Hexbyte Glen Cove Chinese astronomers investigate X-ray bursts of SGR J1935+2154 thumbnail

Hexbyte Glen Cove Chinese astronomers investigate X-ray bursts of SGR J1935+2154

Hexbyte Glen Cove

SGR J1935+2154: Examples of short-duration bursts inspected in the study. Credit: Zou et al., 2021.

By analyzing the data from NASA’s Fermi spacecraft, astronomers from the Hebei Normal University and Nanjing University in China, have investigated X-ray bursting activity of a soft gamma-ray repeater (SGR) known as SGR J1935+2154. Results of the study, published July 9 on arXiv.org, deliver more hints about the properties of X-ray bursts from this source.

SGRs are sources emitting large bursts of gamma-rays and X-rays at irregular intervals. They are known to be magnetars, isolated neutron stars with ultra-strong magnetic fields. SGRs emit X-rays in their quiescent state, and outbursts occur when the intense magnetic field shifts. Based on their brightness, the SGR bursts can be divided into three classes: short-duration bursts, giant flares, and intermediate burts.

At a distance of about 30,000 , SGR J1935+2154 is a soft gamma-ray repeater discovered in 2014 by NASA’s Neil Gehrels Swift Observatory. To date, the source has experienced several periods (windows) of activity in 2014, 2015, 2016, 2019 and 2020. When it comes to April 2020, it was recognized as the most violent bursting month of this SGR so far.

Previous observations of SGR J1935+2154 suggested a periodic window behavior (PWB) for its bursting activity, which means that bursting phases always appear periodically, but there is no periodicity for specific bursts. In order to further investigate this hypothesis, a team of astronomers led by Jin-Hang Zou (Hebei Normal University/Nanjing University) has conducted a systematic search for X-ray bursts of this SGR using the data from the Gamma-ray Burst Monitor (GBM) onboard Fermi, hoping to identify its PWB.

“We performed a systematic search for X-ray bursts of the SGR J1935+2154 using the Fermi Gamma-ray Burst Monitor continuous data dated from Jan 2013 to July 2021,” the researchers wrote in the paper.

By analyzing the Fermi data, Zou’s team has identified eight bursting phases of SGR J1935+2154, consisting of a total of 255 individual bursts. Further analysis of this dataset using two independent methods allowed them to find that the bursts exhibit a period of approximately 237 days with an about 58.6 percent duty cycle.

These results are fully consistent with all the X-ray bursts of SGR J1935+2154 observed by multiple missions to date. Moreover, the findings suggest that the next active windows will occur June-November 2021 and February-July 2022. The first predicted window was confirmed by the current ongoing burst activities of SGR J1935+2154, which started on June 26, 2021.

Trying to explain the physical origin of the identified 237-day period of bursts, the astronomers assume that the most natural way to cause such a period may be the free precession of the magnetar.

“Given that there is no evidence showing SGR J1935+2154 are in a binary system, we focus on the explanations invoking the properties of the magnetar itself,” the authors of the paper wrote in concluding remarks.

More information:
Periodicity Search on X-ray Bursts of SGR J1935+2154 Using 8.5-year Fermi/GBM Data, arXiv:2107.03800 [astro-ph.HE] arxiv.org/abs/2107.03800

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Chinese astronomers investigate X-ray bursts of SGR J1935+2154 (2021, July 19)
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Hexbyte Glen Cove Astronomers inspect black hole X-ray binary MAXI J1348–630 thumbnail

Hexbyte Glen Cove Astronomers inspect black hole X-ray binary MAXI J1348–630

Hexbyte Glen Cove

X-ray and radio light curves of MAXI J1348–630 during its 2019/2020 outburst. Credit: Carotenuto et al., 2021.

An international team of astronomers has carried out a comprehensive radio and X-ray monitoring of a black hole X-ray binary known as MAXI J1348–630. The observational campaign provided important insights into the evolution of the source’s compact and transient jets. The study was presented in a paper published March 22 on arXiv.org.

Black hole X-ray binaries (BHXBs) are binary systems consisting of a black hole orbited by a stellar companion, typically a low-mass, evolved star. In BHXBs, X-rays are produced by material accreting from a secondary companion star onto a black hole primary. Such systems are usually detected in outbursts when the X-ray flux increases significantly.

MAXI J1348–630 was initially detected on January 26, 2019 as a bright X-ray transient by the Monitor of All-sky X-ray Image (MAXI) aboard the International Space Station (ISS). Further observations of this source confirmed that it is a BHXB with a black hole mass of about seven solar masses at a distance of some 7,170 light years away from the Earth.

Almost immediately after the bursting activity of MAXI J1348–630 started, a group of astronomers led by Francesco Carotenuto of the University of Paris, France, commenced a monitoring campaign of this source with the aim of shedding more light on its nature. They observed MAXI J1348–630 in the radio band with the MeerKAT telescope in south Africa and the Australia Telescope Compact Array (ATCA), and also in the X-rays using MAXI and NASA’s Swift spacecraft.

“In this work, we have presented the X-ray and radio monitoring of MAXI J1348–630 during its 2019/2020, discovery outburst. With our X-ray monitoring, we have been able to follow the whole outburst,” the researchers wrote in the paper.

The observations show that during the outburst MAXI J1348–630 exhibited a rather typical X-ray evolution in the first part, completing a whole cycle in the hardness-density diagram (HID), and then showcased a complex sequence of hard-state-only re-brightenings in the second part.

During the outburst, Carotenuto’s team observed the rise, quenching, and re-activation of the compact jets. They also identified two single-sided discrete ejecta, launched about two months apart and traveling away from the black hole. These ejecta had proper motion at a level of some 100 mas/day—the highest proper motion measured so far for such features in BHXBs.

The astronomers found that the first ejection happened during the hard-to-soft state transition of the source, before a strong radio flare. When it comes to the second ejection, it was launched during a short excursion from the soft to the intermediate state.

According to the authors of the paper, the results suggest that MAXI J1348–630 appears to be inside a low-density cavity in the interstellar medium (ISM).

“After traveling with constant speed, the first component underwent a strong deceleration, which was covered with unprecedented detail and suggested that MAXI J1348–630 could be located inside a low-density cavity in the , as already proposed for XTE J1550–564 and H1743–322,” the researchers concluded.

More information:
The black hole transient MAXI J1348–630: evolution of the compact and transient jets during its 2019/2020 outburst, arXiv:2103.12190 [astro-ph.HE] arxiv.org/abs/2103.12190

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Astronomers inspect black hole X-ray binary MAXI J1348–630 (2021, March 30)
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Hexbyte Glen Cove Astronomers detect a black hole on the move thumbnail

Hexbyte Glen Cove Astronomers detect a black hole on the move

Hexbyte Glen Cove

Galaxy J0437+2456 is thought to be home to a supermassive, moving black hole. Credit: Sloan Digital Sky Survey (SDSS).

Scientists have long theorized that supermassive black holes can wander through space—but catching them in the act has proven difficult.

Now, researchers at the Center for Astrophysics | Harvard & Smithsonian have identified the clearest case to date of a supermassive black hole in motion. Their results are published today in the Astrophysical Journal.

“We don’t expect the majority of supermassive black holes to be moving; they’re usually content to just sit around,” says Dominic Pesce, an astronomer at the Center for Astrophysics who led the study. “They’re just so heavy that it’s tough to get them going. Consider how much more difficult it is to kick a bowling ball into motion than it is to kick a soccer ball—realizing that in this case, the ‘bowling ball’ is several million times the mass of our Sun. That’s going to require a pretty mighty kick.”

Pesce and his collaborators have been working to observe this rare occurrence for the last five years by comparing the velocities of supermassive black holes and .

“We asked: Are the velocities of the black holes the same as the velocities of the galaxies they reside in?” he explains. “We expect them to have the same velocity. If they don’t, that implies the black hole has been disturbed.”

For their search, the team initially surveyed 10 distant galaxies and the supermassive black holes at their cores. They specifically studied black holes that contained water within their accretion disks—the spiral structures that spin inward towards the black hole.

As the water orbits around the black hole, it produces a laser-like beam of radio light known as a maser. When studied with a combined network of radio antennas using a technique known as very long baseline interferometry (VLBI), masers can help measure a black hole’s velocity very precisely, Pesce says.

The technique helped the team determine that nine of the 10 supermassive black holes were at rest—but one stood out and seemed to be in motion.

Located 230 million light-years away from Earth, the black hole sits at the center of a galaxy named J0437+2456. Its mass is about three million times that of our Sun.

Using follow-up observations with the Arecibo and Gemini Observatories, the team has now confirmed their initial findings. The supermassive black hole is moving with a speed of about 110,000 miles per hour inside the galaxy J0437+2456.

But what’s causing the motion is not known. The team suspects there are two possibilities.

“We may be observing the aftermath of two supermassive black holes merging,” says Jim Condon, a radio astronomer at the National Radio Astronomy Observatory who was involved in the study. “The result of such a merger can cause the newborn black hole to recoil, and we may be watching it in the act of recoiling or as it settles down again.”

But there’s another, perhaps even more exciting possibility: the black hole may be part of a binary system.

“Despite every expectation that they really ought to be out there in some abundance, scientists have had a hard time identifying clear examples of binary supermassive black holes,” Pesce says. “What we could be seeing in the galaxy J0437+2456 is one of the black holes in such a pair, with the other remaining hidden to our radio observations because of its lack of maser emission.”

Further observations, however, will ultimately be needed to pin down the true cause of this supermassive black hole’s unusual motion.

More information:
Dominic W. Pesce et al, A Restless Supermassive Black Hole in the Galaxy J0437+2456, The Astrophysical Journal (2021). DOI: 10.3847/1538-4357/abde3d

Astronomers detect a black hole on the move (2021, March 12)
retrieved 15 March 2021
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Hexbyte Glen Cove Astronomers spot bizarre, never-before-seen activity from one of the strongest magnets in the universe thumbnail

Hexbyte Glen Cove Astronomers spot bizarre, never-before-seen activity from one of the strongest magnets in the universe

Hexbyte Glen Cove

Artist’s impression of the active magnetar Swift J1818.0-1607. Credit: Carl Knox, OzGrav.

Astronomers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) and CSIRO have just observed bizarre, never-seen-before behavior from a radio-loud magnetar—a rare type of neutron star and one of the strongest magnets in the universe.

Their new findings, published today in the Monthly Notices of the Royal Astronomical Society (MNRAS), suggest magnetars have more complex magnetic fields than previously thought, which may challenge theories of how they are born and evolve over time.

Magnetars are a rare type of rotating neutron star with some of the most in the universe. Astronomers have detected only 30 of these objects in and around the Milky Way—most of them detected by X-ray telescopes following a high-energy outburst.

However, a handful of these magnetars have also been seen to emit radio pulses similar to pulsars—the less-magnetic cousins of magnetars that produce beams of radio waves from their magnetic poles. Tracking how the pulses from these radio-loud magnetars change over time offers a unique window into their evolution and geometry.

In March 2020, a new magnetar named Swift J1818.0-1607 (J1818 for short) was discovered after it emitted a bright X-ray burst. Rapid follow-up observations detected radio pulses originating from the magnetar. Curiously, the appearance of the radio pulses from J1818 were quite different from those detected from other radio-loud magnetars.

Most radio pulses from magnetars maintain a consistent brightness across a wide range of observing frequencies. However, the pulses from J1818 were much brighter at low frequencies than high frequencies—similar to what is seen in pulsars, another more common type of radio-emitting neutron star.

In order to better understand how J1818 would evolve over time, a team led by scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) observed it eight times using the CSIRO Parkes radio telescope (also known as Murriyang) between May and October 2020.

During this time, they found the magnetar underwent a brief identity crisis: In May it was still emitting the unusual pulsar-like pulses that had been detected previously; however, by June, it had started flickering between a bright and a weak state. This flickering behavior reached a peak in July, when the astronomers saw it flickering back and forth between pulsar-like and magnetar-like radio pulses.

“This bizarre behavior has never been seen before in any other radio-loud magnetar,” explains study lead author and Swinburne University/CSIRO Ph.D. student Marcus Lower. “It appears to have only been a short-lived phenomenon, as by our next observation, it had settled permanently into this new magnetar-like state.”

The scientists also looked for pulse shape and brightness changes at different radio frequencies and compared their observations to a 50-year-old theoretical model. This model predicts the expected geometry of a pulsar, based on the twisting direction of its polarized light.

“From our observations, we found that the magnetic axis of J1818 isn’t aligned with its rotation axis,” says Lower. “Instead, the radio-emitting magnetic pole appears to be in its southern hemisphere, located just below the equator. Most other magnetars have magnetic fields that are aligned with their spin axes or are a little ambiguous. This is the first time we have definitively seen a magnetar with a misaligned magnetic pole.”

Remarkably, this magnetic geometry appears to be stable over most observations. This suggests any changes in the pulse profile are simply due to variations in the height the radio pulses are emitted above the neutron star surface. However, the August 1st 2020 observation stands out as a curious exception.

“Our best geometric model for this date suggests that the radio beam briefly flipped over to a completely different magnetic pole located in the northern hemisphere of the magnetar,” says Lower.

A distinct lack of any changes in the magnetar’s profile shape indicate the same lines that trigger the ‘normal’ radio pulses must also be responsible for the pulses seen from the other magnetic .

The study suggests this is evidence that the radio pulses from J1818 originate from loops of connecting two closely spaced poles, like those seen connecting the two poles of a horseshoe magnet or sunspots on the sun. This is unlike most ordinary neutron stars, which are expected to have north and south poles on opposite sides of the star that are connected by a donut-shaped magnetic field.

This peculiar magnetic field configuration is also supported by an independent study of the X-rays pulses from J1818 that were detected by the NICER telescope on board the International Space Station. The X-rays appear to come from either a single distorted region of magnetic field lines that emerge from the magnetar surface or two smaller, but closely spaced, regions.

These discoveries have potential implications for computer simulations of how magnetars are born and evolve over long periods of time, as more complex magnetic field geometries will change how quickly their magnetic fields are expected to decay over time. Additionally, theories that suggest fast radio bursts can originate from magnetars will have to account for radio pulses potentially originating from multiple active sites within their magnetic fields.

Catching a flip between in action could also afford the first opportunity to map the magnetic field of a magnetar.

“The Parkes telescope will be watching the closely over the next year” says scientist and study co-author Simon Johnston, from the CSIRO Astronomy and Space Science.

More information:
M E Lower et al. The dynamic magnetosphere of Swift J1818.0−1607, Monthly Notices of the Royal Astronomical Society (2020). DOI: 10.1093/mnras/staa3789

Marcus E. Lower, et al. The dynamic magnetosphere of Swift J1818.0−1607 arxiv.org/abs/2011.12463 arXiv:2011.12463v2 [astro-ph.HE] T

Astronomers spot bizarre, never-before-seen activity from one of the strongest magnets in the universe (2021, February 1)
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