* Astronomy

Title: Suzaku Observation of the Anomalous X-ray Pulsar CXOU J164710.2--455216
Authors: Sachindra Naik (1), Tadayasu Dotani (1, 2, 3), Nobuyuki Kawai (3), Motohide Kokubun (1), Takayasu Anada (1), Mikio Morii (4), Tatehiro Mihara (5), Teruaki Enoto (6), Madoka Kawaharada (6), Toshio Murakami (7), Yujin E. Nakagawa (8), Hiromitsu Takahashi (9), Yukikatsu Terada (5), Atsumasa Yoshida (8) ((1) ISAS/JAXA, Japan, (2) Space and Astronautical Science, School of Physical Sciences, The Graduate University for Advanced Studies, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan, (3) Department of Physics, Tokyo Institute of Technology, Tokyo, Japan, (4) Rikkyo University, Tokyo, Japan, (5) RIKEN, Japan, (6) University of Tokyo, Japan, (7) Kanazawa University, Kanazawa, Japan, (8) Aoyama Gakuin University, Sagamihara, Kanagawa, Japan, (9) Hiroshima University, Japan)

Suzaku TOO observation of the anomalous X-ray pulsar CXOU J164710.2-455216 was performed on 2006 September 23--24 for a net exposure of 38.8 ks. During the observation, the XIS was operated in 1/8 window option to achieve a time resolution of 1 s. Pulsations are clearly detected in the XIS light curves with a barycenter corrected pulse period of 10.61063(2) s. The XIS pulse profile shows 3 peaks of different amplitudes with RMS fractional amplitude of ~11% in 0.2--6.0 keV energy band. Though the source was observed with the HXD of Suzaku, the data is highly contaminated by the nearby bright X-ray source GX 340+0 which was in the HXD field of view. The 1-10 keV XIS spectra are well fitted by two blackbody components. The temperatures of two blackbody components are found to be 0.61 ±0.01 keV and 1.22 ±0.06 keV and the value of the absorption column density is 1.73 ±0.03 x 10^{22} atoms cm^{-2}. The observed source flux in 1-10 keV energy range is calculated to be 2.6 x 10^{-11} ergs cm^{-2} s^{-1} with significant contribution from the soft blackbody component (kT = 0.61 keV). Pulse phase resolved spectroscopy of XIS data shows that the flux of the soft blackbody component consists of three narrow peaks, whereas the flux of the other component shows a single peak over the pulse period of the AXP. The blackbody radii changes between 2.2-2.7 km and 0.28-0.38 km (assuming the source distance to be 5 kpc) over pulse phases for the soft and hard components, respectively. The details of the results obtained from the timing and spectral analysis is presented.

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Title: Radio observations of the massive stellar cluster Westerlund 1
Authors: Sean M. Dougherty (NRC HIA, Canada), J. Simon Clark (Open University, UK)

High-dynamic range radio observations of Westerlund 1 are presented that detect a total of 21 stars in the young massive stellar cluster, the richest population of radio emitting stars known for any young massive galactic cluster in the Galaxy. We will discuss some of the more remarkable objects, including the highly radio luminous supergiant B[e] star W9, with an estimated mass-loss rate ~10^{-3} solarmass/yr, comparable to that of eta Carina, along with the somewhat unusual detection of thermal emission from almost all the cool red supergiants and yellow hypergiants. There is strong supporting evidence from X-ray observations that each of the WR stars with radio emission are likely to be colliding-wind binaries

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A very massive star, in the constellation Ara, collapsed to form a neutron star and not a black hole as anticipated, according to new results from the Chandra X-ray Observatory. This discovery shows that nature has a harder time making black holes than previously thought.

Scientists found this neutron star - a dense neutron ball about 12 miles in diameter - in an extremely young star cluster. The neutron star revealed itself through periodic X-ray pulsations, every 10.6 seconds. Astronomers were able to use well-determined properties of other stars in the cluster to deduce that the parent star of this neutron star was about 25 and somewhat less than 40 solar masses mass of the sun.

"Our discovery shows that some of the most massive stars do not collapse to form black holes as predicted, but instead form neutron stars" - Michael Muno, University of California, Los Angeles, postdoctoral Hubble fellow. He is lead author of a paper to be published in an upcoming edition of The Astrophysical Journal Letters.


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Position(2000): RA 16h 47m 05.40s Dec -45º 50' 36.70"

The optical image (left) of Westerlund 1 shows a dense cluster of young stars, several with masses of about 40 suns. Some astronomers speculated that repeated collisions between such massive stars in the cluster might have led to formation of an intermediate-mass black hole, more massive than 100 suns. A search of the cluster with Chandra (right) found no evidence for this type of black hole. Instead they found a neutron star (CXO J164710.2-455216), a discovery which may severely limit the range of stellar masses that lead to the formation of stellar black holes.
Credit: NASA/CXC/UCLA/M.Muno et al.


When very massive stars make neutron stars and not black holes, they will have a greater influence on the composition of future generations of stars. When the star collapses to form the neutron star, more than 95 percent of its mass, much of which is metal-rich material from its core, is returned to the space around it.

"This means that enormous amounts of heavy elements are put back into circulation and can form other stars and planets" - J. Simon Clark of the Open University in the United Kingdom.

Astronomers do not completely understand how massive a star must be to form a black hole rather than a neutron star. The most reliable method for estimating the mass of the parent star is to show that the neutron star or black hole is a member of a cluster of stars, all of which are close to the same age.
Because more massive stars evolve faster than less massive ones, the mass of a star can be estimated if its evolutionary stage is known. Neutron stars and black holes are the end stages in the evolution of a star, so their parent stars must have been among the most massive stars in the cluster.

The work described by Muno was based on two Chandra observations on May 22 and June 18, 2005. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA's Science Mission Directorate. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Centre in Cambridge, Mass.

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