Tag Archives: populations

An ultra-magnetized neutron star awakes

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These days, astronomers all over the world sleep with one eye open, keeping a close watch of of the supermassive black hole located in the center of our Milky Way Galaxy: Sagittarius A* (Sgr A*). A mysterious gas cloud called “G2” is on a collision course with our Galactic nucleus and may produce some fireworks in the near future (read all about it here).

Imagine the excitement when on April 24 (2013), our daily observations performed with Swift’s X-ray Telescope suddenly detected enhanced activity at the position of Sgr A*. An Astronomer’s Telegram was readily distributed to instantly notify the astronomical community. To everybody’s surprise, however, rapid follow-up observations at infrared and radio wavelengths did not detect anything out of the ordinary and in stead suggested the supermassive black hole remained quiet as always.

The mystery was resolved when right next day, Swift’s Burst Alert Telescope detected a very short (less than a second) and energetic burst of gamma-ray emission. Together with the detection of a pulsed X-ray signal using the brand-new high-energy telescope NuSTAR, this revealed that an otherwise dormant neutron star, located very close to the supermassive black hole, had been revived. This neutron star, named SGR J1745-29, has an extremely strong magnetic field and belongs to the rare class of “magnetars”. So far it is only the magnetar that continues to show fireworks, whereas Sgr A* remains as quiet as it has ever been.

Kennea et al. 2013, ApJ Letters 770, L24: Swift Discovery of a New Soft Gamma Repeater, SGR J1745-29, near Sagittarius A*

Paper link: ADS

Press item: Sky&Telescope feature

Artist’s concept of an explosion on the surface of a neutron star. Credit: NASA/Dana Berry.

Artist’s concept of an explosion on the surface of a neutron star.
Credit: NASA/Dana Berry.

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.

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.