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Chandrals First Decade of Discovery SPECIAL FEATURE: INTRODUCTION Chandra’s first decade of discovery Douglas A. Swartza,1, Scott J. Wolkb, and Antonella Fruscioneb aUniversities Space Research Association, National Aeronautics and Space Administration Marshall Space Flight Center, Huntsville, AL 35805; and bHarvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 Edited by Harvey D. Tananbaum, Smithsonian Astrophysical Observatory, Cambridge, MA, and approved February 4, 2010 (received for review December 16, 2009) We review some of the many scientific results reported at a symposium held in September 2009 celebrating the 10th anniversary of National Aeronautics and Space Administration’s (NASA’s) Chandra X-ray Observatory. These results were contributed by scientists who were among the more than 300 symposium participants. We highlight those results that most emphasize the unique imaging and spectroscopic capabilities of Chandra. review | X-rays fter many years in the making regions of cosmic ray acceleration. Inves- Compact Objects (1), the Chandra X-ray Obser- tigators can also take advantage of the A SNR often contain central compact A vatory, one of the National strong broad emission lines from SNR to objects (CCOs). These are thought to be Aeronautics and Space Admin- perform spatially resolved spectroscopy neutron stars formed in the core collapse istration’s Great Observatories, was de- with Chandra and, thereby, map the dis- supernova explosions of massive stars. ployed by the STS-93 crew of the space tribution and measure the mass of the These stars are often just tiny dots em- shuttle Columbia on July 23, 1999. abundant elements from C to Fe in the bedded in the extensive X-ray glow of their Chandra’sofficial first light image was re- ejecta. As explained in detail by invited accompanying remnants. Unless they corded less than a month later on August speaker Carles Badenes (3), this mapping pulse, or are extremely bright, or radiate 19, 1999. The ensuing decade has wit- helps researchers achieve the ultimate at dissimilar energies, CCOs can be dif- nessed a profound revolution in our un- goals of constraining the supernova pro- ficult to detect in young SNRs. derstanding of the X-ray universe as a genitors (particularly of Type Ia events) direct consequence of the subarcsecond and understanding fundamentals of the Neutron Stars. The CCO in the Cas A SNR imaging capabilities afforded by Chandra’s explosion mechanism (particularly for was first identified in the Chandra first unique focusing optics. The symposium core-collapse events). light image and was one of Chandra’s first Chandra’s First Decade of Discovery was Jae-Joon Lee (4) described observations scientific discoveries (7). The spectrum held in Boston from September 22–25, of G292.0+1.8, the remnant of a core- is consistent with a blackbody but the 2009, to celebrate this revolution. collapse supernovae with a pulsar in its inferred luminosity of the object implies Here we review some of the many sci- midst. The Chandra image reveals spec- aradiusof<0.5 km, suggesting the entific results contributed by the more than tacular substructures including a torus, jet, emission is perhaps from a hot spot on 300 symposium participants. We try to and an extended central compact nebula a larger compact object. Hot spots should highlight those results that most emphasize or pulsar wind nebula associated with the produce pulsations as the star rotates the unique imaging and spectroscopic embedded pulsar. but pulsations have not been detected. capabilities of Chandra. The review is A much younger remnant, in fact the The Cas A CCO is now used as a pro- organized in a manner similar to the topical youngest remnant known in our galaxy, was totype of a class of likely neutron stars sessions of the symposium. Each topical recently imaged by using Chandra as with no radio pulsations. It has been session had one invited speaker who reported by Kazimierz Borkowski (5). shown (8) that these could be low mag- reviewed the broader aspects of the topic. Borkowski and his colleagues determined netic field objects with hydrogen atmos- These invited speakers have also made the young age of this remnant by measur- pheres if they had radii of ≈5kmbut written contributions to this volume ing its expansion with time. It was first that would require they be (very small) and are cross-referenced throughout this discovered in radio observations in 1985. quark stars of very low mass. Craig review. When Chandra observed it some 22 years Heinke (9) reported an intriguing alter- Unfortunately, not all of the many ex- later it had expanded by ≈16%, a change native model for the neutron star atmo- cellent contributions can be summarized consistent with an age of only about 140 sphere composed of carbon rather than within this short review. The abstracts years (compared with ≈300 years for Cas H or He. This model fitsthedatawell of all of the contributions along with most A and older ages for the historical SNRs and gives a radius consistent with theo- of the slide presentations and videos of like Tycho and Kepler, whose supernova retical predictions for neutron stars. the oral presentations are available at events were observed as bright “guest Heinke suggests the carbon atmosphere http://cxc.harvard.edu/ChandraDecade/ stars” in the days before telescopes were is a signature of youth in that H and proceedings. invented). Chandra has since observed the He accreted from the remnant can burn remnant again to confirm the expansion to C during the first 1,000 years or so Supernova Remnants and young age. before a thick H layer accumulates at We begin with the subject of Chandra’s first There was also a detailed report on the later times. light image, the supernova remnant (SNR) Chandra monitoring campaign of SN Cassiopeia A (Fig. 1; ref. 2). Galactic 1987A in the Large Magellanic Cloud SNRs like Cas A are rich sources of in- presented by Sangwook Park (6). The SN Author contributions: D.A.S., S.J.W., and A.F. wrote the formation because Chandra can resolve so 1987A remnant has expanded to an an- paper. much of the structure and hence map out gular size of only 1.6 inches during its 22- The authors declare no conflict of interest. forward and reverse shocks, separate year lifetime; Chandra is the only X-ray This article is a PNAS Direct Submission. observatory that can measure radial ex- ejecta from presupernova circumstellar 1To whom correspondence should be addressed. E-mail: and interstellar material, and delineate pansion on such miniscule scales. [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.0914464107 PNAS | April 20, 2010 | vol. 107 | no. 16 | 7127–7134 Downloaded by guest on September 24, 2021 before dispersing on ≈100 Myr timescales. Young clusters in the Milky Way are favorite targets of X-ray study because of the many high energy processes connect- ing stars and their natal environments. Several investigators reported on surveys of young star clusters at the meeting. The first of these was an interim report on a wide field survey of the Eta Carina re- gion led by Leisa Townsley (15). Carina is a starburst cluster, an example of the large-scale star formation seen in starburst galaxies. The Carina complex is com- posed of at least 10 young stellar clusters covering a range of about 0.1–3 Myr in age. Carina contains at least 60 massive O and early-B stars, including an O2 star of initial mass well over 100 M⊙. The lu- minous blue variable η Car and bright diffuse X-rays hint that the Carina com- plex might have seen cavity supernovae. These supernovae are often faint in optical light because they occur within hot tenu- ous bubbles or cavities formed by earlier stellar winds and supernovae. Kenji Ha- maguchi (16, 17) has also confirmed a Fig. 1. Energy-colored photon counts image of the Cassiopeia A supernova remnant observed by neutron star exists in the field, suggesting Chandra. Red (0.5–1.5 keV) green, (1.5–2.5 keV), and blue (4.0–6.0 keV) are individually logarithmically that at least one supernova has already scaled to bring out the fine structure. Red and green are thermal radiation from the ejecta that has occurred in this young star complex. passed through the hot reverse shock. The blue outer filamentary structure is dominated by high-energy The Carina program, which involved synchrotron emission from electrons accelerated at the forward shock. archival data and 20 dedicated pointings, resolved >14,350 point sources, making Compact objects can radiate for periods picture (see ref. 13 for details) that in- it, by far, the largest collection of point of time far in excess of the lifetime of cludes two arcs, inner jets along the di- sources detected for a single Chandra their progenitors—and much longer than rection of proper motion, a faint hook program. Because young stars remain the time it takes for their expanding ahead of the compact PWN, and asym- X-ray bright for a relatively short time, remnants to fade. Thus, compact objects metric diffuse emission. most of the sources are expected to be are found in a wide range of environments, Pavlov reported in the meeting on new premain sequence stars associated with not just within the SNRs in which they observations taken as recently as July the multiple generations of star formation were formed. 2009. The observations show a significant in this region. The evidence for young Isolated neutron stars (INSs) are often change of the compact pulsar wind nebula. stellar ages is that 87% of the X-ray point difficult to identify because they are The system is clearly highly dynamic with sources have counterparts in deep infrared small and hot, making them faint at X-ray-emitting structures varying in images.
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