Black Hole Destroys Star and Shoots Jet

Black Hole Destroys Star and Shoots Jet
Black Hole Destroys Star and Shoots Jet
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Astronomers have watched the growth of a jet fueled by a shredded star.

When stars zip too close to a supermassive black hole, they enter dangerous territory. How close “too close” is depends on the black hole, but for one that’s 10 million times the mass of the Sun, any star venturing closer than an astronomical unit is done for: The black hole will rip the star apart. Torn asunder, half the star goes whizzing away, while the other half forms a disk of hot gas around its destroyer. This gas heats and glows, appearing to our telescopes as a long-lasting flare.

Astronomers have detected a few dozen of these tidal disruption events (TDEs), usually in optical, ultraviolet, or X-ray wavelengths. Sometimes — maybe 10% of the time — the TDEs come with jets, beams of plasma powered by the newly formed gas disks. At least, that’s what observers infer based on the emission they see; light from the best-studied of the jet-shooting TDEs traveled some 4 billion years to reach us, much too far away for astronomers to see the jet itself.

Reporting June 14th in Science, Seppo Mattila (University of Turku, Finland) and colleagues say they’ve now done just that, successfully watching a shredded-star jet be born and grow over a decade.

The team stumbled across the event while looking for supernovae. The researchers were studying the galactic pileup Arp 299 (a.k.a. NGC 3690), two glorious spiral galaxies colliding some 140 million light-years away. The ongoing merger is driving gas into the galaxies’ central regions, building a brilliant accretion disk around the black hole in the western galaxy and triggering the creation of countless stars, many of which are massive enough to go supernova.

Black Hole Destroys Star and Shoots Jet
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Mattila and colleagues spotted an infrared flare in January 2005 in the western galaxy’s nucleus, near the active black hole. By July, a compact radio source had joined it. As the team watched over the next decade with various ground- and space-based instruments, this radio source grew and stretched into a clumpy streak. Material in the jet at first moved at almost the speed of light, then quickly slowed down to a mere 22% of light speed as it ran into the surrounding gas and dust.

On its own, the existence of the jet doesn’t mean the flare-up is a TDE, cautions Suvi Gezari (University of Maryland). Active black holes are notoriously variable, flaring unexpectedly. But this event, called Arp 299-B AT1, has a big point in its favor: the jet’s angle. A big doughnut of dusty gas surrounds the black hole, and we see this torus from the side. Any jet fed by it would be oriented straight up-down from our perspective, like a pole stuck through an inner tube.

But Arp 299-B AT1’s jet points toward us, skewed only about 25° to 35° from our line of sight. That’s easily done with a disrupted star. A star can shoot toward the black hole at any angle, and the disk of gas created by its destruction can loop around the black hole and launch a jet unaligned with the original disk feeding the black hole.

“It’s a really nice demonstration that this was a star coming in, rather than some unusual flare,” says Andrew Levan (University of Warwick, UK), who like Gezari has spent years working on TDEs but wasn’t involved with the current study.

Arp 299-B AT1 is unusually unremarkable at optical and X-ray wavelengths. There appears to be a whole lot of gas and dust between the TDE and us, blocking and absorbing this radiation and eventually reemitting it in the infrared. Many galaxies’ cores, including our own, are replete with dust, and being able to spot one of these events behind so much dust could clear the way to finding them in galaxies where we’ve missed them before, Levan explains.

Based on the event’s intrinsic brightness and how much energy the team thinks went into heating the surrounding dust, the researchers estimate that it was the death of a star between 2 and 7 solar masses and unleashed a thousand times more radiation than a standard core-collapse supernova.

What a dramatic way to die.

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Pulsar Limits “Fifth Force” Interactions with Dark Matter

Pulsar Limits “Fifth Force” Interactions with Dark Matter
Pulsar Limits “Fifth Force” Interactions with Dark Matter
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A recent experiment to better understand the nature of dark matter constrains a possible “fifth force” of nature to almost zero.

Scientists recently studied a pulsar binary system to constrain the existence of a hypothetical fifth fundamental force of nature.

We already know about four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. However, there are some effects in the universe that cannot be explained by these forces alone. For example, a 2016 experiment in Hungary showed unexpected behavior in the decay of nuclei in the isotype beryllium-8. (After shooting protons at lithium foil, observers saw more electron-positron pairs ejected at a 140-degree angle, which is difficult to explain with standard nuclear physics theories.)

One possibility is the existence of a “fifth force” of nature, which governs the behavior of elementary particles alongside the other four forces. Some scientists suggest this force could work on dark matter, the unseen substance that makes up most of the universe’s mass. We can see dark matter’s effects on ordinary matter, but direct detection has eluded scientists and what it’s made of remains unknown.

Testing for a Fifth Force

One research group tested for a fifth force using a pulsar and its white dwarf star companion. Pulsars, whose atoms have been compacted into neutrons, are so dense that their extreme gravitational fields could enhance any possible interactions with dark matter. The white dwarf, while still sardine-packing its atoms, isn’t nearly so compact. General relativity predicts that normal matter ought to fall freely toward dark matter, but a fifth force that has the ability to interact with both normal and dark matter could strengthen or diminish dark matter’s pull. If a fifth force does exist, the Milky Way’s dark matter halo, whose density ought to peak in the galactic center, would pull on the neutron star and the white dwarf in different ways, slightly altering their orbit.

The researchers chose binary pulsar PSR J1713+0747, which is 3,800 light-years from Earth, lying in the direction of the galactic center. Dark matter is believed to be more populous towards the heart of the galaxy, so the pulsar binary system provides an ideal test how a fifth force would act on dark matter and standard matter. The researchers wanted to see if the movements of the pulsar and white dwarf would differ as they orbited one another.

“If there is a fifth force that acts between dark matter and standard matter, it would not be universal,” says Lijing Shao (Max Planck Institute for Radio Astronomy, Germany). “It would therefore produce an apparent difference for the neutron star and the white dwarf in their free fall towards dark matter. Thus, the orbit of the neutron star would be different than what is predicted by the general relativity.”

Using 20 years of radio observations of this system, the researchers concluded that if a fifth force does exist, it must have less than 1% of gravity’s strength . (And gravity is already the weakest of the four known forces.) The results appear in Physical Review Letters.

The researchers also discovered that the limits on the density of dark matter at this pulsar system were similar to other tests closer to Earth. In other words, the team didn’t prove or disprove other observations showing that dark matter density increases towards the center of the galaxy.

Beyond Relativity

Aurélien Hees (Observatory of Paris), who was not involved in the study, noted that this work is the first to investigate interactions between a hypothetical fifth force and dark matter in this way. The short rotational period of the pulsar – just 4.6 milliseconds – and its stable rotation made it a good candidate for constraining the effects of the fifth force, he said.

With the fifth force, he explains, “We expect to see something a little bit beyond relativity. We are trying to search for that with all the observations available from Earth.”

Shao says his team hopes to study more binary pulsars closer to the center of the galaxy to better understand the effects of dark matter. Unlike most tests of general relativity, in this case the researchers want to find pulsars moving in relatively slow orbits around their companion The challenge, of course, is finding the pulsars in the first place. He suggested a breakthrough will come when the more sensitive Square Kilometer Array is ready in the 2020s. “Bigger radio telescopes and arrays are better because they more precisely measure the time of the [pulsar signal] arrival,” Shao said.

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