Since 1958, the NASA Explorer Program has conducted low-cost missions that were deemed relevant to the goals of the Science Mission Directorate (SMD), particularly where the study of our Sun and the deeper cosmic mysteries are concerned. Recently, the Explorer Program selected four missions that they considered to be well-suited to these goals, two of which will be selected for launch in the coming years.
Consisting of two astrophysics Small Explorer (SMEX) and two Missions of Opportunity (MO) proposals, these missions are designed to study cosmic explosions and the debris they leave behind, as well as monitor how nearby stellar flares may affect the atmospheres of orbiting planets. After detailed evaluations, two of these missions will be selected next year and will take to space sometime in 2025.
In 1967, NASA scientists noticed something they had never seen before coming from deep space. In what has come to be known as the “Vela Incident“, multiple satellites registered a Gamma-Ray Burst (GRB) that was so bright, it briefly outshined the entire galaxy. Given their awesome power and the short-lived nature, astronomers have been eager to determine how and why these bursts take place.
Decades of observation have led to the conclusion that these explosions occur when a massive star goes supernova, but astronomers were still unsure why it happened in some cases and not others. Thanks to new research by a team from the University of Warwick, it appears that the key to producing GRBs lies with binary star systems – i.e. a star needs a companion in order to produce the brightest explosion in the Universe.
In 2017, LIGO (Laser-Interferometer Gravitational Wave Observatory) and Virgo detected gravitational waves coming from the merger of two neutron stars. They named that signal GW170817. Two seconds after detecting it, NASA’s Fermi satellite detected a gamma ray burst (GRB) that was named GRB170817A. Within minutes, telescopes and observatories around the world honed in on the event.
The Hubble Space Telescope played a role in this historic detection of two neutron stars merging. Starting in December 2017, Hubble detected the visible light from this merger, and in the next year and a half it turned its powerful mirror on the same location over 10 times. The result?
The deepest image of the afterglow of this event, and one chock-full of scientific detail.
When stars reach the end of their lifespan, many undergo gravitational collapse and explode into a supernova, In some cases, they collapse to become black holes and release a tremendous amount of energy in a short amount of time. These are what is known as gamma-ray bursts (GRBs), and they are one of the most powerful events in the known Universe.
Recently, an international team of astronomers was able to capture an image of a newly-discovered triple star system surrounded by a “pinwheel” of dust. This system, nicknamed “Apep”, is located roughly 8,000 light years from Earth and destined to become a long-duration GRB. In addition, it is the first of its kind to be discovered in our galaxy.
On July 2nd, 1967, the U.S. Vela 3 and 4 satellites noticed something rather perplexing. Originally designed to monitor for nuclear weapons tests in space by looking for gamma radiation, these satellites picked up a series of gamma-ray bursts (GRBs) coming from deep space. And while decades have passed since the “Vela Incident“, astronomers are still not 100% certain what causes them.
One of the problems has been that until now, scientists have been unable to study gamma ray bursts in any real capacity. But thanks to a new study by an international team of researchers, GRBs have been recreated in a laboratory for the first time. Because of this, scientists will have new opportunities to investigate GRBs and learn more about their properties, which should go a long away towards determining what causes them.
Until now, the study of GRBs have been complicated by two major issues. On the one hand, GRBs are very short lived, lasting for only seconds at a time. Second, all detected events have occurred in distant galaxies, some of which were billions of light-years away. Nevertheless, there are a few theories as to what could account for them, ranging from the formation of black holes and collisions between neutron stars to extra-terrestrial communications.
For this reason, investigating GRBs is especially appealing to scientists since they could reveal some previously-unknown things about black holes. For the sake of their study, the research team approached the question of GRBs as if they were related to the emissions of jets of particles released by black holes. As Dr. Gianluca Sarri, a lecturer at Queen’s University Belfast, explained in a recent op-ed piece with The Conversation:
“The beams released by the black holes would be mostly composed of electrons and their “antimatter” companions, the positrons… These beams must have strong, self-generated magnetic fields. The rotation of these particles around the fields give off powerful bursts of gamma ray radiation. Or, at least, this is what our theories predict. But we don’t actually know how the fields would be generated.”
With the assistance of their collaborators in the US, France, the UK and Sweden, the team from Queen’s University Belfast relied on the Gemini laser, located at the Rutherford Appleton Laboratory in the UK. With this instrument, which is one of the most powerful lasers in the world, the international collaboration sought to create the first small scale replica of GRBs.
By shooting this laser onto a complex target, the team was able to create miniature versions of these ultra-fast astrophysical jets, which they recorded to see how they behaved. As Dr. Sarri indicated:
“In our experiment, we were able to observe, for the first time, some of the key phenomena that play a major role in the generation of gamma ray bursts, such as the self-generation of magnetic fields that lasted for a long time. These were able to confirm some major theoretical predictions of the strength and distribution of these fields. In short, our experiment independently confirms that the models currently used to understand gamma ray bursts are on the right track.”
This experiment was not only important for the study of GRBs, it could also advance our understanding about how different states of matter behave. Basically, almost all phenomena in nature come down to the dynamics of electrons, as they are much lighter than atomic nuclei and quicker to respond to external stimuli (such as light, magnetic fields, other particles, etc).
“But in an electron-positron beam, both particles have exactly the same mass, meaning that this disparity in reaction times is completely obliterated,” said Dr. Sarri. “This brings to a quantity of fascinating consequences. For example, sound would not exist in an electron-positron world.”
In addition, there is the aforementioned argument that GRBs could in fact be evidence of Extra-Terrestrial Intelligence (ETI). In the Search for Extra-Terrestrial Intelligence (SETI), scientists look for electromagnetic signals that do not appear to have natural explanations. By knowing more about different types of electromagnetic bursts, scientists could be better able to isolate those for which there are no known causes. As Dr. Sarri put it:
“Of course, if you put your detector to look for emissions from space, you do get an awful lot of different signals. If you really want to isolate intelligent transmissions, you first need to make sure all the natural emissions are perfectly known so that they can excluded. Our study helps towards understanding black hole and pulsar emissions, so that, whenever we detect anything similar, we know that it is not coming from an alien civilization.”
Much like research into gravitational waves, this study serves as an example of how phenomena that were once beyond our reach is now open to study. And much like gravitational waves, research into GRBs is likely to yield some impressive returns in the coming years!
Rogue stars are moving so quickly they’re leaving the Milky Way, and never coming back. How in the Universe could this happen?
Stars are built with the lightest elements in the Universe, hydrogen and helium, but they contain an incomprehensible amount of mass. Our Sun is made of 2 x 10^30 kgs of stuff. That’s a 2 followed by 30 zeros. That’s 330,000 times more stuff than the Earth.
You would think it’d be a bit of challenge to throw around something that massive, but there are events in the Universe which are so catastrophic, they can kick a star so hard in the pills that it hits galactic escape velocity.
Rogue, or hypervelocity stars are moving so quickly they’re leaving the Milky Way, and never coming back. They’ve got a one-way ticket to galactic voidsville. The velocity needed depends on the location, you’d need to be traveling close to 500 kilometers per second. That’s more than twice the speed the Solar System is going as it orbits the centre of the Milky Way.
There are a few ways you can generate enough kick to fire a star right out of the park. They tend to be some of the most extreme events and locations in the Universe. Like Supernovae, and their big brothers, gamma ray bursts.
Supernovae occur when a massive star runs out of hydrogen, keeps fusing up the periodic table of elements until it reaches iron. Because iron doesn’t allow it to generate any energy, the star’s gravity collapses it. In a fraction of a second, the star detonates, and anything nearby is incinerated. But what if you happen to be in a binary orbit with a star that suddenly vaporizes in a supernova explosion?
That companion star is flung outward with tremendous velocity, like it was fired from a sling, clocking up to 1,200 km/s. That’s enough velocity to escape the pull of the Milky Way. Huzzah! Onward, to adventure! Ahh, crap… please do not be pointed at the Earth?
Another way to blast a star out of the Milky Way is by flying it too close to Kevin, the supermassive black hole at the heart of the galaxy.
And for the bonus round, astronomers recently discovered stars rocketing away from the galactic core as fast as 900 km/s. It’s believed that these travelers were actually part of a binary system. Their partner was consumed by the Milky Way’s supermassive black hole, and the other is whipped out of the galaxy in a gravitational jai halai scoop.
Interestingly, the most common way to get flung out of your galaxy occurs in a galactic collision. Check out this animation of two galaxies banging together. See the spray of stars flung out in long tidal tails? Billions of stars will get ejected when the Milky Way hammers noodle first into Andromeda.
A recent study suggests half the stars in the Universe are rogue stars, with no galaxies of their own. Either kicked out of their host galaxy, or possibly formed from a cloud of hydrogen gas, flying out in the void. They are also particularly dangerous to Carol Danvers.
Considering the enormous mass of a star, it’s pretty amazing that there are events so catastrophic they can kick entire stars right out of our own galaxy.
What do you think life would be like orbiting a hypervelocity star? Tell us your thoughts in the comments below.
Gamma-ray bursts (GRBs) represent the most powerful explosions in the cosmos, sending out as much energy in a matter of seconds as our Sun will give off during its entire 10-billion-year lifespan.
These powerful explosions are thought to be triggered when dying stars collapse into jet-spewing black holes. Yet no one has ever witnessed a GRB directly. Instead astronomers are left to study their fading light.
But some GRBs mysteriously seem to have no afterglow. Now, observations from the Atacama Large Millimeter/submillimeter Array (ALMA) are shedding light on these so-called dark bursts.
One possible explanation is that dark bursts explode so far away their visible light is extinguished due to the expansion of the Universe. Another possible explanation is that dark bursts explode in galaxies with unusually thick amounts of interstellar dust, which absorb a burst’s light.
Neither explanation, however, seems likely as astronomers anticipate that GRB progenitors — massive stars — are found in active star-forming regions surrounded by large amounts of molecular gas. But unfortunately there has never been an observational result to back up this theory either.
So astronomers have been working hard to better understand GRBs by studying their host galaxies. Now, a Japanese team of astronomers led by Bunyo Hatsukade from the National Astronomical Observatory in Japan, has used ALMA to report the first-ever map of molecular gas and dust in two galaxies that were previously rocked by GRBs.
Hatsukade and colleagues detected the radio emission from molecular gas and dust in two dark host galaxies — GRB 020819B and GRB 051022 — at about 4.3 billion and 6.9 billion light-years away, respectively.
“We have been searching for molecular gas in GRB host galaxies for over 10 years using various telescopes around the world,” said Kotaro Kohno from the University of Tokyo in a press release. “As a result of our hard work, we finally achieved a remarkable breakthrough using the power of ALMA. We are very excited with what we have achieved.”
Watch the video below for an artist concept animation of the environment around GRB 020819B based on ALMA observations:
The telescope’s high sensitivity enabled the team of astronomers to detect the emission from molecular gas, as opposed to most telescopes, which can only probe absorption along the line of sight. This combined with its high spatial resolution provided the first detailed map of the molecular gas and dust throughout a GRB host galaxy.
Surprisingly, less gas was observed than expected, and correspondingly much more dust. The ratio of dust to molecular gas at the GRB site is 10 times higher than in normal environments.
“We didn’t expect that GRBs would occur in such a dusty environment with a low ratio of molecular gas to dust,” said Hatsukade. “This indicates that the GRB occurred in an environment quite different from a typical star-forming region.”
The research team thinks the high proportion of dust compared to molecular gas is likely due to the intense ultraviolet radiation from the young, massive stars, which will break up any molecular gas while leaving the dust relatively undisturbed.
It’s becoming clear that dust absorbs the afterglow radiation, causing these dark gamma-ray bursts. The team plans to carry out further observations and is excited to use ALMA’s incredible sensitivity to probe other host galaxies.
Roughly once a day the sky is lit up by a mysterious torrent of energy. These events — known as gamma-ray bursts — represent the most powerful explosions in the cosmos, sending out as much energy in a fraction of a second as our Sun will give off during its entire lifespan.
Yet no one has ever witnessed a gamma-ray burst directly. Instead astronomers are left to study their fading light.
New research from an international team of astronomers has discovered a puzzling feature within one Gamma-ray burst, suggesting that these objects may behave differently than previously thought.
These powerful explosions are thought to be triggered when dying stars collapse into jet-spewing black holes. While this stage only lasts a few minutes, its afterglow — slowly fading emission that can be seen at all wavelengths (including visible light) — will last for a few days to weeks. It is from this afterglow that astronomers meticulously try to understand these enigmatic explosions.
The afterglow emission is formed when the jets collide with the material surrounding the dying star. They cause a shockwave, moving at high velocities, in which electrons are being accelerated to tremendous energies. However, this acceleration process is still poorly understood. The key is in detecting the afterglow’s polarization — the fraction of light waves that move with a preferred plane of vibration.
“Different theories for electron acceleration and light emission within the afterglow all predict different levels of linear polarization, but theories all agreed that there should be no circular polarization in visible light,” said lead author Klaas Wiersema in a press release.
“This is where we came in: we decided to test this by carefully measuring both the linear and circular polarization of one afterglow, of GRB 121024A, detected by the Swift satellite.”
And to their surprise, the team detected circular polarization, meaning that the light waves are moving together in a uniform, spiral motion as they travel. The gamma-ray burst was 1000 times more polarized than expected. “It is a very nice example of observations ruling out most of the existing theoretical predictions,” said Wiersema.
The detection shows that current theories need to be re-examined. Scientists expected any circular polarization to be washed out. The radiation of so many electrons travelings billions of light-years would erase any signal. But the new discovery suggests that there could be some sort of order in the way these electrons travel.
Of course the possibility remains that this particular afterglow was simply an oddball and not all afterglows behave like this.
Nonetheless “extreme shocks like the ones in GRB afterglows are great natural laboratories to push our understanding of physics beyond the ranges that can be explored in laboratories,” said Wiersema.
Last weekend (April 27, 2013), the Fermi and Swift spacecraft witnessed a “shockingly” bright burst of gamma rays from a dying star. Named GRB 130427A, it produced one of the longest lasting and brightest GRBs ever detected.
Because Swift was able to rapidly determine the GRB’s position in the sky, and also because of the duration and brightness of the burst, the GRB was able to be detected in optical, infrared and radio wavelengths by ground-based observatories. Astronomers quickly learned that the GRB had one other near-record breaking quality: it was relatively close, as it took place just 3.6 billion light-years away.
“This GRB is in the closest 5 percent of bursts, so the big push now is to find an emerging supernova, which accompanies nearly all long GRBs at this distance,” said Neil Gehrels, principal investigator for Swift.
“We have waited a long time for a gamma-ray burst this shockingly, eye-wateringly bright,” said Julie McEnery, project scientist for the Fermi Gamma-ray Space Telescope. “The GRB lasted so long that a record number of telescopes on the ground were able to catch it while space-based observations were still ongoing.”
No two GRBs are the same, but they are usually classified as either long or short depending on the burst’s duration. Long bursts are more common and last for between 2 seconds and several minutes; short bursts last less than 2 seconds, meaning the action can all over in only milliseconds.
This recent event started just after 3:47 a.m. EDT on April 27. Fermi’s Gamma-ray Burst Monitor (GBM) triggered on the eruption of high-energy light in the constellation Leo. The burst occurred as NASA’s Swift satellite was slewing between targets, which delayed its Burst Alert Telescope’s detection by a few seconds.
Fermi’s Large Area Telescope (LAT) recorded one gamma ray with an energy of at least 94 billion electron volts (GeV), or some 35 billion times the energy of visible light, and about three times greater than the LAT’s previous record. The GeV emission from the burst lasted for hours, and it remained detectable by the LAT for the better part of a day, setting a new record for the longest gamma-ray emission from a GRB.
As far as the optical brightness of this event, according to a note posted on the BAUT Forum (the Universe Today and Bad Astronomy forum) data from the SARA-North 1-meter telescope at at Kitt Peak in Arizona at about 04:00 UT on April 29 showed a relative magnitude of about 18.5.
Gamma-ray bursts are the universe’s most luminous explosions, and come from the explosion of massive stars or the collision between two pulsars. Colliding pulsars are usually of short duration, so astronomers can rule out a pulsar collision as causing this event.
If the GRB is near enough, astronomers usually discover a supernova at the site a week or so after the outburst.
NASA said that ground-based observatories are monitoring the location of GRB 130427A and expect to find an underlying supernova by midmonth.
According to astronomer Andrew Levan, there’s an old adage in studying gamma ray bursts: “When you’ve seen one gamma ray burst, you’ve seen … only one gamma ray burst. They aren’t all the same,” he said during a press briefing on April 16 discussing the discovery of a very different kind of GRB – a type that comes in a new long-lasting flavor.
Three of these unusual long-lasting stellar explosions have recently been discovered using the Swift satellite and other international telescopes, and one, named GRB 111209A, is the longest GRB ever observed, with a duration of at least 25,000 seconds, or about 7 hours.
“We have observed the longest gamma ray burst in modern history, and think this event is caused by the death of a blue supergiant,” said Bruce Gendre, a researcher now associated with the French National Center for Scientific Research who led this study while at the Italian Space Agency’s Science Data Center in Frascati, Italy. “It caused the most powerful stellar explosion in recent history, and likely since the Big Bang occurred.”
The astronomers said these three GRBs represent a previously unrecognized class of these stellar explosions, which arise from the catastrophic deaths of supergiant stars hundreds of times larger than our Sun. GRBs are the most luminous and mysterious explosions in the Universe. The blasts emit surges of gamma rays — the most powerful form of light — as well as X-rays, and they produce afterglows that can be observed at optical and radio energies.
Swift, the Fermi telescope and other spacecraft detect an average of about one GRB each day. As to why this type of GRB hasn’t been detected before, Levan explained this new type appears to be difficult to find because of how long they last.
“Gamma ray telescopes usually detect a quick spike, and you look for a burst — at how many gamma rays come from the sky,” Levan told Universe Today. “But these new GRBs put out energy over a long period of time, over 10,000 seconds instead of the usual 100 seconds. Because it is spread out, it is harder to spot, and only since Swift launched do we have the ability to build up images of GBSs across the sky. To detect this new kind, you have to add up all the light over a long period of time.”
Levan is an astronomer at the University of Warwick in Coventry, England.
He added that these long-lasting GRBs were likely more common in the Universe’s past.
Traditionally, astronomers have recognized two types of GRBs: short and long, based on the duration of the gamma-ray signal. Short bursts last two seconds or less and are thought to represent a merger of compact objects in a binary system, with the most likely suspects being neutron stars and black holes. Long GRBs may last anywhere from several seconds to several minutes, with typical durations falling between 20 and 50 seconds. These events are thought to be associated with the collapse of a star many times the Sun’s mass and the resulting birth of a new black hole.
“It’s a very random process and every GRB looks very different,” said Levan during the briefing. “They all have a range of durations and a range of energies. It will take much bigger sample to see if this new type have more complexities than regular gamma rays bursts.”
All GRBs give rise to powerful jets that propel matter at nearly the speed of light in opposite directions. As they interact with matter in and around the star, the jets produce a spike of high-energy light.
Gendre and his colleagues made a detailed study of GRB 111209A, which erupted on Dec. 9, 2011, using gamma-ray data from the Konus instrument on NASA’s Wind spacecraft, X-ray observations from Swift and the European Space Agency’s XMM-Newton satellite, and optical data from the TAROT robotic observatory in La Silla, Chile. The 7-hour burst is by far the longest-duration GRB ever recorded.
Another event, GRB 101225A, exploded on December 25, 2010 and produced high-energy emission for at least two hours. Subsequently nicknamed the “Christmas burst,” the event’s distance was unknown, which led two teams to arrive at radically different physical interpretations. One group concluded the blast was caused by an asteroid or comet falling onto a neutron star within our own galaxy. Another team determined that the burst was the outcome of a merger event in an exotic binary system located some 3.5 billion light-years away.
“We now know that the Christmas burst occurred much farther off, more than halfway across the observable universe, and was consequently far more powerful than these researchers imagined,” said Levan.
Using the Gemini North Telescope in Hawaii, Levan and his team obtained a spectrum of the faint galaxy that hosted the Christmas burst. This enabled the scientists to identify emission lines of oxygen and hydrogen and determine how much these lines were displaced to lower energies compared to their appearance in a laboratory. This difference, known to astronomers as a redshift, places the burst some 7 billion light-years away.
Levan’s team also examined 111209A and the more recent burst 121027A, which exploded on Oct. 27, 2012. All show similar X-ray, ultraviolet and optical emission and all arose from the central regions of compact galaxies that were actively forming stars. The astronomers have concluded that all three GRBs constitute a new kind of GRB, which they are calling “ultra-long” bursts.
“Ultra-long GRBs arise from very large stars,” said Levan, “perhaps as big as the orbit of Jupiter. Because the material falling onto the black hole from the edge of the star has further to fall it takes longer to get there. Because it takes longer to get there, it powers the jet for a longer time, giving it time to break out of the star.”
Levan said that Wolf-Rayet stars best fit the description. “They are born with more than 25 times the Sun’s mass, but they burn so hot that they drive away their deep, outermost layer of hydrogen as an outflow we call a stellar wind,” he said. Stripping away the star’s atmosphere leaves an object massive enough to form a black hole but small enough for the particle jets to drill all the way through in times typical of long GRBs
John Graham and Andrew Fruchter, both astronomers at the Space Telescope Science Institute in Baltimore, provided details that these blue supergiant contain relatively modest amounts of elements heavier than helium, which astronomers call metals. This fits an apparent puzzle piece, that these ultra-long GRBs seem to have a strong intrinsic preference for low metallicity environments that contain just trace amounts of elements other than hydrogen and helium.
“High metalicity long duration GRBs do exist but are rare,” said Graham. “They occur at about 1/25th the rate (per unit of star formation) of the low metallicity events. This is good news for us here on Earth, as the likelihood of this type of GRB going off in our own galaxy is far less than previously thought.”
The astronomers discussed their findings Tuesday at the 2013 Huntsville Gamma-ray Burst Symposium in Nashville, Tenn., a meeting sponsored in part by the University of Alabama at Huntsville and NASA’s Swift and Fermi Gamma-ray Space Telescope missions. Gendre’s findings appear in the March 20 edition of The Astrophysical Journal.