Nearly 10 years of Swift X-ray monitoring the Galactic center

In 2006 February, shortly after its launch, the Swift satellite began monitoring the inner 50 x 50 pc (1.5×10^15 km squared) of our Milky Way with the on board X-Ray Telescope. In the months February-October*, 15-minute X-ray snapshots of our Galactic center were taken every 1-4 days. In nearly 10 years years time (2006 February till 2014 October), this accumulated to nearly 1.3 Ms of total exposure time, equivalent to about 15 days of continuous observing. This legacy program has yielded a wealth of information about the long-term X-ray behavior of the Milky Way’s supermassive black hole Sgr A*, as well as numerous transient X-ray sources that are located in region covered by the campaign.

One of the main discoveries resulting from this campaign was the detection of six bright X-ray flares from Sgr A*. These are mysterious flashes of X-ray emission during which the supermassive black hole brightens by a factor of up to approximately 150 for tens of minutes to hours. Unfortunately, Swift temporarily lost its view of Sgr A* as of April 2013, due to the sudden awakening of a nearby ultra-magnetized neutron star (a “magnetar”). The X-ray emission from this object was about 200 times brighter than that of Sgr A*, hence it outshined our supermassive black hole. However, the notorious magnetar steadily faded over time and in late 2014 we regained view of the supermassive black hole again. Indeed, a seventh X-ray flare from Sgr A* was caught in 2014 September, the brightest ever seen with Swift.

The Swift/XRT monitoring campaign has also been instrumental to understand the nature of a peculiar class of sub-luminous X-ray binaries. These are binary star systems in which a neutron star or a black hole accretes matter from a nearby companion (e.g., Sun-like) star. It has been a puzzle for decades why a small sub-group X-ray binaries are much fainter than accretion theory prescribes. This suggests that these neutron stars/black holes have little appetite, but the question is why. Swift has made a very important contribution in mapping the accretion behavior (“eating patterns”) of this class of objects and also identified 3 new members.

In addition, we  discovered that “normal” X-ray binaries (i.e., ones eating more exorbitantly) sometimes display similar eating patterns as the sub-luminous X-ray binaries, giving important clues about the underlying mechanism producing low-level accretion events (i.e., fasting periods). For example, we found that in some cases the neutron stars/black holes sub-luminous X-ray binaries were likely caught enjoying an occasional midnight snack, and should be feasting on a larger banquet at other times. However, the behavior of one particular neutron star suggests that it may have a relatively strong magnetic field that sometimes simply impedes the infall (i.e., accretion) of material that is transferred from its companion. This object is therefore a candidate X-ray binary/millisecond radio pulsar transitional object.

After nearly 10 successful years, continuation of Swift’s Galactic center monitoring program remains highly valuable. For example, this would allow the collection of even more X-ray flares from our supermassive black hole, and to investigate whether the eating patterns of Sgr A* change in any way after the gaseous object “G2” has swung by.

Paper link: ADS

Swift/XRT monitoring campaign website: www.swift-sgra.com

*Due to proximity of the Sun the center of our Galaxy cannot be observed with Swift in November-January.

gc_swift_2006_2014_withnames

Swift/XRT image of the inner ~50×50 pc of the Milky Way using 1.3 Ms of data accumulated in 2006-2014. Red represents 0.3-1.5 keV energies, green 1.5-3.0 keV and blue 3.0-10 keV.

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The accretion flow around a black hole

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.

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

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

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).

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).

Staring at the center of the Milky Way

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.

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.

Chasing the faint ASCA X-ray sources

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.

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.