Black hole on a diet creates a 'changing look' quasar

This artist's rending shows "before" and "after" images of a changing look quasar.
Credit: By Jim Shelton

Yale University astronomers have identified the first “changing look” quasar, a gleaming object in deep space that appears to have its own dimmer switch.

The discovery may offer a glimpse into the life story of the universe’s great beacons.

Quasars are massive, luminous objects that draw their energy from black holes. Until now, scientists have been unable to study both the bright and dim phases of a quasar in a single source.

As described in an upcoming edition of The Astrophysical Journal, Yale-led researchers spotted a quasar that had dimmed by a factor of six or seven, compared with observations from a few years earlier.

“We’ve looked at hundreds of thousands of quasars at this point, and now we’ve found one that has switched off,” said C. Megan Urry, Yale’s Israel Munson Professor of Astronomy and Astrophysics, and the study’s co-author. “This may tell us something about their lifetimes.”

Stephanie LaMassa, a Yale associate research scientist and principal investigator for the study, noticed the phenomenon during an ongoing probe of Stripe 82 — a sliver of the sky found along the Celestial Equator. Stripe 82 has been scanned in numerous astronomical surveys, including the Sloan Digital Sky Survey.

“This is like a dimmer switch,” LaMassa said. “The power source just went dim. Because the life cycle of a quasar is one of the big unknowns, catching one as it changes, within a human lifetime, is amazing.”

Even more significant for astronomers was the weakening of the quasar’s broad emission lines. Visible on the optical spectrum, these broad emission lines are signatures of gas that is too distant to be consumed by a black hole, yet close enough to be “excited” by energy from material that does fall into a black hole.

The change in the emission lines is what told researchers that the black hole had essentially gone on a diet, and was giving off less energy as a result. That’s when the “changing look” quasar hit its dimmer switch, and most of its broad emission lines disappeared.

The Yale team analyzed a variety of observation data, including recent optical spectra information and archival optical photometry and X-ray spectra information. They needed to rule out the possibility the quasar merely appeared to lose brightness, due to a gas cloud or other object passing in front of it.

The findings may prove invaluable on several fronts. First, they provide direct information about the intermittent nature of quasar activity; even more intriguingly, they hint at the sporadic activity of black holes.

“It makes a difference to know how black holes grow,” Urry said, noting that all galaxies have black holes, and quasars are a phase that black holes go through before becoming dormant. “This perhaps has implications for how the Milky Way looks today.”

Additionally, there is the chance the quasar may fire up again, showing astronomers yet another changing look.

“Even though astronomers have been studying quasars for more than 50 years, it’s exciting that someone like me, who has studied black holes for almost a decade, can find something completely new,” LaMassa said.

(Illustration by Michael S. Helfenbein)

Source: Yale university

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Black holes follow the rules

Artist's impression of a black hole at the centre of a galaxy. Credit: Gabriel Pérez Díaz.

Rather than having random sizes, massive black holes seem to follow a predictable rule in relation to the physical properties of the galaxy in which they are located.

Research at Swinburne University of Technology has shown that it is possible to predict the masses of black holes in galaxies for which it was previously thought not possible.

In large galaxies, the central black hole is related to the mass of the spheroid-shaped distribution of stars at the centre of the galaxy, known as the galaxy’s 'bulge'.

Some astronomers have claimed that the size of black holes at the centres of galaxies with small bulges was unrelated to the bulge.

Even the four million solar mass black hole in the bulge of our Milky Way galaxy was thought to be arbitrarily low relative to trends defined by their more massive, and therefore easier to detect, counterparts.

However, in previous work Swinburne Professor Alister Graham, lead-author of the current research, identified a new relationship involving black holes in galaxies with small bulges.  He demonstrated that the black hole in the bulge of the Milky Way was not set by chance but instead followed an astronomical rule.

“The formula is quadratic, in that the black hole mass quadruples every time the bulge mass doubles,” Professor Graham said. “Therefore, if the bulge mass increases 10 times, the black hole mass increases 100 times.”

Now, after studying more than 100 galaxies with black holes 4 to 40 times less massive than our Milky Way's black hole, they too have been found to follow this same rule.

"It turns out that there is yet more order in our Universe than previously appreciated,” Professor Graham said.

"This is exciting not just because it provides further insight into the mechanics of black hole formation, but because of the predictions it allows us to make."

The gravitational collapse of massive stars can produce black holes up to a few tens of times the mass of our Sun. And black holes that are one-tenth of a million to ten billion times the mass of our Sun have been identified at the centres of giant galaxies. However, there is a missing population of intermediate-mass black holes.

Astronomers don't know if this is because of observational difficulties in finding them, or if the massive black holes at the centres of galaxies start life as 100,000 solar mass seed black holes that formed in the early Universe.

This latest result, which extends the new rule to 40-times lower masses, gives astronomers some confidence that it may extend even further, so the smallest bulges might host these missing intermediate-mass black holes. 

"If confirmed, it would imply tremendous black hole appetites", co-author of the study, Dr Nicholas Scott, said. "There would need to be a dramatic growth of these small black holes relative to their host bulge, with the bulges growing via the creation of stars out of gas clouds while the black holes devour both gas and stars."

The researchers have identified a few dozen candidate galaxies in which they think intermediate-mass black holes may be hiding.  Future observations, with facilities such as the Square Kilometre Array and space-based X-ray telescopes, are expected to help resolve this black hole mystery.

Source: Swinburne

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NASA's Swift mission probes an exotic object: 'Kicked' black hole or mega star?

Using the Keck II telescope in Hawaii, researchers obtained high-resolution images of Markarian 177 and SDSS1133 using a near-infrared filter. Twin bright spots in the galaxy's center are consistent with recent star formation, a disturbance that hints this galaxy may have merged with another. Credit: W. M. Keck Observatory/M. Koss (ETH Zurich) et al.

An international team of researchers analyzing decades of observations from many facilities, including NASA's Swift satellite, has discovered an unusual source of light in a galaxy some 90 million light-years away.

The dwarf galaxy Markarian 177 (center) and its unusual source SDSS1133 (blue) lie 90 million light-years away. The galaxies are located in the bowl of the Big Dipper, a well-known star pattern in the constellation Ursa Major.

The object's curious properties make it a good match for a supermassive black hole ejected from its home galaxy after merging with another giant black hole. But astronomers can't yet rule out an alternative possibility. The source, called SDSS1133, may be the remnant of a massive star that erupted for a record period of time before destroying itself in a supernova explosion.

"With the data we have in hand, we can't yet distinguish between these two scenarios," said lead researcher Michael Koss, an astronomer at ETH Zurich, the Swiss Federal Institute of Technology. "One exciting discovery made with NASA's Swift is that the brightness of SDSS1133 has changed little in optical or ultraviolet light for a decade, which is not something typically seen in a young supernova remnant."

In a study published in the Nov. 21 edition of Monthly Notices of the Royal Astronomical Society, Koss and his colleagues report that the source has brightened significantly in visible light during the past six months, a trend that, if maintained, would bolster the black hole interpretation. To analyze the object in greater detail, the team is planning ultraviolet observations with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope in October 2015.

Whatever SDSS1133 is, it's persistent. The team was able to detect it in astronomical surveys dating back more than 60 years.

The mystery object is part of the dwarf galaxy Markarian 177, located in the bowl of the Big Dipper, a well-known star pattern within the constellation Ursa Major. Although supermassive black holes usually occupy galactic centers, SDSS1133 is located at least 2,600 light-years from its host galaxy's core.

In June 2013, the researchers obtained high-resolution near-infrared images of the object using the 10-meter Keck II telescope at the W. M. Keck Observatory in Hawaii. They reveal the emitting region of SDSS1133 is less than 40 light-years across and that the center of Markarian 177 shows evidence of intense star formation and other features indicating a recent disturbance.

"We suspect we're seeing the aftermath of a merger of two small galaxies and their central black holes," said co-author Laura Blecha, an Einstein Fellow in the University of Maryland's Department of Astronomy and a leading theorist in simulating recoils, or "kicks," in merging black holes. "Astronomers searching for recoiling black holes have been unable to confirm a detection, so finding even one of these sources would be a major discovery."

The collision and merger of two galaxies disrupts their shapes and results in new episodes of star formation. If each galaxy possesses a central supermassive black hole, they will form a bound binary pair at the center of the merged galaxy before ultimately coalescing themselves.

Merging black holes release a large amount of energy in the form of gravitational radiation, a consequence of Einstein's theory of gravity. Waves in the fabric of space-time ripple outward in all directions from accelerating masses. If both black holes have equal masses and spins, their merger emits gravitational waves uniformly in all directions. More likely, the black hole masses and spins will be different, leading to lopsided gravitational wave emission that launches the black hole in the opposite direction.

The kick may be strong enough to hurl the black hole entirely out of its home galaxy, fating it to forever drift through intergalactic space. More typically, a kick will send the object into an elongated orbit. Despite its relocation, the ejected black hole will retain any hot gas trapped around it and continue to shine as it moves along its new path until all of the gas is consumed.

If SDSS1133 isn't a black hole, then it might have been a very unusual type of star known as a Luminous Blue Variable (LBV). These massive stars undergo episodic eruptions that cast large amounts of mass into space long before they explode. Interpreted in this way, SDSS1133 would represent the longest period of LBV eruptions ever observed, followed by a terminal supernova explosion whose light reached Earth in 2001.

The nearest comparison in our galaxy is the massive binary system Eta Carinae, which includes an LBV containing about 90 times the sun's mass. Between 1838 and 1845, the system underwent an outburst that ejected at least 10 solar masses and made it the second-brightest star in the sky. It then followed up with a smaller eruption in the 1890s.

In this alternative scenario, SDSS1133 must have been in nearly continual eruption from at least 1950 to 2001, when it reached peak brightness and went supernova. The spatial resolution and sensitivity of telescopes prior to 1950 were insufficient to detect the source. But if this was an LBV eruption, the current record shows it to be the longest and most persistent one ever observed. An interaction between the ejected gas and the explosion's blast wave could explain the object's steady brightness in the ultraviolet.

Whether it's a rogue supermassive black hole or the closing act of a rare star, it seems astronomers have never seen the likes of SDSS1133 before.

Source: NASA/Goddard Space Flight Center

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Unravelling the mystery of gamma-ray bursts with kilometer-scale microphones

This is an illustration of how a neutron star might orbit a black hole. Credit: NASA

A team of scientists hopes to trace the origins of gamma-ray bursts with the aid of giant space 'microphones'.

Researchers at Cardiff University are trying to work out the possible sounds scientists might expect to hear when the ultra-sensitive LIGO and Virgo detectors are switched on in 2015.
It's hoped the kilometre-scale microphones will detect gravitational waves created by black holes, and shed light on the origins of the Universe.

Researchers Dr Francesco Pannarale and Dr Frank Ohme, in Cardiff University's School of Physics and Astronomy, are exploring the potential of seeing and hearing events that astronomers know as short gamma-ray bursts.

These highly energetic bursts of hard radiation have been seen by gamma-ray satellites such as Fermi and Swift, but the exact origin of these quickly disappearing flashes of gamma-rays remains unknown.

"By picking up the gravitational waves associated with these events, we will be able to access precious information that was previously hidden, such as whether the collision of a star and a black hole has ignited the burst and roughly how massive these objects were before the impact," explained Dr Ohme, who has focused his research on predicting the exact shape of the gravitational wave signals scientists are expecting to see.

Dr Pannarale added: "A possible scenario that could produce gamma-ray bursts involves a neutron star, the most compact star in the Universe, being ripped apart by a black hole while orbiting it. The remaining matter would be accelerated so much it could cause the energy bursts we are observing today.

"In some cases, by observing both electro-magnetic and gravitational wave signatures of the same event, we will be able to better understand the behaviour of material in the highest density region we know in our Universe, so that we will start to rule out various theoretical models that have been proposed but cannot be tested otherwise."

Source: Cardiff University

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'Eye of Sauron' provides new way of measuring distances to galaxies

This image shows the spiral galaxy NGC 4151. Credit: X-ray: NASA/CXC/CfA/J.Wang et al.; Optical: Isaac Newton Group of Telescopes, La Palma/Jacobus Kapteyn Telescope; Radio: NSF/NRAO/VLA.

A team of scientists, led by Dr Sebastian Hoenig from the University of Southampton, have developed a new way of measuring precise distances to galaxies tens of millions of light years away, using the W. M. Keck Observatory near the summit of Mauna Kea in Hawaii.

The method is similar to what land surveyors use on Earth, by measuring the physical and angular, or 'apparent', size of a standard ruler in the galaxy, to calibrate the distance from this information.

The research, which is published in the journal Nature, was used to identify the accurate distance of the nearby NGC4151 galaxy, which wasn't previously available. The galaxy NGC 4151, which is dubbed the 'Eye of Sauron' by astronomers for its similarity to the film depiction of the eye of the character in The Lord of the Rings, is important for accurately measuring black hole masses.

Recently reported distances range from 4 to 29 megaparsecs, but using this new method the researchers calculated the distance of 19 megaparsecs to the supermassive black hole.
Indeed, as in the famous saga, a ring plays a crucial role in this new measurement. All big galaxies in the universe host a supermassive black hole in their centre and in about a tenth of all galaxies, these supermassive black holes are growing by swallowing huge amounts of gas and dust from their surrounding environments. In this process, the material heats up and becomes very bright -- becoming the most energetic sources of emission in the universe known as active galactic nuclei (AGN).

The hot dust forms a ring around the supermassive black hole and emits infrared radiation, which the researchers used as the ruler. However, the apparent size of this ring is so small that the observations were carried out using infrared interferometry to combine W. M. Keck Observatory's twin 10-meter telescopes, to achieve the resolution power of an 85m telescope.

To measure the physical size of the dusty ring, the researchers measured the time delay between the emission of light from very close to the black hole and the infrared emission. This delay is the distance the light has to travel (at the speed-of-light) from close to the black hole out to the hot dust.

By combining this physical size of the dust ring with the apparent size measured with the data from the Keck interferometer, the researchers were able to determine a distance to the galaxy NGC 4151.

Dr Hoenig says: "One of the key findings is that the distance determined in this new fashion is quite precise -- with only about 10 per cent uncertainty. In fact, if the current result for NGC 4151 holds for other objects, it can potentially beat any other current methods to reach the same precision to determine distances for remote galaxies directly based on simple geometrical principles. Moreover, it can be readily used on many more sources than the current most precise method."

"Such distances are key in pinning down the cosmological parameters that characterise our universe or for accurately measuring black hole masses. Indeed, NGC 4151 is a crucial anchor to calibrate various techniques to estimate black hole masses. Our new distance implies that these masses may have been systematically underestimated by 40 per cent."

Dr Hoenig, together with colleagues in Denmark and Japan, is currently setting up a new program to extend their work to many more AGN. The goal is to establish precise distances to a dozen galaxies in this new way and use them to constrain cosmological parameters to within a few per cent. In combination with other measurements, this will provide a better understanding of the history of expansion of our universe.

Source: University of Southampton

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Pulsars with black holes could hold the 'Holy Grail' of gravity

Discovering a pulsar orbiting a black hole could be the ‘holy grail’ for testing gravity.
Credit: SKA Organisation/Swinburne Astronomy Productions

The intermittent light emitted by pulsars, the most precise timekeepers in the universe, allows scientists to verify Einstein's theory of relativity, especially when these objects are paired up with another neutron star or white dwarf that interferes with their gravity. However, this theory could be analysed much more effectively if a pulsar with a black hole were found, except in two particular cases, according to researchers from Spain and India.

Pulsars are very dense neutron stars that are the size of a city (their radius approaches ten kilometres), which, like lighthouses for the universe, emit gamma radiation beams or X-rays when they rotate up to hundreds of times per second. These characteristics make them ideal for testing the validity of the theory of general relativity, published by Einstein between 1915 and 1916.

"Pulsars act as very precise timekeepers, such that any deviation in their pulses can be detected," Diego F. Torres, ICREA researcher from the Institute of Space Sciences (IEEC-CSIC), explains. "If we compare the actual measurements with the corrections to the model that we have to use in order for the predictions to be correct, we can set limits or directly detect the deviation from the base theory."

These deviations can occur if there is a massive object close to the pulsar, such as another neutron star or a white dwarf. A white dwarf can be defined as the stellar remnant left when stars such as our Sun use up all of their nuclear fuel. The binary systems, composed of a pulsar and a neutron star (including double pulsar systems) or a white dwarf, have been very successfully used to verify the theory of gravity.

Last year, the very rare presence of a pulsar (named SGR J1745-2900) was also detected in the proximity of a supermassive black hole (Sgr A*, made up of millions of solar masses), but there is a combination that is still yet to be discovered: that of a pulsar orbiting a 'normal' black hole; that is, one with a similar mass to that of stars.

Until now scientists had considered this strange pair to be an authentic 'holy grail' for examining gravity, but there exist at least two cases where other pairings can be more effective. This is what is stated in the study that Torres and the physicist Manjari Bagchi, from the International Centre of Theoretical Sciences (India) and now postdoc at the IEEC-CSIC, have published in the Journal of Cosmology and Astroparticle Physics. The work also received an Honourable Mention in the 2014 Essays of Gravitation prize.

The first case occurs when the so-called principle of strong equivalence is violated. This principle of the theory of relativity indicates that the gravitational movement of a body that we test only depends on its position in space-time and not on what it is made up of, which means that the result of any experiment in a free fall laboratory is independent of the speed of the laboratory and where it is found in space and time.

The other possibility is if one considers a potential variation in the gravitational constant that determines the intensity of the gravitational pull between bodies. Its value is G = 6.67384(80) x 10-11 N m2/kg2. Despite it being a constant, it is one of those that is known with the least accuracy, with a precision of only one in 10,000.

In these two specific cases, the pulsar-black hole combination would not be the perfect 'holy grail', but in any case scientists are anxious to find this pair, because it could be used to analyse the majority of deviations. In fact, it is one of the desired objectives of X-ray and gamma ray space telescopes (such as Chandra, NuStar or Swift), as well as that of large radio telescopes that are currently being built, such as the enormous 'Square Kilometre Array' (SKA) in Australia and South Africa.

Source: Plataforma SINC

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