NuSTAR Probes Black Hole Jet Mystery

This artist's concept shows a black hole with an accretion disk -- a flat structure of material orbiting the black hole - and a jet of hot gas, called plasma. Credit: NASA/JPL-Caltech.  › Larger view

Black holes are famous for being ravenous eaters, but they do not eat everything that falls toward them. A small portion of material gets shot back out in powerful jets of hot gas, called plasma, that can wreak havoc on their surroundings. Along the way, this plasma somehow gets energized enough to strongly radiate light, forming two bright columns along the black hole's axis of rotation. Scientists have long debated where and how this happens in the jet.

Astronomers have new clues to this mystery. Using NASA's NuSTAR space telescope and a fast camera called ULTRACAM on the William Herschel Observatory in La Palma, Spain, scientists have been able to measure the distance that particles in jets travel before they "turn on" and become bright sources of light. This distance is called the "acceleration zone." The study is published in the journal Nature Astronomy.

Scientists looked at two systems in the Milky Way called "X-ray binaries," each consisting of a black hole feeding off of a normal star. They studied these systems at different points during periods of outburst -- which is when the accretion disk -- a flat structure of material orbiting the black hole -- brightens because of material falling in.

One system, called V404 Cygni, had reached nearly peak brightness when scientists observed it in June 2015. At that time, it experienced the brightest outburst from an X-ray binary seen in the 21st century. The other, called GX 339-4,was less than 1 percent of its maximum expected brightness when it was observed. The star and black hole of GX 339-4 are much closer together than in the V404 Cygni system.

Despite their differences, the systems showed similar time delays - about one-tenth of a second -- between when NuSTAR first detected X-ray light and ULTRACAM detected flares in visible light slightly later. That delay is less than the blink of an eye, but significant for the physics of black hole jets.

"One possibility is that the physics of the jet is not determined by the size of the disc, but instead by the speed, temperature and other properties of particles at the jet's base," said Poshak Gandhi, lead author of the study and astronomer at the University of Southampton, United Kingdom.

The best theory scientists have to explain these results is that the X-ray light originates from material very close to the black hole. Strong magnetic fields propel some of this material to high speeds along the jet. This results in particles colliding near light-speed, energizing the plasma until it begins to emit the stream of optical radiation caught by ULTRACAM.

Where in the jet does this occur? The measured delay between optical and X-ray light explains this. By multiplying this amount of time by the speed of the particles, which is nearly the speed of light, scientists determine the maximum distance traveled.

This expanse of about 19,000 miles (30,000 kilometers) represents the inner acceleration zone in the jet, where plasma feels the strongest acceleration and "turns on" by emitting light. That's just under three times the diameter of Earth, but tiny in cosmic terms, especially considering the black hole in V404 Cygni weighs as much as 3 million Earths put together.

"Astronomers hope to refine models for jet powering mechanisms using the results of this study," said Daniel Stern, study co-author and astronomer based at NASA's Jet Propulsion Laboratory, Pasadena, California.

Making these measurements wasn't easy. X-ray telescopes in space and optical telescopes on the ground have to look at the X-ray binaries at exactly the same time during outbursts for scientists to calculate the tiny delay between the telescopes' detections. Such coordination requires complex planning between the observatory teams. In fact, coordination between NuSTAR and ULTRACAM was only possible for about an hour during the 2015 outburst, but that was enough to calculate the groundbreaking results about the acceleration zone.

The results also appear to connect with scientists' understanding of supermassive black holes, much bigger than the ones in this study. In one supermassive system called BL Lacertae, weighing 200 million times the mass of our Sun, scientists have inferred time delays millions of times greater than what this study found. That means the size of the acceleration area of the jets is likely related to the mass of the black hole.

"We are excited because it looks as though we have found a characteristic yardstick related to the inner workings of jets, not only in stellar-mass black holes like V404 Cygni, but also in monster supermassive ones," Gandhi said.

The next steps are to confirm this measured delay in observations of other X-ray binaries, and to develop a theory that can tie together jets in black holes of all sizes.

"Global ground and space telescopes working together were key to this discovery. But this is only a peek, and much remains to be learned. The future is really bright for understanding the extreme physics of black holes," said Fiona Harrison, principal investigator of NuSTAR and professor of astronomy at Caltech in Pasadena.

NuSTAR is a Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington. NuSTAR was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences Corp., Dulles, Virginia. NuSTAR's mission operations center is at UC Berkeley, and the official data archive is at NASA's High Energy Astrophysics Science Archive Research Center. ASI provides the mission's ground station and a mirror archive. Caltech manages JPL for NASA.

For more information on NuSTAR, visit: https://www.nasa.gov/nustar - http://www.nustar.caltech.edu/

News Media Contact

Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
Elizabeth.landau@jpl.nasa.gov

Source: JPL-Caltech/News

Herschel gets to the bottom of black-hole jets

 Artist's impression of the GX 339-4 black-hole binary system. 

Credit: ESA/ATG medialab 

Herschel image of the GX 339-4 black-hole binary system. 

Credit: ESA/Herschel/PACS/S.Corbel et al.

Astronomers using ESA's Herschel space observatory have detected emission from the base of black-hole jets for the first time. While studying the black-hole binary system GX 339-4 in a multi-wavelength observation campaign, they noticed changes in the source's X-ray and radio emissions signalling the onset of powerful jets being released from the black hole's vicinity. This prompted the astronomers to observe the source at far-infrared wavelengths with Herschel. As the first observation of emission from jets in a black-hole binary system at these wavelengths, the data have allowed the astronomers to probe the jets down to their base, where the far-infrared emission originates. Herschel's contribution to the multi-wavelength observations has proved a crucial addition to the understanding of black-hole jets and of the physical processes that take place very close to a black hole.

When black holes – the densest objects in the Universe – accrete matter from their surroundings, they also trigger the release of powerful jets of highly-energetic particles that stem from the accretion disc into outer space. This phenomenon happens both at the stellar-mass black holes that derive from the death of massive stars and at the supermassive black holes lurking at the centre of massive galaxies. The physical mechanisms underlying the outburst of jets and their connection with the accretion process, however, are still unclear although astronomers have been studying them for decades, first via observations at radio wavelengths and, more recently, across the entire electromagnetic spectrum

Stellar-mass black holes that are accreting mass from a companion star in a binary system are of great help to astronomers interested in the dynamics of jets. Since they are much smaller than their supermassive counterparts, stellar-mass black holes give rise to jets whose properties change on relatively short time scales – of the order of a few hours or days – providing astronomers with a great opportunity to study their evolution and, possibly, the ignition mechanisms that trigger the appearance of jets.

"One of the best-studied stellar-mass black holes is the one hosted in the binary system GX 339-4: we can monitor its evolution quite closely because the source gives rise to bright outbursts every couple of years," explains Stéphane Corbel from Laboratoire AIM, France. Corbel led a new study of this system based on far-infrared (FIR) data from ESA's Herschel Space Observatory, as well as on observations performed at X-ray, optical, near-infrared (NIR) and radio wavelengths.

"The multi-wavelength approach is essential for us to explore the vicinity of black holes, as different regions radiate at different wavelengths. Broadly speaking, the accretion disc shines most brightly in X-rays, whereas the jets emit mainly radio waves. But there is more: the base of the jets – closer to the black hole – emit light at shorter wavelengths than radio waves, up to the infrared: this is where Herschel's contribution proved crucial," Corbel adds.

While GX 339-4 has been studied extensively at radio, NIR, optical and X-ray wavelengths, astronomers had rarely observed it in the vast portion of the spectrum between radio and NIR wavelengths. In fact, until now hardly any data from any stellar-mass black hole had been collected in this broad wavelength range.

The astronomers requested to observe GX 339-4 with Herschel after they detected changes to its X-ray emission signalling that the outburst phase of this source, which had been going on for several months, was about to cease. Since timing was essential, the observations were performed under Director's Discretionary Time.

"We believe that black-hole binaries give rise to outbursts when enough material has piled up in the accretion disc: then, just like a dam that bursts because it can no longer hold any more water, the material is accreted onto the black hole, giving rise to an enormous increase of the source's emission at soft X-ray wavelengths," explains Corbel.

The outburst phase, also known as 'soft' state, is accompanied by the release of 'ballistic jets' – jets that are very bright at radio wavelengths, consist of multiple ejections and extend up to 10 000 Astronomical Units (AU). When the outburst is over and the source evolves to the so-called 'hard' state, the appearance of the jets changes: with weaker radio emission and an extent up to only about 10 AU, these are known as 'compact jets'.

"We had been monitoring GX 339-4's outburst across the electromagnetic spectrum for several months. When we saw that it was transitioning to a more quiescent state, we were extremely curious to see what would happen to the jets," says Corbel.

"It is the first time that we could witness the onset of compact jets and follow their evolution," he adds. "By combining radio observations with Herschel's FIR data, we could probe the jet emission down to the base, very close to the black hole."

The Herschel data confirmed the current view, based on radio observations, which explains the emission from jets as synchrotron radiation released by highly-energetic electrons. In particular, the most energetic electrons, present at the base of the jets, radiate at FIR wavelengths, whilst the lower-energy ones, which are more abundant at larger distances from the black hole, give rise to radio emission.

The new data, however, raise questions about what causes the emission detected at NIR and optical wavelengths; this emission is also associated with the jets but does not seem to have the same origin as the radio and FIR emission. Since the optical and NIR emission follows that at radio and FIR wavelengths, one of the possible explanations is that radio and FIR photons emitted in the jets are then reflected off the disc, gaining energy in the process and thus radiating at shorter wavelengths.

"With this result, Herschel has filled a long-standing gap in the monitoring of stellar-mass black-hole jets across the electromagnetic spectrum, bridging observations performed at radio and near-infrared wavelengths," comments Göran Pilbratt, Herschel Project Scientist at ESA. "This new view complements our current picture of these fascinating objects, while highlighting, at the same time, how their emission is even more complex than previously thought," he concludes.

Notes for editors

The study presented here is based on observations of the black-hole binary system GX 339-4 performed at 70 and 160 microns with the Photodetector Array Camera and Spectrometer (PACS) on board ESA's Herschel Space Observatory. The data were gathered under Director's Discretionary Time on 25 February and 6 March 2011.

GX 339-4 is a binary system hosting a 7-solar-mass black hole that accretes mass from its companion, a low-mass star. The system is located at a distance of about 26 000 light years.

In this study, the Herschel observations were combined with data from a multi-wavelength campaign aimed at monitoring GX 339-4 across the electromagnetic spectrum; the team of astronomers used data from NASA's Rossi X-Ray Timing Explorer (RXTE) and Swift satellites to study the source's X-ray emission, from the SMARTS 1.3-m telescope at Cerro Tololo Inter-American Observatory, located in Chile, to observe the source at optical and near-infrared wavelengths, and the Australia Telescope Compact Array to study its radio emission.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. The PACS instrument contains an imaging photometer (camera) and an imaging spectrometer. The camera operates in three bands centred on 70, 100, and 160 μm, respectively, and the spectrometer covers the wavelength range between 51 and 220 μm. PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAF-IFSI/OAA/OAP/OAT, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI/INAF (Italy), and CICYT/MCYT (Spain).

Related publications

S. Corbel, et al., "Formation of the compact jets in the black hole GX 339-4", 2013, Monthly Notices of the Royal Astronomical Society, in press

Contacts

Stéphane Corbel
Laboratorie AIM (CEA/IRFU, CNRS/INSU, and Université Paris Diderot)
Gif-sur-Yvette, France
Email: stephane.corbel@cea.fr
Phone: +33-1-6908-4562

Göran Pilbratt
Herschel Project Scientist
Research and Scientific Support Department
Science and Robotic Exploration Directorate
ESA, The Netherlands
Email: gpilbratt@rssd.esa.int
Phone: +31 71 565 3621

NASA's WISE Mission Captures Black Hole's Wildly Flaring Jet

This artist's concept illustrates what the flaring black hole called GX 339-4 might look like. Infrared observations from NASA's Wide-field Infrared Survey Explorer (WISE) reveal the best information yet on the chaotic and extreme environments of this black hole's jets. Image credit: NASA. Full image and caption

PASADENA, Calif. -- Astronomers using NASA's Wide-field Infrared Survey Explorer (WISE) have captured rare data of a flaring black hole, revealing new details about these powerful objects and their blazing jets.

Scientists study jets to learn more about the extreme environments around black holes. Much has been learned about the material feeding black holes, called accretion disks, and the jets themselves, through studies using X-rays, gamma rays and radio waves. But key measurements of the brightest part of the jets, located at their bases, have been difficult despite decades of work. WISE is offering a new window into this missing link through its infrared observations.

"Imagine what it would be like if our sun were to undergo sudden, random bursts, becoming three times brighter in a matter of hours and then fading back again. That's the kind of fury we observed in this jet," said Poshak Gandhi, a scientist with the Japan Aerospace Exploration Agency (JAXA). He is the lead author of a new study on the results appearing in the Astrophysical Journal Letters. "With WISE's infrared vision, we were able to zoom in on the inner regions near the base of the stellar-mass black hole's jet for the first time and the physics of jets in action."

The black hole, called GX 339-4, had been observed previously. It lies more than 20,000 light-years away from Earth near the center of our galaxy. It has a mass at least six times greater than the sun. Like other black holes, it is an ultra-dense collection of matter, with gravity that is so great even light cannot escape. In this case, the black hole is orbited by a companion star that feeds it. Most of the material from the companion star is pulled into the black hole, but some of it is blasted away as a jet flowing at nearly the speed of light.

"To see bright flaring activity from a black hole, you need to be looking at the right place at the right time," said Peter Eisenhardt, the project scientist for WISE at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "WISE snapped sensitive infrared pictures every 11 seconds for a year, covering the whole sky, allowing it to catch this rare event."

Observing the jet's variability was possible because of images taken of the same patch of sky over time, a feature of NEOWISE, the asteroid-hunting portion of the WISE mission. WISE data enabled the team to zoom in on the very compact region around the base of the jet streaming from the black hole. The size of the region is equivalent to the width of a dime seen at the distance of our sun.

The results surprised the team, showing huge and erratic fluctuations in the jet activity on timescales ranging from 11 seconds to a few hours. The observations are like a dance of infrared colors and show that the size of the jet's base varies. Its radius is approximately 15,000 miles (24,140 kilometers), with dramatic changes by as large as a factor of 10 or more.

"If you think of the black hole's jet as a firehose, then it's as if we've discovered the flow is intermittent and the hose itself is varying wildly in size," Poshak said.

The new data also allowed astronomers to make the best measurements yet of the black hole's magnetic field, which is 30,000 times more powerful than the one generated by Earth at its surface. Such a strong field is required for accelerating and channeling the flow of matter into a narrow jet. The WISE data are bringing astronomers closer than ever to understanding how this exotic phenomenon works.

A video showing variations of the black hole jet, as seen via WISE observations, is online at http://www.astro.isas.jaxa.jp/~pgandhi/wise_gx339/wise_blackhole_anim.html .

Poshak Gandhi is supported by the JAXA International Top Young Fellowship program. Other authors of the paper include: A.W. Blain of the University of Leicester, United Kingdom; D.M. Russell and S. Markoff of the University of Amsterdam; P. Casella of the University of Southampton, United Kingdom; J. Malzac of Centre National de la Recherche Scientifique and Université de Toulouse, France; S. Corbel of Université Paris Diderot and Commissariat à l'énergie atomique Saclay, France; P. D'Avanzo of Istituto Nazionale di Astrofisica, Italy; F.W. Lewis of Faulkes Telescope Project, Wales; M. Cadolle Bel of the European Space Astronomy Centre, Spain; P. Goldoni of Laboratoire Astroparticule et Cosmologie, France and Commissariat à l'énergie atomique Saclay, France; S. Wachter of the California Institute of Technology, Pasadena, Calif.; D. Khangulyan of the Japan Aerospace Exploration Agency; and A. Mainzer of JPL.

JPL manages and operated WISE for NASA's Science Mission Directorate in Washington. The spacecraft was put into hibernation mode after it scanned the sky twice, completing its main objectives. The mission was selected under NASA's Explorers Program, which is managed by the agency's Goddard Space Flight Center in Greenbelt, Md. The science instrument was built by the Space Dynamics Laboratory in Logan, Utah; and the spacecraft was built by Ball Aerospace and Technologies Corp., in Boulder, Colo. Science operations and data processing take place at the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. More information is online at http://www.nasa.gov/wise and http://wise.astro.ucla.edu and http://jpl.nasa.gov/wise .

Whitney Clavin 818-354-4673 Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

Trent Perrotto 202-358-0321 NASA Headquarters, Washington
Trent.j.perrotto@nasa.gov

Suzaku Catches Retreat of a Black Hole's Disk

GX 339-4, illustrated here, is among the most dynamic binaries in the sky, with four major outbursts in the past seven years. In the system, an evolved star no more massive than the sun orbits a black hole estimated at 10 solar masses. Credit: ESO/L. Calçada

Full size version

Studies of one of the galaxy's most active black-hole binaries reveal a dramatic change that will help scientists better understand how these systems expel fast-moving particle jets.

Binary systems where a normal star is paired with a black hole often produce large swings in X-ray emission and blast jets of gas at speeds exceeding one-third that of light. What fuels this activity is gas pulled from the normal star, which spirals toward the black hole and piles up in a dense accretion disk.

"When a lot of gas is flowing, the dense disk reaches nearly to the black hole," said John Tomsick at the University of California, Berkeley. "But when the flow is reduced, theory predicts that gas close to the black hole heats up, resulting in evaporation of the innermost part of the disk." Never before have astronomers shown an unambiguous signature of this transformation.

To look for this effect, Tomsick and an international group of astronomers targeted GX 339-4, a low-mass X-ray binary located about 26,000 light-years away in the constellation Ara. There, every 1.7 days, an evolved star no more massive than the sun orbits a black hole estimated at 10 solar masses. With four major outbursts in the past seven years, GX 339-4 is among the most dynamic binaries in the sky.

In September 2008, nineteen months after the system's most recent outburst, the team observed GX 339-4 using the orbiting Suzaku X-ray observatory, which is operated jointly by the Japan Aerospace Exploration Agency and NASA. At the same time, the team also observed the system with NASA's Rossi X-ray Timing Explorer satellite.

Instruments on both satellites indicated that the system was faint but in an active state, when black holes are known to produce steady jets. Radio data from the Australia Telescope Compact Array confirmed that GX 339-4's jets were indeed powered up when the satellites observed.

Despite the system's faintness, Suzaku was able to measure a critical X-ray spectral line produced by the fluorescence of iron atoms. "Suzaku's sensitivity to iron emission lines and its ability to measure the shapes of those lines let us see a change in the accretion disk that only happens at low luminosities," said team member Kazutaka Yamaoka at Japan's Aoyama Gakuin University.

X-ray photons emitted from disk regions closest to the black hole naturally experience stronger gravitational effects. The X-rays lose energy and produce a characteristic signal. At its brightest, GX 339-4's X-rays can be traced to within about 20 miles of the black hole. But the Suzaku observations indicate that, at low brightness, the inner edge of the accretion disk retreats as much as 600 miles.

"We see emission only from the densest gas, where lots of iron atoms are producing X-rays, but that emission stops close to the black hole -- the dense disk is gone," explained Philip Kaaret at the University of Iowa. "What's really happening is that, at low accretion rates, the dense inner disk thins into a tenuous but even hotter gas, rather like water turning to steam."

The dense inner disk has a temperature of about 20 million degrees Fahrenheit, but the thin evaporated disk may be more than a thousand times hotter.

The study, which appears in the Dec. 10 issue of The Astrophysical Journal Letters, confirms the presence of low-density accretion flow in these systems. It also shows that GX 339-4 can produce jets even when the densest part of the disk is far from the black hole.

"This doesn't tell us how jets form, but it does tell us that jets can be launched even when the high-density accretion flow is far from the black hole," Tomsick said. "This means that the low-density accretion flow is the most essential ingredient for the formation of a steady jet in a black hole system."

Francis Reddy
NASA's Goddard Space Flight Center

Violent flickering in Black Holes

ESO PR Photo 36/08
Star-Forming Region NGC 366
Credit: ESO/L. Calçada

VLT and Rossi XTE satellite probe violently variable black holes

Unique observations of the flickering light from the surroundings of two black holes provide new insights into the colossal energy that flows at their hearts. By mapping out how well the variations in visible light match those in X-rays on very short timescales, astronomers have shown that magnetic fields must play a crucial role in the way black holes swallow matter.

Like the flame from a candle, light coming from the surroundings of a black hole is not constant — it flares, sputters and sparkles. "The rapid flickering of light from a black hole is most commonly observed at X-ray wavelengths," says Poshak Gandhi, who led the international team that reports these results. "This new study is one of only a handful to date that also explore the fast variations in visible light, and, most importantly how these fluctuations relate to those in X-rays."

The observations tracked the shimmering of the black holes simultaneously using two different instruments, one on the ground and one in space. The X-ray data were taken using NASA's Rossi X-ray Timing Explorer satellite. The visible light was collected with the high speed camera ULTRACAM, a visiting instrument at ESO's Very Large Telescope (VLT), recording up to 20 images a second. ULTRACAM was developed by team members Vik Dhillon and Tom Marsh. "These are among the fastest observations of a black hole ever obtained with a large optical telescope," says Dhillon.

To their surprise, astronomers discovered that the brightness fluctuations in the visible light were even more rapid than those seen in X-rays. In addition, the visible-light and X-ray variations were found not to be simultaneous, but to follow a repeated and remarkable pattern: just before an X-ray flare the visible light dims, and then surges to a bright flash for a tiny fraction of a second before rapidly decreasing again.

None of this radiation emerges directly from the black hole, but from the intense energy flows of electrically charged matter in its vicinity. The environment of a black hole is constantly being reshaped by a riotous mêlée of strong and competing forces such as gravity, magnetism and explosive pressure. As a result, light emitted by the hot flows of matter varies in brightness in a muddled and haphazard way. "But the pattern found in this new study possesses a stable structure that stands out amidst an otherwise chaotic variability, and so, it can yield vital clues about the dominant underlying physical processes in action," says team member Andy Fabian.

The visible-light emission from the neighbourhoods of black holes was widely thought to be a secondary effect, with a primary X-ray outburst illuminating the surrounding gas that subsequently shone in the visible range. But if this were so, any visible-light variations would lag behind the X-ray variability, and would be much slower to peak and fade away. "The rapid visible-light flickering now discovered immediately rules out this scenario for both systems studied," asserts Gandhi. "Instead the variations in the X-ray and visible light output must have some common origin, and one very close to the black hole itself."

Strong magnetic fields represent the best candidate for the dominant physical process. Acting as a reservoir, they can soak up the energy released close to the black hole, storing it until it can be discharged either as hot (multi-million degree) X-ray emitting plasma, or as streams of charged particles travelling at close to the speed of light. The division of energy into these two components can result in the characteristic pattern of X-ray and visible-light variability.

More Information

The two black holes studied here, GX 339-4 and SWIFT J1753.5-0127, are the remnants of massive dead stars in the Milky Way. They are embedded in separate "binary" stellar systems, where the black hole is bound to a normal star that is losing matter to its dark companion. Both black holes have masses of around ten times that of our Sun, yet the size of their orbits is only a few million kilometres, much more compact than the orbit of Mercury around our Sun.

Apart from Gandhi, Dhillon, Durant, Fabian, and Marsh, the other members of the team are Kazuo Makishima at the University of Tokyo, Japan, Jon Miller at the University of Michigan, USA, Tariq Shahbaz at the Instituto de Astrofisica de Canarias, Spain, and Henk Spruit of the Max-Planck-Institute for Astrophysics, Germany.

Gandhi, P., Makishima, K., Durant, M., Fabian, A. C., Dhillon, V. S., Marsh, T. R., Miller, J. M., Shahbaz, T. & Spruit, H. C., Rapid optical and X-ray timing observations of GX 339-4: flux correlations at the onset of a low/hard state, Monthly Notices of the Roy. Astron. Soc. Letters, 390, L29 (2008), astro-ph/0807.1529
Durant, M., Gandhi, P., Shahbaz, T., Fabian, A., Miller, J., Dhillon, V. S. & Marsh, T. R,. SWIFT J1753.5-0127: a surprising optical/X-ray cross-correlation function,The Astrophysical Journal, 682, L45 (2008), astro-ph/0806.2530

Contacts

Poshak Gandhi
RIKEN Cosmic Radiation Lab
Wako, Saitama, Japan
Phone: +81 48 467 9334
E-mail: pg (at) crab.riken.jp

Martin Durant
Instituto de Astrofísica de Canarias
La Laguna, Tenerife, Spain
Phone: +34 922 605 388
E-mail: durant (at) iac.es

Vik Dhillon
University of Sheffield, UK
Phone: +44 114 222 4528
Email: Vik.Dhillon (at) sheffield.ac.uk

Tom R. Marsh
University of Warwick, UK
Phone: +44 247 657 4739
Email: t.r.marsh (at) warwick.ac.uk

Andy Fabian
Institute of Astronomy
Cambridge, UK
Phone: +44 1223 337548
Email: acf (at) ast.cam.ac.uk

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