rays, published April 14 in Physical Review Letters, Ajello and his team show that blazars and other discrete sources can account for nearly all of this emission.

"There is very little room left for signals from exotic sources in the extragalactic gamma-ray background, which in turn means that any contribution from these sources must be quite small," Ajello said. "This information may help us place limits on how often WIMP particles collide or decay."

Although these latest studies have come up empty-handed, the quest to find dark matter continues both in space and in ground-based experiments. Fermi is joined in its search by NASA's Alpha Magnetic Spectrometer, a particle detector on the International Space Station.

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

For more information about NASA's Fermi Gamma-ray Space Telescope, visit:  www.nasa.gov/fermi

By Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Md.

Editor: Ashley Morrow

 Source: NASA/Dark Energy/Matter

A mysterious ring of microwaves

A mysterious ring of microwaves
Copyright: ESA and the Planck Collaboration
Hi-res image

Fifty years ago, astronomers discovered a mystery. They called it Loop I. Today, we still have not fully resolved the mystery of how this giant celestial structure formed but we do now have the best image of it, thanks to ESA’s Planck satellite.

Loop I is a nearly circular formation that covers one third of the sky. In reality, it is probably a spherical ‘bubble’ that stretches to more than 100º across, making it wider than 200 full Moons. Its absolute size, however, is extremely uncertain because astronomers do not know how close it is to us: estimates to the centre of the bubble vary from 400 light-years to 25 000 light-years.

What they do know is that the structure shows up in many different wavelengths, from radio waves to gamma rays. Planck sees Loop I in microwaves. This image’s colours reflect the polarisation – the direction in which the microwaves are oscillating.

Our eyes are not sensitive to this information in the visible light, where we perceive only the intensity and colour. Planck, however, can detect all three of these characteristics in the microwaves it targets.

The microwaves detected by Planck are emitted by electrons that are being accelerated by the Galaxy’s magnetic field.

Loop I is most visible in the sky’s northern hemisphere. Astronomers refer to this portion as the north polar spur. It can be seen in this image as the yellow arc. This fades to purple and can be traced into the southern hemisphere, completing the circle. The blue band spanning the image horizontally is the Galactic Plane.

The most popular interpretation places Loop I close to us. If this is correct, it could be related to the ‘Scorpius–Centaurus OB Association’, a region of high-mass star formation that has been active for over 10 million years. Loop I could well be a supernova remnant, a giant bubble hollowed out by the explosion of stars in the OB association.

The stars responsible for Loop I have long since dispersed, so what we see is the ‘smoke’ rather than the ‘fire’ of the explosions.

High-mass stars burn their nuclear fuel so quickly that they live only a few million years before exploding. As these titanic supernovas bloom, their blast waves carve bubbles in the surrounding gas. This compresses the Galaxy’s magnetic field into the bubble ‘walls’, making it stronger and more efficient at accelerating the electrons to produce the observed radiation.

Loop I could well be the combined super-bubble from a number of such cataclysms. As the electrons lose energy and diffuse into the wider Galaxy, so Loop I will eventually fade and disappear. This is likely to take a few million years.

If the loop is more distant, then it could conceivably be the result of an outburst from around the black hole at the centre of the Galaxy.

(A version of the image showing the position of Loop I is available here. The colour represents the direction of polarisation, while the brightness of the colour measures the intensity of polarisation.)

Source: ESA/Planck

VERITAS Detects Gamma Rays from Galaxy Halfway Across the Visible Universe

This artist's conception shows a blazar – the core of an active galaxy powered by a supermassive black hole. The VERITAS array has detected gamma rays from a blazar known as PKS 1441+25. Researchers found that the source of the gamma rays was within the relativistic jet but surprisingly far from the galaxy's black hole. The emitting region is at least a tenth of a light-year away, and most likely is 5 light-years away. Credit: M. Weiss/CfA.   High Resolution (jpg) - Low Resolution (jpg)

Cambridge, MA - In April 2015, after traveling for about half the age of the universe, a flood of powerful gamma rays from a distant galaxy slammed into Earth's atmosphere. That torrent generated a cascade of light - a shower that fell onto the waiting mirrors of the Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Arizona. The resulting data have given astronomers a unique look into that faraway galaxy and the black hole engine at its heart.

Gamma rays are photons of light with very high energies. These gamma rays came from a galaxy known as PKS 1441+25, which is a rare type of galaxy known as a blazar. At its center it hosts a supermassive black hole surrounded by a disk of hot gas and dust.

As material from the disk swirls toward the black hole, some of it gets channeled into twin jets that blast outward like water from a fire hose only much faster - close to the speed of light. One of those jets is aimed nearly in our direction, giving us a view straight into the galaxy's core.

"We're looking down the barrel of this relativistic jet," explains Wystan Benbow of the Harvard-Smithsonian Center for Astrophysics (CfA). "That's why we're able to see the gamma rays at all."

One of the unknowns in blazar physics is the exact location of gamma-ray emission. Using data from VERITAS, as well as the Fermi Gamma-Ray Space Telescope, the researchers found that the source of the gamma rays was within the relativistic jet but surprisingly far from the galaxy's black hole. The emitting region is at least a tenth of a light-year away, and most likely is 5 light-years away. (A light-year is the distance light travels in one year, or about 6 trillion miles.)

Moreover, the region emitting gamma rays was larger than typically seen in an active galaxy, measuring about a third of a light-year across. "These jets tend to have clumps in them. It's possible that two of those clumps may have collided and that's what generated the burst of energy," says co-author Matteo Cerruti of the CfA.

Measuring high-energy gamma rays at all was a surprise. They tend to be either absorbed at the source or on their long journey to Earth. When the galaxy flared to life, it must have generated a huge flood of gamma rays.

The finding also provides insight into a phenomenon known as extragalactic background light or EBL, a faint haze of light that suffuses the universe. The EBL comes from all the stars and galaxies that have ever existed, and in a sense can track the history of the universe.

The EBL also acts like a fog to high-energy gamma rays, absorbing them as they travel through space. This new measurement sets an indirect limit on how abundant the EBL can be - too much, and it would have absorbed the gamma-ray flare. The results complement previous measurements based on direct observations.

These results have been accepted for publication in The Astrophysical Journal Letters and are available online.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, 

Contact:

Christine Pulliam
Media Relations Manager
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu

Source: Harvard-Smithsonian Center for Astrophysics (CfA)