#Today I learned about transient luminous events | Explore Tumblr Posts and Blogs

spacetimewithstuartgary · 3 years

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AAS NOVA FRBs Are Spiraling Out of Control By Astrobites First discovered in 2007, fast radio bursts (FRBs) have been the talk of the town for the last few years. As their name suggests, FRB sources emit very fast (millisecond-long) bursts of radiation at radio frequencies. Due to their very high dispersion measures (DMs), we know that FRBs are typically located in galaxies outside of the Milky Way, although the first galactic FRB was detected in March 2020. While the majority of FRBs are one-off events, some FRBs repeat, and two even repeat periodically! Even with the huge effort to study FRBs over the last few years, however, we still have much to learn about these objects. In particular, while hundreds of FRBs have been discovered, we have only localized about a dozen FRBs to their respective host galaxies. Localizing FRBs is very important for auxiliary science, such as the missing baryon problem, and for determining what produces FRBs. By comparing FRB host environments with the hosts of other transients, we can hopefully determine whether FRBs might come from similar origins. This is what today’s authors explore! Hubble for the Win! The authors use infrared (IR) and ultraviolet (UV) observations from the Hubble Space Telescope to study the host galaxies of eight different FRBs. Six of these observations are newly presented, while the host galaxy of the infamous FRB 121102 (the first repeating FRB) and FRB 180916 (the first periodic FRB) were reported in previous studies. So What Is Cooking for These Eight FRBs? First, the authors locate the FRBs within their host galaxies. Of the eight FRBs, five are located within spiral galaxies as shown below in centre image. This is not unexpected, as ~60% of the observed galaxies in our universe are spiral galaxies. While they are located within the spiral arms of their hosts, the five FRBs are not actually located at the brightest points of the spiral arms (but note that they have huge error regions!). The authors are also able to convert the UV light at the position of the FRB to the star formation rate density at this position, and the IR light at the position of the FRB to the stellar mass surface density at this location. As shown below in lower image, only FRB 121102 and FRB 180916 lie in areas with a lot of star formation (and the star formation rate for FRB 180916 is an upper limit). The other FRBs tend to lie in more moderate, although slightly higher than average, regions. This is a bit surprising as young magnetars, a very popular origin for FRBs, are typically located very close to points of high star formation. There is almost a perfect 1:1 correlation between the stellar mass surface density at the location of the FRB and the average for the FRB’s galaxy, suggesting that FRBs lie in typical surface mass density regions within their host galaxies. The authors also use the properties of the FRBs within their host galaxies to try to identify (or rule out) possible mechanisms for the origins of FRBs. In particular, they focus on five properties: 1. The distance between the FRB and the center of the host galaxy, i.e., the radial offset. 2. The distance between the FRB and the center of the host galaxy after normalizing by the size of the galaxy, i.e., the normalized radial offset. 3. The brightness of the galaxy at the location of the FRB as compared to the brightness throughout the galaxy in the UV band. The UV light can be used as a proxy for star formation within a galaxy. 4. The brightness of the galaxy at the location of the FRB as compared to the brightness throughout the galaxy in the IR band. The IR light can be used as a proxy for stellar mass within the host. 5. The amount of light within the galaxy that is interior to the FRB’s location, known as the enclosed flux. The authors compare these properties of FRBs within their hosts to that of six different transient phenomena: long duration gamma-ray bursts (LGRBs) short-duration gamma-ray bursts (SGRBs), Ca-rich transients, Type Ia supernovae (Type Ia SNe), core-collapse SNe (CCSNe), and super-luminous SNe (SLSNe). While it can be a bit difficult to remember all of these different transients, they can generally be lumped into three different categories: 1. Massive star explosions (LGRBs, CCSNe, SLSNE) 2. Explosions involving older stellars objects such as neutron star mergers (SGRBs) or white dwarfs (Type Ia SNe), and 3. Objects with unknown origins (Ca-rich transients). From their comparisons, the authors find that the properties of LGRBs, SGRBs, SLSNe and Ca-rich transients are inconsistent with those of FRBs. The table below indicates which properties are inconsistent for each of these transients (marked with X). The authors cannot rule out CCSNe and Type Ia SNE as being associated with FRBs. This is similar to another recent result that finds FRBs are consistent with CCSNe (and hence magnetars born from CCSNe) but different from this result that finds it unlikely that Type Ia SNe are associated with every FRB. For the fifth property, the enclosed flux, the authors don’t compare the distribution from the FRBs to that of the other transients. Instead, they only conclude that the FRBs appear to trace the distribution of light within their host galaxies. So Where Does This Leave Us? If there is one key transient associated with FRBs, then this work suggests that it is not LGRBs, SGRBs, SLSNe, or Ca-rich transients. However, there is one very important caveat to the authors’ work, which is that they group repeating and non-repeating FRBs together in their sample. So it is possible that there is not just one but multiple mechanisms responsible for the FRBs within their sample. Thus, the mystery of what produces FRBs continues… TOP IMAGE....Hubble Space Telescope infrared image of the spiral host galaxy of FRB 180916. [Mannings et al. 2021] CENTRE IMAGE....Host galaxy images for each of the FRBs with the FRB location circled. The FRBs that reside in spiral arms are labelled using a blue “S.” [Mannings et al. 2021] LOWER IMAGE....Left: Comparison between the star formation rate density for different FRBs as compared with the average for the host. There is no clear relation between the two, but we note that many of the observations are upper limits (indicated with triangles). Right: Comparison between the stellar mass surface density for different FRBs as compared with the average for the host. There is almost a perfect 1:1 relation between the two. [Mannings et al. 2021] Title: A High-Resolution View of Fast Radio Burst Host Environments Authors: Alexandra G. Mannings et al. First Author’s Institution: University of California, Santa Cruz Status: Submitted to ApJ 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. Original astrobite edited by Viraj Karambelkar. About the author, Alice Curtin: I’m a second year MsC student at McGill University studying Fast Radio Burts and pulsars using the Canadian Hydrogen Mapping Experiment (CHIME). My work mainly focuses on characterizing radio frequency interference, investigating multi-wavelength counterparts to FRBs, and using pulsars as calibrators of future instruments. When not doing research, I typically find myself teaching physics to elementary school students, spending time with friends, or doing something active outside.

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spacetimewithstuartgary · 4 years

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New class of astrophysical events THE ‘COW’ MYSTERY STRIKES BACK: TWO MORE RARE, EXPLOSIVE EVENTS CAPTURED; DISCOVERIES REVEAL NEW CLASS OF FAST BLUE OPTICAL TRANSIENT EVENTS The ‘Cow’ is not alone; with the help of W. M. Keck Observatory on Maunakea in Hawaii, astronomers have discovered two more like it -- the ‘Koala’ and a similar mysterious bright object called CSS161010. This trio of fast blue optical transients (FBOTs) appear to be relatives, all belonging to a highly-luminous family that has a track record for surprising astronomers with their fast, powerful bursts of energy. The ‘Koala,’ which is a nickname derived from the tail end of its official name ZTF18abvkwla, suddenly appeared as a bright new source in the optical sky before disappearing within just a few nights. A team of astronomers at Caltech realized this behavior was similar to the ‘Cow’ and requested radio observations to see if the two were connected. “When I reduced the data, I thought I made a mistake,” said Anna Ho, graduate student of astronomy at Caltech and lead author of the study. “The ‘Koala’ resembled the ‘Cow’ but the radio emission was ten times brighter -- as bright as a gamma-ray burst!” Ho and her research team’s paper is published in today’s issue of The Astrophysical Journal. Another cosmic explosion of this type, CSS161010, fascinated a team of astronomers led by Northwestern University. Based on radio observations, they calculated this transient launched material into space faster than 0.55 times the speed of light. “This was unexpected,” said Deanne Coppejans, postdoctoral associate at Northwestern University and lead author of the study on CSS161010. “We know of energetic stellar explosions that can eject material at almost the speed of light, specifically gamma-ray bursts, but they only launch a small amount of mass -- about 1 millionth the mass of the Sun. CSS161010 launched 1 to 10 percent the mass of the Sun to relativistic speeds -- evidence that this is a new class of transient!” Coppejans and her team’s paper is published in today’s issue of The Astrophysical Journal Letters. These three strange events make up a new subtype of FBOTs, which first dazzled the world in the summer of 2018 when the ‘Cow,’ short for AT2018cow, exploded in the sky. Three months later, Ho’s team captured the ‘Koala.’ Though the ‘Cow’ was the first to make world headlines, CSS161010 was actually the first FBOT discovered with luminous radio and X-ray emission, but astronomers did not know how to interpret these findings yet. “At that time, there was really no theoretical model that predicted bright radio emission from bright FBOTs,” said Coppejans. “It wasn’t until we conducted follow-up radio and X-ray observations that the true nature of CSS161010 revealed itself. Seeing it at these wavelengths is important because the data showed we were looking at something new and highly energetic.” What makes these luminous FBOTs strange is they look like supernova explosions, but flare up and vanish much faster. They’re also extremely hot, making them appear bluer in color than your standard supernovae. Also, while these new FBOTs explosions are just as violent as long gamma-ray bursts (GRBs) and can also launch outflows at relativistic velocities, their observational signatures are different in that they are surrounded by a lot of circumstellar matter. And unlike GRBs, the ‘Cow’ and CSS161010 contain hydrogen. “We don’t see these two elements in GRB-supernova spectra because we think GRBs come from dying stars that were ‘stripped’ of their hydrogen and helium envelopes prior to collapsing into a new black hole,” said Ho. Origin of Luminous FBOTS The two teams used Keck Observatory’s Low Resolution Imaging Spectrometer (LRIS) and DEep Imaging and Multi-Object Spectrograph (DEIMOS) to characterize the host galaxies of the ‘Koala’ and CSS161010; they found both FBOTs come from low-mass dwarf galaxies, just like the ‘Cow.’ “The host galaxy of CSS161010 is so small that only a 10-meter class telescope like Keck can collect enough light to allow us to physically model the emission,” said co-author Giacomo Terreran, postdoctoral associate at Northwestern University’s CIERA (Center for Interdisciplinary Exploration and Research for Astrophysics). “Remarkably, the Keck data showed the host galaxies of CSS161010, the ‘Koala,’ and the ‘Cow,’ while tiny, are actively forming stars, indicating their home base has a very small stellar mass typical of dwarf galaxies.” “This likely indicates the dwarf galaxy properties, such as the metallicity or formation history, might allow some very rare evolutionary paths of stars that lead to the most violent explosions,” said Coppejans. While both teams attribute the explosions of massive stars as the most likely cause of these new FBOTs, another possibility still under consideration is they originate from stars being devoured by black holes. If so, this new class of FBOTs could be key in the hunt for medium-sized black holes, which have yet to be detected. In general, the more massive a galaxy is, the heavier its central black hole; following this trend, it is expected that dwarf galaxies are candidates for hosting intermediate mass black holes. “One idea is that FBOTs could be the flare of a star being ripped apart by an intermediate mass black hole. If this is the case, then they could potentially be beacons to help find these elusive black holes,” said CSS161010 co-author Rafaella Margutti, assistant professor of physics and astronomy at Northwestern University and faculty member of Northwestern’s CIERA. While the origin of this type of FBOT is still hotly debated, the new data provide fresh insight on how they may have formed. “The observations prove the most luminous FBOTs have a ‘central engine’ -- a source like a neutron star or black hole that powers the transient,” said Margutti. “It’s not yet clear if these bright FBOTs are rare supernovae, stars being shredded by black holes, or other energetic phenomena. Multi-wavelength observations of more FBOTs and their environment will answer this question.” Methodology and Next Steps Due to their extremely rapid rise to maximum light, these rare FBOTs are difficult to detect. But recent developments in high-cadence optical surveys scanning huge swaths of the sky every night make the hunt for rare, short-duration transients more feasible. The key to determining their true nature is to conduct follow-up multi-wavelength observations. The ‘Koala’ was first detected using the Zwicky Transient Facility at Palomar Observatory. Ho’s team then used the Hale Telescope to obtain spectra, followed by the Very Large Array (VLA) and the Giant Metrewave Radio Telescope (GMRT) to conduct radio observations. CSS161010 was first captured by the Catalina Real-time Transient Survey and independently discovered by the All-Sky Automated Survey for Supernovae. Coppejans and her team then conducted follow-up radio observations with the VLA and GMRT, and X-ray observations with NASA’s Chandra X-ray Observatory. The radio emission is produced by the shock wave of the material slamming into the surrounding medium at more than 0.55 times the speed of light, but the X-ray emission cannot be explained this way. The team speculates they might be directly seeing the central engine in X-rays, like in the ‘Cow.’ “One lesson learned is while FBOTs have proven rarer and harder to find than some of us were hoping, in the radio band they’re also much more luminous than we’d guessed, allowing us to provide quite comprehensive data even on events that are far away,” said Daniel Perley, senior lecturer at Liverpool John Moores University’s Astrophysics Research Institute and co-author of the ‘Koala’ study. “These observations of the ‘Koala’ and CSS161010 show how much we can learn from radio and X-ray observations of FBOTs,” said Ho. “The challenge going forward is to delineate different FBOT subtypes and to develop more precise vocabulary. It’s exciting to help investigate a new and unexpected phenomenon. In science, you sometimes don’t find what you were expecting to find, but along the way you uncover new directions.” TOP IMAGE....An artist's illustration of a fast blue optical transient, or FBOT. credit: bill saxton, nrao/aui/nsf CENTRE IMAGES....Artist’s illustration comparing FBOTs to normal supernovae and gamma-ray bursts. Credit: Bill Saxton, NRAO/AUI/NSF LOWER IMAGE....A direct image of CSS161010’s host galaxy taken with W. M. Keck Observatory’s DEIMOS instrument, shown in the bottom square and magnified in the larger top square. Observations show it is a dwarf galaxy located 500,000,000 light years away in the direction of the constellation Eridanus. Image credit: G. Terreran, Northwestern University BOTTOM IMAGE....Artist’s illustration detailing the structure of FBOTs. Image credit: Bill Saxton, NRAO/AUI/NSF

#space #science #Astronomy #physics #black holes

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