Category Archives: accretion

Blown away by an accretion disk wind

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GRO J1744-28 is among the most puzzling neutron star X-ray binaries known in our Galaxy. How could the companion star have possibly reduced to such its current small size without spinning the neutron star up to very high rotation rate or without strongly reducing its magnetic field? High-resolution Chandra X-ray observations may have solved this puzzle.

After remaining dormant for nearly 20 years, GRO J1744-28 suddenly exhibited a new accretion outburst in 2014. This provided the excellent opportunity to study this extraordinary X-ray binary with modern X-ray telescopes. Using high-resolution Chandra Grating Spectroscopic observations, we found evidence for X-ray absorption lines that could arise from a dense wind blowing off the accretion disk. Such winds are commonly seen for black hole X-ray binaries, although it is not established yet whether similarly strong winds can also form in the accretion disks that surround neutron stars.

If the absorption features in GRO J1744-28 indeed origin in a disk wind, then energetic considerations suggest that this wind may carry away a considerable amount of matter that is being transferred from the companion star. Therefore, a considerable amount of matter may be lost from the binary without accreting onto the neutron star. This could potentially solve the long-standing puzzle of how to reconcile the very low mass of the companion star in GRO J1744-28 with the slow rotation period and high magnetic field of the neutron star primary.

Degenaar, Miller, Harrison, Kennea, Kouveliotou, Younes 2014, ApJ Letters 796, L9: High-resolution X-Ray Spectroscopy of the Bursting Pulsar GRO J1744-28

Paper link: ADS

Artist impression of an X-ray binary with a strong disk wind. Credit: NASA’s Goddard Space Flight Center / Chandra X-ray observatory / M. Weiss

Artist impression of an X-ray binary with a strong disk wind.
Credit: NASA’s Goddard Space Flight Center / Chandra X-ray observatory / M. Weiss

A split-personality neutron star?

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The universe is full of remarkable objects. PSR J1824-2452 is a neutron star that is located in the globular cluster M28 and emits pulsed radio emission. This energy is powered by its rapid rotation: the pulsar spins around its own axis about 15,000 times per second (it takes only 3.9 milliseconds to complete one rotation). In 2013, however, its radio pulsations disappeared and instead its X-ray emission increased by 5 orders of magnitude, it lighted off a thermonuclear X-ray burst, and exhibited X-ray pulsations power by the accretion of matter: The neutron star had suddenly become active as an X-ray binary! After about 2 months the X-rays faded and it returned to its life as radio pulsar like nothing had happened. For the first time in the history of astronomy a neutron star was caught in the act of  switching identity.

Low-mass X-ray binaries and millisecond radio pulsars are two different manifestations of neutron stars in binary systems that are thought to be evolutionarily linked. In an X-ray binary, the outer gaseous layers of a small companion star (that has a mass less than that of our Sun) are stripped off and accreted by the neutron star. The large amount of energy that is liberated during the accretion process makes these interacting binaries shine bright in X-rays. After millions-billions of years the companion star will stop feeding the compact primary. The rapidly rotating neutron star, spun up to millisecond periods by gaining angular momentum during the accretion process, may now emit pulsed radio emission so that the binary is observed as a millisecond radio pulsar.

The discovery that neutron stars may rapidly switch identity between these two manifestations opens up a new avenue to study their evolutionary link. It is therefore of prime interest to identify other X-ray binary/radio pulsar transitional objects. The M28 source displayed remarkable X-ray spectral properties and X-ray flux variability, which can potentially serve as a template for such searches. Based on that, we identified one possible candidate: the peculiar X-ray source XMM J174457-2850.3 that is located at a projected distance of about 14 arcminutes from the Milky Way’s supermassive black hole Sgr A*.

XMM J174457-2850.3 is just on the edge of the region surveyed in Swift’s Galactic center monitoring program, which has taken almost daily X-ray snapshots since 2006. For years we were puzzled by the remarkable X-ray variability of XMM J174457-2850.3: instead of spending remaining dim for most of its time and making occasional excursions to bright X-ray states, we often found the source lingering in between its quiescent and outburst levels. This behavior is not typical for low-mass X-ray binaries, leaving us to ponder about the exact nature of this peculiar X-ray source. We got our answer in late 2012, when Swift suddenly caught a rare, very energetic thermonuclear X-ray burst from XMM J174457-2850.3. This conclusively established that the source is, in fact, a neutron star low-mass X-ray binary.

By investigating 12 years of Swift, XMM-Newton and Chandra data of the Galaxy center (obtained between 2000 and 2012), we found that XMM J174457-2850.3 exhibits three different X-ray luminosity states, and has an X-ray spectrum that is much harder (that is, relatively more photons are emitted at higher energies) than commonly seen in low-mass X-ray binaries. These properties are strikingly similar to the M28 X-ray binary/radio pulsar transitional object. Its unusual X-ray properties are explained as interactions between the magnetic field of the neutron star with the surrounding accretion flow. A similar mechanism may be at work in the peculiar Galactic center source XMM J174457-2850.3.

Degenaar, Wijnands, Renolds et al. 2014, MNRAS 792, 109: The Peculiar Galactic Center Neutron Star X-Ray Binary XMM J174457-2850.3

Paper link: ADS

Press item on the M28 neutron star: NASA

ns_pulsar_xrb

Artist’s conception of a neutron star switching faces: a radio pulsar (top) and an X-ray binary (bottom). Credit: NASA’s Goddard Space Flight Center

The accretion flow around a black hole

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Black holes are infamous for their relentless gravitational pull through which they drain matter and energy from their surroundings. However, with their enormous power, these tantalizing objects also blast matter back into space via ultra-fast collimated jets and dense winds. Understanding the exact connection between how black holes accrete from – and supply feedback to – their environment is one of the outstanding challenges of modern astrophysics.

X-ray binaries are excellent laboratories to study the eating habits of black holes. In these binary star systems a black hole orbits a Sun-like star close enough to pull off and accrete the outer layers of its unfortunate companion. This accretion process liberates enormous amounts of energy that is emitted across the electromagnetic spectrum. Studying the accretion flow in X-ray binaries thus warrants a multi-wavelength approach.

We recently performed such a study for the newly discovered X-ray binary Swift J1910.2-0546.  In 2012 May the Swift satellite suddenly discovered a new, bright X-ray point source in the sky and very soon it became clear that the X-ray emission was powered by accretion onto a black hole. Using the X-ray and UV telescopes onboard Swift, we continued to monitor this new X-ray binary for about three months. To complement these observations, the source was also closely followed at optical and infrared wavelengths (B, V, R, I, J, H, and K filters) using the 1.3-m SMARTS telescope located at the Cerro Tololo Inter-American Observatory (CTIO) in Chile. Finally, a high-resolution X-ray spectroscopic observation was obtained with the Chandra satellite.

This monitoring campaign allowed us to map out the accretion morphology around the black hole in Swift J1910.2-0546. Firstly, X-ray spectroscopy revealed two peculiarities: although disk winds appear to be ubiquitous in black hole X-ray binaries when they are at their brightest, our Chandra observations did not reveal any emission or absorption features that are the imprints of an accretion disk wind. Since such winds are thought to be concentrated in the equatorial plane, this may imply that we are viewing the binary at relatively low inclination. Moreover, even during the brightest stages of its outburst tracked by Swift, the temperature of the accretion disk did not reach above 0.5 keV (about 6 million degrees Kelvin), whereas most black hole disks are much hotter with temperatures above 1 keV. This could plausibly be a geometrical effect, again suggesting that the inclination angle of the binary is relatively low.

Comparing the overall light curves of the outburst in different wavebands revealed two other striking features. A sharp and prominent flux dip appeared in the X-rays almost one week later than at UV, optical and infrared wavelengths. The detailed properties of this flux dip appear to point to a global change in accretion flow geometry, possibly related to the formation of a collimated jet or the condensation of the inner part of the accretion disk. In addition, when the activity of the black hole started to cease, we found that the X-rays steadily decreased whereas the UV emission suddenly was rising again. The observed strong anti-correlation between the X-ray/UV flux also indicates a global change in accretion flow.

Degenaar, Maitra, Cackett et al. 2014, ApJ 784, 122: Multi-wavelength Coverage of State Transitions in the New Black Hole X-Ray Binary Swift J1910.2-0546

Paper link: ADS

Artist impression of the accretion flow around a black hole. Credit: NASA/Dana Berry, SkyWorks Digital

Artist impression of the accretion flow around a black hole.
Credit: NASA/Dana Berry, SkyWorks Digital

Awaiting activity of the Milky Way supermassive black hole

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Supermassive black holes lurk at the centers of every Galaxy. Our own Milky Way harbors a black hole of approximately 4 million Solar masses, whose electromagnetic counterpart is known as Sagittarius A* (Sgr A*). Most surprisingly, its luminosity is about 9 orders of magnitude lower than the maximum brightness that a black hole of this mass can reach. Nevertheless, observational features such as the gigantic “Fermi Bubbles” and “light echoes” from molecular clouds near the Galactic center suggest that Sgr A* has not always been dormant, but instead evidences a wild and glorious past.

We may now find ourselves at the dawn of a reactivation phase of our supermassive black hole, which is foreshadowed by the discovery of a cold gas cloud (a.k.a “G2”) that is on a collision course with Sgr A* and is predicted to impact in late 2013 or early 2014. The cloud may become disrupted due to tidal forces and parts of the shredded gas could then be accreted onto the black hole. However, whether this interaction leads to fireworks remains to be seen. Right from the start of G2’s reported discovery in early 2012, there has been ongoing discussion regarding the nature, origin, and hence the faith of this tantalizing gas cloud that seems to have come out of nowhere. It remains uncertain as to whether G2 harbors a central object (e.g., a young star or a binary) that is keeping the cloud gravitationally bound. If so, G2 may survive its doom-trail, keeping any observable effects on the emission of Sgr A* to a bare minimum.

Astronomers all over the world are at the ready in case Sgr A* becomes revived, armed with monitoring campaigns utilizing ground-based and space-based facilities, and target-of-opportunity programs covering the entire electromagnetic spectrum. And so they should: this has the potential to be a unique, once-in-a-lifetime opportunity to observe a disruption event in our own backyard and have an unprecedented view of the feeding process of our Galactic nucleus. Me and my co-workers occupy a front seat and are in place to follow this historic event at infrared and X-ray wavelengths.

We have recently embarked on a monitoring program employing the infrared-imager FourStar mounted on 6.5-m Magellan-Baade telescope, located at the Las Campanas Observatory in Chili. Between July and October, as long as the Galactic center is observable from this site, we are monitoring our Galactic nucleus nearly weekly using in the infra-red J, H and Ks wavebands. This allows us to detect any possible changes in the infrared emission of Sgr A*, which might signal an enhancement of the accretion flow due to the shredded gas cloud.

Our Magellan infrared campaign is complemented by intensive X-ray monitoring. Utilizing the unique flexibility of the Swift satellite, we observe the center of our Galaxy every day with the onboard X-ray telescope. This program has been running since 2006, and has provided us with valuable insight into the long-term X-ray behavior of the supermassive black hole. This serves as an important calibration point to assess if, and how, the X-ray properties of Sgr A* change as a result of its interaction with G2. Moreover, Swift is the only observatory that can accommodate daily X-ray observations, and may therefore turn out to be the first to detect any action and thereby serve as a trigger for other observatories.

Given the uniqueness of this astronomical event and the broad scientific and public interest, we have set up an automated reduction and analysis pipeline for the daily X-ray observations obtained with Swift. New data is downloaded the instant that it becomes available; generally this is within a mere 3 hours after an observation was taken. Quick-look images and light curves are then produced and immediately uploaded onto a website (www.swift-sgra.com), followed by an instant e-mail notice distributed to subscribers. This allows the scientific community to optimally benefit and promptly respond, in case our Galactic nucleus awakens.

Our Swift Monitoring Campaign website: www.swift-sgra.com

The dedicated wiki-page about the gas cloud “G2”: MPE

Selection of press:

NRC news item, 2014 March (Dutch news paper)

BBC science news, 2014 January

NY Times science news, 2014 January

NASA/Swift press release, 2014 January

HEAPOW, 2014 January

Michigan Astronomy feature, 2014 January

allesoversterrenkunde.nl, 2013 September (Dutch science site)

An artist impression of an Active Galactic Nucleus (AGN). Credit: ESA/NASA, the AVO project and Paolo Padovani.

An artist impression of an Active Galactic Nucleus (AGN).
Credit: ESA/NASA, the AVO project and Paolo Padovani.

The table manners of the Milky Way’s supermassive black hole

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Understanding accretion onto supermassive black holes and the associated feedback to their environment lies at the basis of understanding their formation, growth and evolution, the chemical enrichment of the interstellar medium, galaxy evolution, and the formation of large scale structures in the universe. Sagittarius A* (a.k.a. Sgr A*) is a supermassive black hole that forms the dynamical center of our Milky Way Galaxy. Being the most nearby Galactic nucleus, it allows for an unparalleled study of the fueling process of supermassive black holes.

Surprisingly enough, the bolometric luminosity of Sgr A* is about 8-9 orders of magnitude lower than the maximum radiation (the Eddington limit) that can be emitted from the environment of a supermassive black hole with a mass of 4 million times that of our Sun. Its faintness is particularly puzzling because nearby dense star cluster are thought to supply enough matter to serve as a grant banquet for Sgr A*. However, it appears that our Galactic nucleus is on a diet.

Nevertheless, it appears to crave for an occasional snack; the relatively steady quiescent radiation of Sgr A* is, however, occasionally punctured by hours-long flares during which the X-ray emission increases by 1-2 orders of magnitude. These events are likely related to small accretion events or magnetic processes. Most excitingly, the time scale involved with these phenomena suggest that they must be originating very close to the black hole (within approximately 15 Schwarzschild radii). A few dozens of X-ray flares have been detected from Sgr A* by using the Chandra and XMM-Newton satellites. The far majority of these are relatively weak; only on 4 occasions was the emission observed to increase more than 100 times the steady base level.

We investigated nearly 800 observations of the center of our Galaxy that were obtained with the X-ray Telescope onboard the Swift telescope between 2006 and 2012. In these 6 years of monitoring data we discovered a total of 6 bright X-ray flares from Sgr A* during which the emission increased by a factor of 100. Owing to its uniquely dense sampling, the Swift campaign more than double the number of observed bright X-ray flares from our supermassive black hole. This allowed to constrain the recurrence rate of these events, and made an unbiased comparative study of their spectral properties possible for the first time. Having mapped out the long-term X-ray behavior of Sgr A* with Swift provides an important calibration point to assess whether the activity of our supermassive black hole is going to change as the result of its interaction with an approaching gas cloud (read more about this upcoming exciting event here).

Degenaar, Miller, Kennea, Gehrels, Reynolds, Wijnands 2013, ApJ 769, 155: The X-Ray Flaring Properties of Sgr A* during Six Years of Monitoring with Swift

Paper link: ADS

The Swift monitoring website: www.swift-sgra.com

Press: German radio interview

Three-color accumulated Swift X-ray Telescope Image of the Galactic center (2006-2014).

Three-color accumulated Swift X-ray Telescope Image of the Galactic center (2006-2014).

The effects of a violent thermonuclear burst

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Thermonuclear X-ray bursts manifest themselves as intense flashes of X-ray emission that have a duration of seconds to hours, during which a total energy of approximately 1039 to 1042 erg is radiated. These events are caused by unstable thermonuclear burning that transforms hydrogen and/or helium that has falling onto the surface of a neutron star into heavier chemical elements. X-ray bursts are a unique signature of neutron stars in low-mass X-ray binaries. In such interacting binary star systems, a Roche-lobe overflowing late-type companion star feeds matter to the compact object via an accretion disk.

There is a delicate connection between the properties of X-ray bursts and that of the accretion flow. On the one hand, the rate at which mass is accreted onto the neutron star determines the duration, recurrence time, and radiated energy of the X-ray bursts, whereas the accretion geometry can strongly influence the observable properties. On the other hand, it has been proposed that particularly powerful X-ray bursts may be able to influence the accretion flow.

IGR J17062-6143 is an X-ray source that was discovered in 2006, but remained unclassified until the Burst Alert Telescope onboard the Swift satellite detected an X-ray burst in 2012. This unambiguously identified IGR J17062-6143 as a neutron star low-mass X-ray binary. But not just any ordinary neutron star. The X-ray burst was highly energetic (classifying as a so-called intermediately long X-ray burst), and displayed three very unique features that indicate that the explosion was violent enough to disrupt the accretion disk surrounding the neutron star.

Firstly, the 18-min long X-ray burst displayed dramatic, irregular intensity variations that were clearly visible in the X-ray light curve. Similar fluctuations have only been seen on a handful of occasions. They are likely caused by swept-up clouds of gas or puffed-up structures in the accretion disk. The time-scale of the fluctuations suggest this gas is located at a distance of approximately 103 km from the neutron star. Secondly, the X-ray spectrum of the X-ray burst showed a highly significant emission line around an energy of 1 keV (most likely a Fe-L shell line). This emission feature can be explained as irradiation of relatively cold gas. The width of the line suggests that this material is located at a distance of 103 km from the neutron star. Thirdly, significant absorption features near 8 keV were present in the X-ray spectrum (in the Fe-K band). These likely result from hot, ionized gas along the line of sight. Fitting these features with photo-ionization models points towards a similar radial distance as inferred from the emission line and the light curve fluctuations. Spectral emission and absorption features have never been (unambiguously) detected during an X-ray burst, making this a highly exciting discovery.

In conclusion, three independent observational features suggest that the energetic X-ray burst from IGR J17062-6143 swept up gas along our line of sight, out to a distance of roughly 1000 km from the neutron star (approximately 50 gravitational radii). This provides strong evidence  that powerful X-ray bursts can indeed disrupt the (inner) accretion disk.

Degenaar, Miller, Wijnands, Altamirano, Fabian 2013, ApJ 767, L37: X-Ray Emission and Absorption Features during an Energetic Thermonuclear X-Ray Burst from IGRJ17062-6143

Paper link: ADS

An artist impression of an interacting binary. Image credits: David. A. Hardy / STFC

An artist impression of an interacting binary.
Image credits: David. A. Hardy / STFC

Staring at the center of the Milky Way

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The region around Sagittarius A*, the supermassive black hole that represents the dynamical center of our Milky Way Galaxy, harbors a large number of accreting neutron stars and black holes. Between 2005 and 2008, we targeted this region every few months using the X-ray instruments onboard the Chandra and XMM-Newton satellites. The main objective of this monitoring campaign was to study the behavior of transient X-ray binaries. These spend most of their time in a dim quiescent state, during which they often can not be detected, but experience occasional outbursts of bright X-ray emission when the neutron star or black hole pulls off and accretes matter from its companion star.

Our observations covered a region of 1.2 square degree around Sagittarius A* that contains 17 known X-ray transients, 8 of which were active during our campaign. We performed a detailed study of the energy distribution and temporal variations of their X-ray emission. From one of the active neutron stars we detected two thermonuclear explosions, which occurred within a time interval of only 3.8 minutes. Such a short repetition time is only rarely seen and poses a challenge for theoretical models. In addition, we discovered a previously unknown X-ray source, which we tentatively classify as an accreting white dwarf.

Most remarkably, the majority of X-ray transients located near Sagittarius A* are considerably fainter during outburst than is usually seen for accreting neutron stars and black holes. One possible explanation for their sub-luminous character is that these X-ray binaries have very small orbits, in which the compact primary and their companion revolve around each other in less than two hours. Finding such binaries is of particular interest, because they are thought to be strong sources of gravitational waves. The existence of gravitational waves is one of the predictions of Einstein’s theory of General Relativity, which future space-missions hope to prove.

Degenaar, Wijnands, Cackett et al. 2012, A&A 545, 49: A four-year XMM-Newton/Chandra monitoring campaign of the Galactic centre: analysing the X-ray transients

Paper link: ADS

Chandra X-ray image of the center of our Milky Way Galaxy. Credit: NASA/Wang et al. 2002.

Chandra X-ray image of the center of our Milky Way Galaxy.
Credit: NASA/Wang et al. 2002.

Quiescent but not quite?

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The X-ray binary Swift J1749.4-2807 contains a neutron star that rotates around its own axis at a dazzling rate of 518 times per second. To date, only 14 of such fast spinning accreting X-ray pulsars are known. Amongst these, Swift J1749.4-2807 is the only one that shows eclipses: a temporary dramatic drop in the X-ray emission that lasts for approximately 36 minutes and repeats every 8.8 hours. These are caused by the companion star that periodically moves into our line of sight, thereby blocking the X-ray bright central part of the binary.

The unique combination of X-ray pulsations and eclipses makes Swift J1749.4-2807 a particularly promising target to precisely constrain the mass of the neutron star. This is one of the key objectives of modern astrophysics. We used the European satellite XMM-Newton to study the source in quiescence, when the accretion is thought to have switched off and the surface of the neutron star may become directly visible. Quiescent X-ray observations are an important aspect of the challenge to accurately constrain the mass of the neutron star.

Contrary to that seen for the majority of neutron stars, we found that the quiescent X-ray spectrum of Swift J1749.4-2807 consists primarily of high-energy (> 2 keV) photons and shows no evidence for heat radiation that comes from the surface of the neutron star. Its unusual properties can possibly be explained if matter continues to fall onto the neutron star in quiescence. This severely complicates the determination of its mass. It is of utmost importance to understand whether quiescent accretion is common amongst neutron star X-ray binaries.

Degenaar, Patruno, Wijnands 2012, ApJ 756, 148: The Quiescent X-Ray Properties of the Accreting Millisecond X-Ray Pulsar and Eclipsing binary Swift J1749.4-2807

Paper link: ADS

Discovery of eclipses in Swift J1749.4-2807 (2010): NASA press release

Schematic representation of the eclipsing binary Swift J1749.4-2807. Credit: NASA/GSFC.

Schematic representation of the eclipsing binary Swift J1749.4-2807.
Credit: NASA/GSFC.

Chasing the faint ASCA X-ray sources

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In 1993, the Japanese Advanced Satellite for Cosmology and Astrophysics (ASCA) was successfully launched. This satellite was operated for 7 years (until 2000) and was the first mission that provided X-ray imaging capabilities in a relatively broad energy band (0.3-10 keV). During its lifetime, ASCA carried out two dedicated surveys of the Galactic Center and Plane, where it discovered around 200 distinct X-ray sources.

Up to date, about 1/3 of the ASCA-discovered X-ray sources could not be classified. They have relatively faint X-ray intensities that can trace a variety of Astronomical objects such as strongly magnetized neutron stars (called ‘magnetars’), bright accreting white dwarfs (‘polars’ and ‘intermediate polars’), sub-luminous accreting neutron stars and black holes, X-ray emitting massive stars, as well as foreground stars and background active galaxies (‘active galactic nuclei’).

In 2006, we launched a program to observe 35 of the unclassified ASCA-sources with the Swift satellite. The goal of this program was to study the X-ray spectrum of these objects, to find possible indications of temporal variations in the X-ray intensity and to obtain more accurate X-ray positions that would aid in conducting follow-up observations at other wavelengths (optical, infra-red, radio). With this approach we aim to gain more insight into the nature of the faint unclassified ASCA sources.

With our Swift observations we were able to tentatively identify three accreting compact objects: one likely magnetized white dwarf, one neutron star and one object that is likely a neutron star or a black hole. In addition, we found that three objects are possibly nearby X-ray emitting stars. Finally, we found evidence that two of the ASCA-detected sources likely undergo strong variations in their X-ray intensity, since these were not detected during our Swift observations.

Degenaar, Starling, Evans et al. 2012, A&A 540, 22: Swift follow-up observations of unclassified ASCA sources

Paper link: ADS

X-ray image from the ASCA survey of the Galactic Centre. Credit: Sugizaki et al. 2001.

X-ray image from the ASCA survey of the Galactic Centre.
Credit: Sugizaki et al. 2001.

Now you see me, now you don’t

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Neutron stars in X-ray binaries often accrete matter only for a few weeks, after which the accretion stops and the binary remains quiescent for several years. As the naming suggests, it is generally assumed that accretion has completely stopped in quiescence. Yet, the binary still emits X-ray emission (albeit orders of magnitude lower than during the active phase), which is thought to result from the radiation of heat from the neutron star.

The neutron star X-ray binary EXO 1745-248 is located in the globular cluster Terzan 5 (see images) and has been studied in quiescence using Chandra observations obtained in 2003. Unlike the majority of neutron stars, surprisingly, its quiescent emission did not resemble thermal emission. This poses a puzzle for the origin of the quiescent X-ray emission of this X-ray binary.

We used three additional Chandra observations taken in 2009 and 2011 to further study the quiescent X-ray emission of EXO 1745-248. While in 2009 the neutron star was detected at a similar brightness as previously seen, the source had disappeared in 2011! The implied large variation in the quiescent X-ray intensity can possibly be explained if the accretion did not fully stop and the neutron star continued to slowly accumulate matter. Alternatively the 2011 disappearance might be caused by a temporarily obscuration of the X-ray source, for example by the outer edge of the accretion disk.

Degenaar & Wijnands, 2011, MNRAS 422, 581: Strong X-ray variability in the quiescent state of the neutron star low-mass X-ray binary EXO 1745-248

Paper link: ADS

Three-color images of the globular cluster Terzan 5, obtained with the Chandra X-ray satellite.

Three-color images of the globular cluster Terzan 5, obtained with the Chandra X-ray satellite.