New Findings from NuSTAR: A New X-Ray View of the “Hand of God” and More

One star player in this week’s findings out of the 223rd meeting of the American Astronomical Society has been the Nuclear Spectroscopic Telescope Array Mission, also known as NuSTAR. On Thursday, researchers revealed some exciting new results and images from the mission, as well as what we can expect from NuSTAR down the road.

NuSTAR was launched on June 13th, 2012 on a Pegasus XL rocket deployed from a Lockheed L-1011 “TriStar” aircraft flying near the Kwajalein Atoll in the middle of the Pacific Ocean.

Part of a new series of low-cost missions, NuSTAR is the first of its kind to employ a space telescope focusing on the high energy X-ray end of the spectrum centered around 5-80 KeV.

Daniel Stern, part of the NuSTAR team at JPL Caltech, revealed a new X-ray image of the now-famous supernova remnant dubbed “The Hand of God.” Discovered by the Einstein X-ray observatory in 1982, the Hand is home to pulsar PSR B1509-58 or B1509 for short, and sits about 18,000 light years away in the southern hemisphere constellation Circinus. B1509 spins about 7 times per second, and the supernova that formed the pulsar is estimated to have occurred 20,000 years ago and would’ve  been visible form Earth about 2,000 years ago.

A diagram of the NuSTAR satellite. (NASA/JPL/Caltech)
A diagram of the NuSTAR satellite. (NASA/JPL/Caltech)

While the Chandra X-ray observatory has scrutinized the region before, NuSTAR can peer into its very heart. In fact, Stern notes that views from NuSTAR take on less of an appearance of a “Hand” and more of a “Fist”. Of course, the appearance of any nebula is a matter of perspective. Pareidolia litter the deep sky, whether it’s the Pillars of Creation to the Owl Nebula.  We can’t help but being reminded of the mysterious “cosmic hand” that the Guardians of Oa of Green Lantern fame saw when they peered back at the moment of creation. Apparently, the “Hand” is also rather Simpson-esque, sporting only three “fingers!”

Credit:
An diagram of the Hand of God. Credit: NASA/JPL/Caltech/McGill).

NuSTAR is the first, and so far only, focusing hard X-ray observatory deployed in orbit. NuSTAR employs what’s known as grazing incidence optics in a Wolter telescope configuration, and the concentric shells of the detector look like layers on an onion. NuSTAR also requires a large focal length, and employs a long boom that was deployed shortly after launch.

The hard X-ray regime that NuSTAR monitors is similar to what you encounter in your dentist’s office or in a TSA body scanner. Unlike the JEM-X monitor aboard ESA’s INTERGRAL or the Swift observatory, which have a broad resolution of about half a degree to a degree, NuSTAR has an unprecedented resolution of about 18 arc seconds.

The first data release from NuSTAR was in late 2013. NuSTAR is just begging to show its stuff, however, in terms of what researchers anticipate that it’s capable of.

“NuSTAR is uniquely able to map the Titanium-44 emission, which is a radioactive tracer of (supernova) explosion physics,” Daniel Stern told Universe Today.

NuSTAR will also be able to pinpoint high energy sources at the center of our galaxy. “No previous high-energy mission has had the imaging resolution of NuSTAR,” Stern told Universe Today. ”Our order-of-magnitude increase in image sharpness means that we’re able to map out that very rich region of the sky, which is populated by supernovae remnants, X-ray binaries, as well as the big black hole at the center of our Galaxy, Sagittarius A* (pronounced “A-star).”

NuSTAR identifies new black hole canidates (in blue) in the COSMOS field. Overlayed on previous black holes spotted by Chandra in the same field denoted in red and green. (Credit-NASA/JPL-Caltech/Yale University).
NuSTAR identifies new black hole candidates (in blue) in the COSMOS field. The discoveries in the image above are overlayed on previous black holes spotted by Chandra in the same field, which are denoted in red and green. (Credit-NASA/JPL-Caltech/Yale University).

Yale University researcher Francesca Civano also presented a new image from NuSTAR depicting black holes that were previously obscured from view.  NuSTAR is especially suited for this, gazing into the hearts of energetic galaxies that are invisible to observatories such Chandra or XMM-Newton. The image presented covers the area of Hubble’s Cosmic Evolution Survey, known as COSMOS in the constellation Sextans. In fact, Civano notes that NuSTAR has already seen the highest number of obscured black hole candidates to date.

“This is a hot topic in astronomy,” Civano said in a recent press release. “We want to understand how black holes grew and the degree to which they are obscured.”

To this end, NuSTAR researchers are taking a stacked “wedding cake” approach, looking at successively larger slices of the sky from previous surveys. These include looking at the quarter degree field of the Great Observatories Origins Deep Survey (GOOD-S) for 18 days, the two degree wide COSMOS field for 36 days, and the large four degree Swift-BAT fields for 40 day periods hunting for serendipitous sources.

Interestingly, NuSTAR has also opened the window on the hard X-ray background that permeates the universe as well. This peaks in the 20-30 KeV range, and is the combination of the X-ray emissions of millions of black holes.

“For several decades already, we’ve known what the sum total emission of the sky is across the X-ray regime,” Stern told Universe Today. “The shape of this cosmic X-ray background peaks strongly in the NuSTAR range. The most likely interpretation is that there are a large number of obscured black holes out there, objects that are hard to find in other energy bands. NuSTAR should find these sources.”

And NuSTAR may just represent the beginning of a new era in X-ray astronomy. ESA is moving ahead with its next generation flagship X-ray mission, known as Athena+, set to launch sometime next decade. Ideas abound for wide-field imagers and X-ray polarimeters, and one day, we may see a successor to NuSTAR dubbed the High-Energy X-ray Probe or (HEX-P) make it into space.

But for now, expect some great science out of NuSTAR, as it unlocks the secrets of the X-ray universe!

Chandra’s Verdict on the Demise of a Star: “Death by Black Hole”

This week, astronomers announced the detection of a rare event, a star being torn to shreds by a massive black hole in the heart of a distant dwarf galaxy. The evidence was presented Wednesday January 8th at the ongoing 223rd meeting of the American Astronomical Society being held this week in Washington D.C.

Although other instances of the death of stars at the hands of black holes have been witnessed before, Chandra may have been the first to document an intermediate black hole at the heart of a dwarf galaxy “in the act”.

The results span observations carried out by the space-based Chandra X-ray observatory over a period spanning 1999 to 2005. The search is part of an archival study of observations, and revealed no further outbursts after 2005.

“We can’t see the star being torn apart by the black hole, but we can track what happens to the star’s remains,” said University of Alabama’s Peter Maksym in a recent press release. A comparison of with similar events seen in larger galaxies backs up the ruling of “death by black hole.”  A competing team led by Davide Donato also looked at archival data from Chandra and the Extreme Ultraviolet Explorer (EUVE), along with supplementary observations from the Canada-France-Hawaii Telescope to determine the brightness of the host galaxy, and gained similar results.

The dwarf galaxy in the Abell 1795 cluster that was observed has the name WINGS J134849.88+263557.5, or WINGS J1348 for short. The Abell 1795 cluster is about 800 million light years distant.

WINGS denotes the galaxy’s membership in the WIde-field Nearby Galaxy-cluster Survey, and the phone number-like designation is the galaxy’s position in the sky in right ascension and declination.

Like most galaxies associated with galaxy clusters, WINGS J1348 a dwarf galaxy probably smaller than our own satellite galaxy known as the Large Magellanic Cloud. The Abell 1795 cluster is located in the constellation Boötes, and WINGS J1348 has an extremely faint visual magnitude of +22.46.

Optical
An optical view of the Abell 1795 galaxy cluster. Credit- NASA/CFHT/D. Donato et al.

“Scientists have been searching for these intermediate mass black holes for decades,” NASA’s Davide Donato said in a recent press release “We have lots of evidence for small black holes and very big ones, but these medium-sized ones have been tough to pin down.”

Maksym notes in an interview with Universe Today that this isn’t the first detection of an intermediate-mass black hole, which are a class of black holes often dubbed the “mostly” missing link between stellar mass and super massive black holes.

The mass range for intermediate black holes is generally pegged at 100 to one million solar masses.

What makes the event witnessed by Chandra in WINGS J1348 special is that astronomers managed to capture a rare tidal flare, as opposed to a supermassive black hole in the core of an active galaxy.

A bright, long duration flare may be the first recorded event of a black hole destroying a star in a dwarf galaxy. The dwarf galaxy is located in the galaxy cluster Abell 1795, about 800 million light years from Earth. A composite image of the cluster shows Chandra data in blue and optical data from the Canada-France-Hawaii Telescope in red, green and blue. An inset centered on the dwarf galaxy shows Chandra data taken between before and after 2005. The X-ray flare provides evidence that a large black hole has pulled in debris from a star that was torn apart by tidal forces.
A closeup view of the bright, long duration flare witnessed by Chandra pre-2005. Credit- NASA/CXC/University of Alabama/W.P. Maksym et al.

“Most of the time, black holes eat very little, so they can hide very well,” Maksym said in the AAS meeting on Wednesday.

This discovery pushes the limits on what we know of intermediate black holes. By documenting an observed number of tidal flare events, it can be inferred that a number of inactive black holes must be lurking in galaxies as well. The predicted number of tidal events that occur also have implications for the eventual detection of gravity waves from said mergers.

And more examples of these types of X-ray flare events could be waiting to be uncovered in the Chandra data as well.

“Chandra has taken quite a few pictures over the past 13+ years, and collaborators and I have an ongoing program to look for more tidal flares,” Maksym told Universe Today. “We’ve found one other this way, from a larger galaxy, and hope to find more. Abell 1795 was a particularly good place to look because as a calibration source, there were tons of pictures.”

Use of Chandra data was also ideal for the study because its spatial resolution allowed researchers to pinpoint an individual galaxy in the cluster. Maksym also notes that while it’s hard to get follow-up observations of events based on archival data, future missions dedicated to X-ray astronomy with wider fields of view may be able to scour the skies looking for such tidal flaring events.

The NuSTAR satellite was the latest X-Ray observatory  to launch in 2012.  NASA’s Extreme Ultraviolet Explorer picked up a strong ultraviolet source in 1998 right around the time of the tidal flare event, and ESA’s XMM-Newton satellite may have detected the event in 2000 as well.

This was also one of the smallest galaxies ever observed to contain a black hole. Maksym noted in Wednesday’s press conference that an alternative explanation could be a super-massive black hole in a tiny galaxy that just “nibbled” on a passing star, but said that new data from the Gemini observatory does not support this.

“It would be like looking into a dog house and finding a large ogre crammed in there,” Maksym said at Wednesday’s press conference.

This discovery provides valuable insight into the nature of intermediate mass black holes and their formation and behavior. What other elusive cosmological beasties are lying in wait to be discovered in the archives?

Congrats to Maksym and teams on this exciting new discovery, and the witnessing of a rare celestial event!

 

Gravitational Lens Seen for the First Time in Gamma Rays

An exciting new discovery was unveiled early this week at the 223rd  meeting of the American Astronomical Society being held in Washington D.C., when astronomers announced that a gravitational lens was detected for the first time at gamma-ray wavelengths.

The study was conducted using NASA’s Fermi Gamma Ray Space Telescope, and promises to open a new window on the universe, giving astrophysicists another tool to study the emission regions that exist near supermassive black holes.

But the hunt wasn’t easy. A gravitational lens occurs when a massive foreground object, such as a galaxy, bends the light from a distant background object. In the case of this study, researchers targeted a blazar known as B0218+357, a energetic source located 4.35 billion light years away in the direction of the constellation Triangulum.

Blazar and quasar sources are named using their respective coordinates in the sky. Think of “0218+357” as translating into “Right Ascension 2 Hours 18 Minutes, Declination +35.7 degrees north” in backyard astronomer-speak.  A blazar is a compact form of quasar that results from a supermassive black hole at the heart of an active galaxy. The term blazar was first coined by Edward Spiegel in 1978. The first quasar discovered was 3C 273 in 1970, which was also later found to be a blazar. 3C 273 is visible in Virgo using a large backyard telescope.

A foreground spiral galaxy seen face on lies along our line of sight between our vantage point and B0218+357. At 4 billion light years distant, the two have the smallest angular separation of any gravitationally lensed system so far identified at less than a third of an arc second across.

“We began thinking about the possibility of making this observation a couple of years after Fermi launch, and all of the pieces finally came together in late 2012,” said Naval Research Laboratory astrophysicist and lead scientist on the study Teddy Cheung in a recent NASA Goddard Spaceflight Center press release.

Observations of the blazar suggested that it would be flaring in September 2012, making it a prime target for the study. In fact, B0218+357 was the brightest extra-galactic gamma-ray source at the time. Cheung was granted time spanning late September into October 2012 to use Fermi’s Large Area Telescope (LAT) instrument to study the blazar in outburst.

Fermi‘s LAT instrument doesn’t have the resolution possessed by radio and optical instruments to catch the blazar in single images. Instead, the team exploited a phenomenon known as the “delayed playback effect” to catch the blazar in action.

“One light path is slightly longer than the other, so when we detect flares in one image we try and catch them days later when they replay in the other image,” Said team member Jeff Scargle, astrophysicist based at NASA’s Ames Research Center.

Cheung presented the findings of the study Monday at the American Astronomical Society meeting, which included three distinct flaring episodes from the background blazar that demonstrated the tell-tale delayed playback events with a period spanning 11.46 days.

A Hubble Space Telescope image of the gravitational lensing of B0218+357. Credit: NASA/ESA and the Hubble Legacy Archive.
A Hubble Space Telescope image of the gravitational lensing of B0218+357. Credit: NASA/ESA and the Hubble Legacy Archive.

Follow-up observations in radio and optical wavelengths supported the key observations, and demonstrate that Fermi’s LAT imager did indeed witness the event. Interestingly, the delay for the gamma-rays from the lensed blazar takes about a day longer than radio waves to reach the Earth. B0218+357 is also about four times brighter in gamma-rays than in radio wavelengths.

This occurs because the gamma-rays are emanating from a slightly different region than radio waves generated by the blazar, and are taking a different path though the gravitational field of the foreground galaxy. This demonstrates that assets like Fermi can be used to probe the heart of the distant energetic galactic nuclei which harbor supermassive black holes. This opens the hot topic of gravitationally lensed blazars and their role in extra-galactic astronomy up to the gamma-ray spectrum, and gives cosmologists another gadget for their tool box.

“Over the course of a day, one of these flares can brighten the blazar by 10 times in gamma-rays but only 10 percent in visible light and radio, which tells us that the region emitting gamma-rays is very small compared to those emitting at lower energies,” Said Stockholm University team member Stefan Larsson in the recent press release.

Using the analysis of lensing systems at gamma-ray wavelengths will not only help to probe these enigmatic cosmological beasts, but it may also assist with refining the all-important Hubble Constant, which measures the rate at which the universe is expanding.

But Fermi may just beginning to show its stuff when it comes to hunting for extra-galactic sources. The really exciting breakthrough, researchers say, would be the discovery of an energetic extra-galactic source being lensed by a foreground galaxy in gamma-rays that hasn’t been seen been seen at other wavelengths. This recent finding has certainly demonstrated how Fermi can “see” these tell-tale flashes via a clever method. Expect more news in the coming years!

Read the entire paper on the arViv server titled Fermi-LAT Detection of Gravitational Lens Delayed Gamma-ray Flares from Blazar B0218+357.