Guide to Transient Astronomy

by Jason Hinkle and Jessie Thwaites

What are transients?

Transients are astrophysical phenomena that change their brightness over a relatively short time. These can be caused by a number of different astrophysical sources, and each of these transient events has particular characteristics (for example, we typically detect Fast Radio bursts in the radio, but kilonova through gravitational wave detectors). 

Some of these phenomena happen in our galaxy (galactic) while others are outside the Milky Way (extragalactic). We’ll explore a few different types of transients in this guide, but this list is not exhaustive of all transients that can be studied.

Extragalactic Transients

Supernovae

Supernovae are the explosive end stages of the life of a star. They are very important to chemical evolution in the universe, as they create most of the heavy elements (what astronomers call metals) we see around us. After supernovae explode, these metals are incorporated into new generations of stars and may go on to form planets and life. Supernovae are very intrinsically bright, and in most cases they even outshine their own host galaxy! They can also drive large-scale galactic winds, helping to regulate the evolution of star formation within galaxies.

For galaxies like the Milky Way, we expect supernovae to occur about once every one hundred years on average. Supernovae come in two main observational classes, Type I (which do not show lines of hydrogen in their spectra) and Type II (which do show hydrogen). In the decades since supernovae were first recognized, we have been able to link subtypes of supernovae to physical origins. 

Thermonuclear (Type Ia):

Type Ia supernovae are the deaths of white dwarfs. A white dwarf is formed from a low mass star (less than ~8 solar masses) as it ends its life. White dwarfs are essentially cinders of the dead star and are supported by electron degeneracy pressure. The majority of a white dwarf’s life is spent slowly cooling down over time, but if they have a binary companion, they can go supernova.

There are two possible scenarios for a Type Ia supernova, known as single degenerate and double degenerate. In the single degenerate scenario, a regular star serves as the companion to the white dwarf and slowly loses mass to the white dwarf until the white dwarf explodes. Conversely, the double degenerate scenario consists of two white dwarfs, which eventually merge, triggering the explosion. While many of the details on these explosions remain topics of active research, it appears that the large majority of SNe Ia come from the double degenerate channel

Type Ia supernovae are important for cosmology as they are a type of standardizable candle. In fact, it was Type Ia supernovae that were responsible for our discovery of dark energy and the accelerating universe!

Core-collapse (Types Ib, Ic, and II):

Massive stars (greater than about 8 solar masses) will end their lives as a core-collapse supernova (CCSN). The supernova occurs when the core of the star is made of iron, and can no longer continue to create energy from fusion. At this point, both the core and the outer layers of the star rapidly collapse due to the lack of internal pressure support. The core then collapses enough that neutron degeneracy pressure takes over and the core cannot be compressed further. As the outer layers hit the core, they then “bounce” off creating a shock wave and releasing energy, which we see as the supernova.

There are several flavors of core-collapse supernovae, related to the properties of the star that died (also called the progenitor star). Type II supernovae progenitor stars retain their hydrogen envelope and therefore show strong lines of hydrogen in their spectra. Types Ib and Ic however do not show H lines and are referred to as “stripped-envelope” supernovae. The distinction between Ib and Ic is the presence of helium lines, which are not seen for Type Ic supernovae. 

Despite being bright in electromagnetic radiation (light), CCSNe are even “brighter” in neutrinos, as roughly 99% of the total supernova energy is emitted through neutrinos. However, neutrinos are notoriously difficult to detect given their lack of interaction with other particles. For example, for even the nearest recent supernovae (SN1987A in the Large Magellanic Cloud) only about 20 neutrinos were detected.

Depending on the mass of the star, the core will either be left as a ball of neutrons (otherwise known as a neutron star) or be compressed even further by the shock wave to become a black hole. CCSNe are important to study not only to understand the end stages of massive stars, but also the formation of compact objects like neutron stars and black holes.

Superluminous:

There is a rare class of supernovae roughly 100 times brighter than typical supernovae, that has been aptly named superluminous supernovae (SLSNe). These also come in two main types, SLSNe-I (without H lines) and SLSNe-II (with H lines). SLSNe-II are thought to be powered by interactions between the material ejected by the supernova and the circumstellar medium. In addition to their extreme brightness, SLSNe also have extreme durations, living hundreds of days (as opposed to the tens of days of most other supernovae).

SLSNe-I on the other hand are likely powered by highly magnetized neutron stars (known as magnetars). These magnetars are initially spinning incredibly fast, often on the order of several hundred rotations per second. As the magnetar spins down, it injects energy into the outflowing supernova material, causing it to glow brightly. SLSNe give us insight into some of the most energetic events in the universe and the formation of compact objects.

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Gamma-ray bursts

Gamma-ray bursts are some of the most energetic events in the universe. They were first discovered in 1967 (by accident), and since that time thousands of bursts have been detected. GRBs were first observed by the US Vela nuclear detection satellites, built to detect gamma-rays from possible nuclear testing in space. They detected a flash of gamma-rays unlike any ever seen before, and continued iterations of the Vela satellites allowed for the discovery and later the localization of more GRBs.

The very short duration of GRBs initially made it difficult to determine if they were galactic or extragalactic objects. But, with the launch of the Compton Gamma-ray Observatory, GRBs were found to be distributed uniformly across the sky, and this suggested that GRBs are primarily extragalactic objects.

There are two main types of GRBs, with the main distinction based on their duration: short and long GRBs. Short GRBs typically last shorter than 2 seconds and have been associated with the merger of compact objects (such as black holes and neutron stars). Long GRBs last longer than 2 seconds and are produced during the death of rapidly-rotating extremely massive stars (occasionally called hypernovae, but this term isn’t used as often). However, these classes do not always have clear-cut distinctions, and there may be some overlap in these classes. There is also a possibility that some GRBs are produced in other types of sources, such as tidal disruption events (TDEs), magnetar giant flares, or other sources.

Short Gamma-ray bursts:

On August 17th, 2017, a short gamma-ray burst, GRB170817A was observed by Fermi and INTEGRAL just 1.7s after a gravitational wave event (GW170817) was observed by the LIGO-Virgo gravitational wave detectors. This association of the GRB and GW confirmed that short GRBs can be produced in neutron star mergers.

Long Gamma-ray bursts:

Long GRBs are generally associated with the deaths of massive stars. All GRBs close enough for us to observe have an associated Type Ib/c-BL supernova, sometimes called GRB-SN. These tend to be extremely energetic, and are often associated with an afterglow, which is continued emission. Initially the afterglow emission is highly energetic, but as the jet slows the emission becomes less energetic and longer wavelengths are emitted (this can be in x-rays, optical, and eventually radio).

Why are GRBs so interesting?

GRBs are an incredibly energetic class of transients, which (depending on the type) can emit all across the electromagnetic spectrum. They give us insight into the deaths of the most massive stars, such as hypernovae or core-collapse supernovae

GRBs are also very interesting from a multi-messenger astronomy standpoint. GRBs are made up of electromagnetic radiation, but their extremely high energy nature for both long and short GRBs could accelerate ultra-high energy cosmic rays and produce neutrinos as well. In addition, we expect to also observe gravitational waves for short GRBs from the compact object merger(and possibly also for long GRBs as well) meaning that GRBs may be one of the few cases where all 4 messengers (electromagnetic radiation, cosmic rays, neutrinos, and gravitational waves) can be seen from the same source!

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Active Galactic Nuclei (AGN)

At the center of every massive galaxy lives a supermassive back hole (SMBH). SMBHs are notoriously hard to study as they emit no light of their own and are only detectable through their gravitational influence on nearby stars and/or gas. Only recently has the Event Horizon Telescope (EHT) (see this astrobite) allowed us to directly observe the regions immediately surrounding a SMBH.

However, there is a class of objects that allows us to indirectly infer properties of black holes, called active galactic nuclei (AGN). Residing in a few percent of galaxies, an AGN is an actively-accreting SMBH. As material falls onto the SMBH it is heated and glows. Observationally, AGN are distinct from normal galaxies in that there is a strong point source of light at the center that cannot be explained by star formation processes alone.

AGN are extremely diverse in their observational and physical characteristics. AGN can either be radio-quiet (i.e. little to no radio emission) or radio-loud (strong radio emission caused by a relativistic jet). In both classes the angle at which we view the central regions of the AGN strongly affects the observed emission. However, one similarity for nearly all AGN is that they are multi-wavelength objects, emitting strong X-ray, UV, optical, and infrared emission.

AGNs are best known for their variability in time, as well as strong narrow and sometimes broad emission lines in their spectra, caused by gas orbiting in the strong gravity of the SMBH.

AGNs are important laboratories for many types of high-energy physics including accretion, the launching of jets, and high-energy neutrino production. They also allow us to study SMBHs throughout cosmic time.

Cartoon image of an active galactic nucleus. at the center is a black dot, representing the supermassive black hole, and around it are different arrows at various angles showing the different types of AGN.
Cartoon depiction of a typical AGN geometry. Various arrows indicate the observed class of AGN for that viewing angle.
Image credit: Emma Alexander, CC BY 4.0, via Wikimedia Commons

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Tidal Disruption Events (TDEs)

A tidal disruption event (TDE) occurs when a star passes too close to a SMBH and the tidal forces from the black hole overwhelm the gravity holding the star together. When this happens, about half of the stellar mass falls back onto the SMBH, causing a luminous flare. These flares are extremely hot, with temperatures a few times the temperature of our Sun. In addition to the strong UV and optical emission from TDEs, many also exhibit strong soft X-ray emission. TDEs also often show extremely broad emission lines of hydrogen and helium without any of the narrow lines seen in AGN.

Tidal disruption events may also be promising targets for multi-messenger astrophysics. One TDE has already been claimed as a source of astrophysical neutrinos and several studies suggest that advanced gravitational wave observatories may be able to detect TDEs.

Unlike AGN, which can exist for millions of years, a TDE is a relatively short event with a lifetime of only a few years. Because of this, TDEs represent a unique way to study SMBHs that are otherwise quiescent (i.e. not accreting material). Nevertheless, it is only with the recent growth in astronomical surveys that TDEs have been studied in detail and much remains to be learned about the physics powering them. 

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Fast Radio Bursts (FRBs)

Fast Radio Bursts are a relatively new type of transient source, which were first discovered in 2007 by Prof. Duncan Lorimer and his student David Narkevic, in archival data from the Parkes telescope. The first FRB is thus often referred to as the Lorimer burst, after Prof. Lorimer. Now, hundreds of FRBs have been reported and cataloged on the Transient Name Server (TNS) by many experiments. The largest FRB catalog to be released is CHIME Catalog 1, which was released in early 2021 and contained 536 individual bursts observed by the CHIME telescope in a year. 

FRBs are very energetic sources (~10X stronger than the Sun), which usually only last on the order of milliseconds. There appear to be two different types: FRBs that repeat (repeaters) and those that have only one burst (non-repeaters). A few repeaters are very active repeaters, and some are even periodic e.g. FRB121102 (repeats about every 157 days), and FRB20190520B (repeats 4.5 times per hour).

A few bursts have also been localized to a given galaxy, or even a given progenitor within a galaxy. For example, an FRB was localized to a globular cluster in the M81 galaxy, and another FRB-like burst came from magnetar SGR 1935+2154. However, the origins of FRBs are still unknown! The leading progenitor models for FRBs are magnetars, but exactly how a magnetar produces an FRB, and whether magnetars are the only source of FRBs, remain open questions. Additionally, FRBs have only been detected in the radio, but they might very well emit high-energy photons or even  produce neutrinos, although neither of these have been observed yet. 

FRBs are an exciting short transient, because so many are expected to be seen — the CHIME/FRB Collaboration estimates a rate of >500 occurring over the full sky every day. This means the number of FRBs observed could massively increase in the next few years, as more bursts are found by many telescopes.

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Kilonovae

A kilonova happens when two neutron stars (or one neutron star and a black hole) merge. When this happens, there is some matter that is flung out of the system, which then undergoes rapid nucleosynthesis and glows brightly. The name kilonova comes from the fact that these bright bursts are about 1000 times more bright than a classical nova, but are otherwise very different processes from novae (or supernovae).

Compact objects generally refer to black holes, neutron stars (2-3 times the mass of the sun) , and white dwarfs (up to ~1.4 times the mass of our sun). Any of these objects merging can produce gravitational waves; however, we usually hear most about mergers including black holes or neutron stars. This is because the LIGO-Virgo-KAGRA detectors are most sensitive at higher frequencies, which are produced primarily by binary black hole (BBH), neutron star/black hole (NSBH), and binary neutron star (BNS) mergers, while white dwarf mergers produce lower frequency signatures of up to a few milli-Hertz (mHz).

What is the difference between a kilonova and a general compact object merger?

A kilonova is specifically the bright electromagnetic radiation that happens when at least one of the compact objects is a neutron star. A compact object merger refers to any time that two compact objects of any type merge together, producing gravitational waves.

When we see a kilonova we would also expect to see gravitational waves, since both occur from a compact object merger. However, if we see gravitational waves this is not necessarily a kilonova, since we can have a binary black hole merger, which may not produce the bright burst of electromagnetic radiation expected for a kilonova.

Artist rendition of a neutron star merger. There are ripples from gravitational waves moving away, and a bright jet of material traveling away from the merging objects.
Artist rendition of a binary neutron star merger, such as GW170817. This GW event has been observed in both gravitational waves and light. Image credit: 
NSF/LIGO/Sonoma State University/A. Simonnet, CC BY 4.0, via Wikimedia Commons

The case of GW170817 – a success story in multi-messenger astronomy

GW170817 was a gravitational wave event, observed by the LIGO-Virgo gravitational wave detectors on August 17, 2017. It was a collision of two neutron stars, and 1.7 seconds after the merger Fermi-GBM observed a short gamma ray burst, GRB170817A, which was associated with the same object. A few hours later, SSS17a, the first optical counterpart to a GW event, was observed.

This was the first case of observations of one transient source using two messengers, gravitational waves and electromagnetic radiation, making it an exciting event for multi-messenger astronomy.

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Galactic Transients

Novae

(Classical) Novae are bright outbursts that happen in a binary system that has a white dwarf and a main sequence star. They orbit each other closely and have an extremely strong gravitational pull between them, which causes matter from the star to be pulled into an accretion disk and eventually deposited on the white dwarf (the mechanism for this is called Roche lobe overflow). 

As this happens, more and more material is accumulated onto the white dwarf, increasing its pressure and temperature. Eventually, this will cause a runaway fusion reaction, which releases a huge amount of energy and ejects some of the hydrogen on the surface of the white dwarf. 

An artist’s rendition of a nova. We can see the white dwarf (bluish-white on the right) accreting matter from its companion star (orange-red, on left).
Image credit: NASA/CXC/M.Weiss, Public domain, via Wikimedia Commons

This releases a bright burst of light, which is the nova. A nova can have an increase in luminosity that typically ranges from 8.6-12 magnitudes, depending on the type of nova, but can brighten by up to 19 magnitudes. (A magnitude in astronomy is a unitless measure of brightness, with each magnitude being about 2.5 times brighter than the last).

There are some novae that are seen to reoccur, and thus are called recurrent novae. Because the process described above does not destroy the binary system or the white dwarf, the same process could occur multiple times, leading to a recurrent nova.

Novae were originally named nova stella (Latin for new star), which is what early astronomers thought they were seeing. 

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M-dwarf flares

M-dwarf stars (sometimes called red dwarfs) are the most common stars in the universe. They are between roughly 0.08 and 0.6 solar masses and have temperatures between 2400 and 4000 K. Due to their small sizes, low masses, and high abundances they are promising candidates for which to search for habitable exoplanets. Indeed, the TESS mission is optimized to search for planets around low mass dwarfs. 

Studies of M-dwarfs with recent space-based missions like Kepler, K2, and TESS along with ground-based surveys like ASAS-SN and Evryscope have shown that M-dwarfs often exhibit large-scale brightness increases called flares. These flares are thought to be caused by magnetic reconnection, a form of magnetic activity occurring on the star. For the most active M-dwarfs, these stars can exhibit flares at least once per day. Flares are much hotter than the surfaces of the stars on which they occur and emit significant amounts of energy.

Flares are important both for understanding the magnetic fields of stars, but also crucially for the formation of life. Life likely requires enough UV flux to promote the formation of complex molecules, but too much UV flux can strip exoplanet atmospheres. Given the low quiescent UV fluxes of M-dwarfs, the UV emission from flares may be important towards allowing the formation of life on planets orbiting low mass stars.

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Variable Stars

To the human eye, and even to many telescopes, stars appear constant in brightness over time. While this may be true for many stars, there is a fraction of stars that changes in brightness over time, known as variable stars. There are two main types of variable stars, intrinsic and extrinsic, with brightness changes due to internal and external processes respectively.

Intrinsic Variables:

There are two main types of intrinsic variables: pulsating and eruptive. Pulsating stars show periodic changes in their brightness caused by mechanisms that change the radius and temperature of the star. Common examples include Cepheid, RR Lyrae, and Mira variables, which show similar behavior despite different effective temperatures and pulsation periods. These stars can also be used as distance indicators based on well-known relations between their periods and luminosities.

Eruptive variables show large changes in brightness due to outbursts. Some eruptive variables, like novae, cataclysmic variables, and flare stars show increases in brightness during these events. Others, like the rare R Coronae Borealis stars, show dramatic decreases in brightness due to an outburst of dust blocking significant amounts of light. Eruptive variables can teach us important information about accretion, magnetic fields, and dust formation.

Extrinsic Variables:

The two main types of extrinsic variables are eclipsing binaries and rotational variables. Eclipsing binaries are systems of two (or more) stars in which the stars pass in front of each other during their orbits. This causes a periodic dip in brightness, one for each star in the binary per orbital period. Eclipsing binaries are very important to our understanding of stars, as they give strong constraints on stellar radii and masses from the eclipse depths and orbital periods.

Rotational variables show period variations in brightness due to starspots on their surface. Because starspots are cooler than the surrounding stellar surface, they are fainter. As the spots rotate in and out of view as the star rotates, we see the brightness of the star modulated at the rotational period. In fact, our very own Sun is a rotational variable, but at a very low amplitude. Rotation can be an important indicator of age, especially for low-mass stars that evolve slowly and are difficult to age through other means.

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Rotating Radio Transients (RRATs)

Radio pulsars are a type of rapidly rotating neutron star, which produces highly beamed radio waves directed at the observer. Although we often think of pulsars as reliably and regularly repeating sources, like those that Jocelyn Bell Burnell first found when she discovered the first pulsars, not all pulsars repeat on a consistent schedule. These inconsistent and somewhat erratic pulses, often discovered through occasional isolated pulses, are called Rotating Radio Transients, or RRATs

RRATs were first found using the Parkes Multibeam pulsar survey in 2006, which identified 11 RRATs. Similar to pulsars, emission from RRATs lasts for a few milliseconds. However, unlike pulsars, the time between subsequent radio emission is sporadic, and can range from a few seconds to hours. 

Could there be a link between RRATs and FRBs?

Fast Radio Bursts (FRBs) and Rotating Radio Transients (RRATs) appear to have very similar characteristics – so what is the difference between these classes? 

For starters, most FRBs are extragalactic objects (although see this Astrobite for an FRB-like burst from our own galaxy), while RRATs are galactic rotating neutron stars.  Additionally, FRBs are much more energetic (~10 orders of magnitude more energetic!) than most RRATs (see Fig. 3 in this Astrobite).

FRBs do, however, show weird, sporadic emission. Some FRB sources repeat periodically, some FRB sources repeat but with no periodicity detected, and some FRB sources have only emitted a single burst. This kind of sounds like RRATs, which similarly emit sporadically. Additionally, some RRATs have been found to be periodic when ultra-sensitive telescopes are used, suggesting that maybe we just aren’t sensitive enough to probe the necessary energy levels to detect the periodicities.

So, RRATs might be one clue for figuring out the nature of FRBs, but they definitely aren’t the only clue!  

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Why study transients?

Studying transients tells us about some of the most variable and most explosive objects in our universe! Transients are an important group of objects to study in order to understand the dynamics and evolution of our universe. Transients also play an important role in understanding the physics of non-transient sources, like using supernovae to understand stellar evolution. Regardless of how transients are being used to probe the universe around us, they will continue to be a large part of astronomy in the future as more and more telescopes and surveys are being built to study the ever-changing universe around us.

Astrobite edited by: Briley Lewis, Alice Curtin, Huei Sears, Lindsay DeMarchi

Featured Image Credit: ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray) Derivative work including grading and crop: Julian Herzog, CC BY 4.0, via Wikimedia Commons

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