Cornelia Chesley Lang

A CATALOG OF X-RAY POINT SOURCES FROM TWO MEGASECONDS OF CHANDRA OBSERVATIONS OF THE GALACTIC CENTER

We present a catalog of 9017 X-ray sources identified in Chandra observations of a 2 ◦ × 0. 8 field around the Galactic center. This enlarges the number of known X-ray sources in the region by a factor of 2.5. The catalog incorporates all of the ACIS-I observations as of 2007 August, which total 2.25 Ms of exposure. At the distance to the Galactic center (8 kpc), we are sensitive to sources with luminosities of 4 × 10 32 erg s −1 (0.5–8.0 keV; 90% confidence) over an area of 1 deg 2 , and up to an order of magnitude more sensitive in the deepest exposure (1.0 Ms) around Sgr A ∗ . The positions of 60% of our sources are accurate to <1 �� (95% confidence), and 20% have positions accurate to < 0. 5. We search for variable sources, and find that 3% exhibit flux variations within an observation, and 10% exhibit variations from observation-to-observation. We also find one source, CXOUGC J174622.7−285218, with a periodic 1745 s signal (1.4% chance probability), which is probably a magnetically accreting cataclysmic variable. We compare the spatial distribution of X-ray sources to a model for the stellar distribution, and find 2.8σ evidence for excesses in the numbers of X-ray sources in the region of recent star formation encompassed by the Arches, Quintuplet, and Galactic center star clusters. These excess sources are also seen in the luminosity distribution of the X-ray sources, which is flatter near the Arches and Quintuplet than elsewhere in the field. These excess point sources, along with a similar longitudinal asymmetry in the distribution of diffuse iron emission that has been reported by other authors, probably have their origin in the young stars that are prominent at l ≈ 0. 1.

A 62 Day X-Ray Periodicity and an X-Ray Flare from the Ultraluminous X-Ray Source in M82

In 240 days of X-ray monitoring of M82, we have discovered an X-ray periodicity at 62.0 ? 2.5 days with a peak-to-peak amplitude corresponding to an isotropic luminosity of 2.4 ? 1040 ergs s-1 in M82 and an X-ray flare reaching a peak luminosity of 9.8 ? 1040 ergs s-1. The periodicity and flare likely originate from the ultraluminous X-ray source (ULX) in M82, which has been identified as a possible intermediate-mass black hole. We suggest that the 62 day modulation is due to orbital motion within an X-ray binary with a Roche lobe overflowing companion star, which would imply that the average density of the companion star is near 5 ? 10-5 g cm-3 and is therefore a giant or supergiant. Chandra observations just after the flare show an energy spectrum that is consistent with a power law with no evidence of a thermal component or line emission. Radio observations made with the VLA during the flare allow us to rule out a blazar identification for the source and place strong constraints on relativistically beamed models of the X-ray emission. The Chandra observations reveal that a second X-ray source reached a flux of 4.4 ? 10-12 ergs cm-2 s-1 in the 0.3-7 keV band, which is dramatically higher than any flux previously seen from this source and corresponds to an isotropic luminosity of 1.1 ? 1040 ergs s-1. This source is a second ultraluminous X-ray source in M82 and may give rise to the QPOs detected from the central region of M82.

A Radio Polarimetric Study of the Galactic Center Threads

The Very Large Array has been used to carry out a multifrequency, polarimetric study of the non-thermal filaments (NTF’s), G0.08+0.15, and G359.96+0.09, also known as the Northern and Southern Threads (Morris & Yusef-Zadeh 1985). These linear structures have been observed at �=20, 6, 3.6 and 2 cm, with high enough spatial resolution to be resolved for the first time at �6 and �3.6 cm. The �20 cm image reveals a wealth of new detail in the radio sources lying within the inner 60 pc of the Galaxy. The Southern Thread has a prominent split along its length, similar to splitting at the ends of previously studied NTF’s. In addition to the prominent Northern and Southern Threads, there are several elongated features that resemble NTF’s, but extend for only 5 7 pc. With resolutions as fine as 2 ′′ , the �3.6 and �6 cm images reveal a high degree of continuity and little substructure internal to the filament. However, the width of the Northern Thread varies along its length between �4 ′′ and �12 ′′ (0.15 0.5 pc), and becomes markedly diffuse at its NW extremity. The spectral index of the Northern Thread has been determined over a broad range of frequencies. Its flux density falls with frequency, (�= 0.5) between �90 and �6 cm (where S� / � � ), and becomes much steeper (�= 2.0) between �6 and �2 cm. The spectral index does not vary significantly along the length of the Northern Thread, which implies either that the diffusion timescale for the emitting electrons is less than their synchrotron lifetime, or that the emitting electrons are reaccelerated continuously at multiple positions along the filament. Because of the lack of spectral index variation, we have not located the source of relativistic electrons. Polarization observations at �6 and �3.6 cm confirm the non-thermal nature of the emission from the Northern Thread. The fractional polarization in the Northern Thread reaches 70% in some regions, although the polarized emission is patchy. Large rotation measures (RM � 2000 rad m −2 ) have been observed with irregular variations across the filament; typical values of RM are 100 2000 rad m −2 . The lack of any apparent pattern in the distribution of RM suggests that the Faraday rotating medium is not physically assocated with the Northern Thread since the filament itself is so highly ordered. The intrinsic magnetic field in the Northern Thread is predominantly aligned along its long axis. The data on the Southern Thread were less conclusive: the Southern Thread was not detected in total intensity at �6 or �3.6 cm, while the polarized emission at �3.6 cm arising from the Southern Thread is evident, with a bifurcated structure similar to that detected in total �20 cm intensity.

The interplay between star formation and the nuclear environment of our Galaxy: deep X-ray observations of the Galactic centre Arches and Quintuplet clusters

The Galactic centre (GC) provides a unique laboratory for a detailed examination of the interplay between massive star formation and the nuclear environment of our Galaxy. Here, we present a 100-ks Chandra Advanced CCD Imaging Spectrometer (ACIS) observation of the Arches and Quintuplet star clusters. We also report on a complementary mapping of the dense molecular gas near the Arches cluster made with the Owens Valley Millimeter Array. We present a catalogue of 244 point-like X-ray sources detected in the observation. Their number-flux relation indicates an overpopulation of relatively bright X-ray sources, which are apparently associated with the clusters. The sources in the core of the Arches and Quintuplet clusters are most likely extreme colliding wind massive star binaries. The diffuse X-ray emission from the core of the Arches cluster has a spectrum showing a 6.7-keV emission line and a surface intensity profile declining steeply with radius, indicating an origin in a cluster wind. In the outer regions near the Arches cluster, the overall diffuse X-ray enhancement demonstrates a bow shock morphology and is prominent in the Fe Kα 6.4-keV line emission with an equivalent width of ∼ 1.4 keV. Much of this enhancement may result from an ongoing collision between the cluster and the adjacent molecular cloud, which have a relative velocity? 120 km -1 . The older and less-compact Quintuplet cluster contains much weaker X-ray sources and diffuse emission, probably originating from low-mass stellar objects as well as a cluster wind. However, the overall population of these objects, constrained by the observed total diffuse X-ray luminosities, is substantially smaller than expected for both clusters, if they have normal Miller & Scalo initial mass functions. This deficiency of low-mass objects may be a manifestation of the unique star formation environment of the GC, where high-velocity cloud-cloud and cloud-cluster collisions are frequent.

ISOLATED WOLF-RAYET STARS AND O SUPERGIANTS IN THE GALACTIC CENTER REGION IDENTIFIED VIA PASCHEN-α EXCESS

We report the discovery of 19 hot, evolved, massive stars near the Galactic center region (GCR). These objects were selected for spectroscopy owing to their detection as strong sources of Paschen-α (Pα) emission-line excess, following a narrowband imaging survey of the central 0°.65 × 0°.25 (l, b) around Sgr A* with the Hubble Space Telescope. Discoveries include six carbon-type (WC) and five nitrogen-type (WN) Wolf-Rayet stars, six O supergiants, and two B supergiants. Two of the O supergiants have X-ray counterparts having properties consistent with solitary O stars and colliding-wind binaries. The infrared photometry of 17 stars is consistent with the Galactic center distance, but 2 of them are located in the foreground. Several WC stars exhibit a relatively large infrared excess, which is possibly thermal emission from hot dust. Most of the stars appear scattered throughout the GCR, with no relation to the three known massive young clusters; several others lie near the Arches and Quintuplet clusters and may have originated within one of these systems. The results of this work bring the total sample of Wolf-Rayet (WR) stars in the GCR to 88. All sources of strong Pα excess have been identified in the area surveyed with HST, which implies that the sample of WN stars in this region is near completion, and is dominated by late (WNL) types. The current WC sample, although probably not complete, is almost exclusively dominated by late (WCL) types. The observed WR subtype distribution in the GCR is a reflection of the intrinsic rarity of early subtypes (WNE and WCE) in the inner Galaxy, an effect that is driven by metallicity.

HST/NICMOS Paschen-α Survey of the Galactic Centre: Overview

We have recently carried out the first wide-field hydrogen Paschen-α line imaging survey of the Galactic Centre using the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument aboard the Hubble Space Telescope. The survey maps out a region of 2253 pc^2 (416 arcmin^2) around the central supermassive black hole (Sgr A*) in the 1.87 and 1.90 μm narrow bands with a spatial resolution of ~0.01 pc (0.2 arcsec full width at half-maximum) at a distance of 8 kpc. Here, we present an overview of the observations, data reduction, preliminary results and potential scientific implications, as well as a description of the rationale and design of the survey. We have produced mosaic maps of the Paschen-α line and continuum emission, giving an unprecedentedly high-resolution and high-sensitivity panoramic view of stars and photoionized gas in the nuclear environment of the Galaxy. We detect a significant number of previously undetected stars with Paschen-α in emission. They are most likely massive stars with strong winds, as confirmed by our initial follow-up spectroscopic observations. About half of the newly detected massive stars are found outside the known clusters (Arches, Quintuplet and Central). Many previously known diffuse thermal features are now resolved into arrays of intriguingly fine linear filaments indicating a profound role of magnetic fields in sculpting the gas. The bright spiral-like Paschen-α emission around Sgr A* is seen to be well confined within the known dusty torus. In the directions roughly perpendicular to it, we further detect faint, diffuse Paschen-α emission features, which, like earlier radio images, suggest an outflow from the structure. In addition, we detect various compact Paschen-α nebulae, probably tracing the accretion and/or ejection of stars at various evolutionary stages. Multiwavelength comparisons together with follow-up observations are helping us to address such questions as where and how massive stars form, how stellar clusters are disrupted, how massive stars shape and heat the surrounding medium, how various phases of this medium are interspersed and how the supermassive black hole interacts with its environment.

Very Large Array Detection of the Ionized Stellar Winds Arising from Massive Stars in the Galactic Center Arches Cluster

The Galactic center Arches stellar cluster, detected and studied until now only in the near-infrared, is comprised of at least 100 massive (M ≥ 20 M☉) stars. Here we report the detection at centimeter wavelengths of radio continuum emission from eight radio sources associated with the cluster. Seven of these radio sources have rising spectral indices between 4.9 and 8.5 GHz and coincide spatially with the brightest stars in the cluster, as determined from JHK photometry and Brα and Brγ spectroscopy. Our results confirm the presence of powerful ionized winds in these stars. The eighth radio source has a nonthermal spectrum, and its nature is yet unclear, but it could be associated with a lower mass young star in the cluster.

A radio transient 0.1 parsecs from sagittarius A

We report the discovery of a transient radio source 27 (0.1 pc projected distance) south of the Galactic center massive black hole, Sgr A*. The source flared with a peak of at least 80 mJy in 2004 March. The source was resolved by the Very Large Array into two components with a separation of ~07 and characteristic sizes of ~02. The two components of the source faded with a power-law index of 1.1 ± 0.1. We detect an upper limit to the proper motion of the eastern component of ~3 × 103 km s-1 relative to Sgr A*. We detect a proper motion of ~104 km s-1 for the western component relative to Sgr A*. The transient was also detected at X-ray wavelengths with the Chandra X-Ray Observatory and XMM-Newton and given the designation CXOGC J174540.0-290031. The X-ray source falls in between the two radio components. The maximum luminosity of the X-ray source is ~1036 ergs s-1, significantly sub-Eddington. The radio jet flux density predicted by the X-ray/radio correlation for X-ray binaries is orders of magnitude less than the measured flux density. We conclude that the radio transient is the result of a bipolar jet originating in a single impulsive event from the X-ray source and interacting with the dense interstellar medium of the Galactic center.

Stellar Winds and Embedded Star Formation in the Galactic Center Quintuplet and Arches Clusters: Multifrequency Radio Observations

A multifrequency, multiconfiguration study has been made of the compact radio sources in the Galactic center Quintuplet and Arches stellar clusters using the Very Large Array. Ten radio sources have been detected in the Quintuplet cluster. The majority of these radio sources have rising spectral indices and are positionally coincident with young massive stars that are known to have powerful stellar winds. We conclude that the three most compact of these sources are produced by stellar wind emission; thus, mass-loss rates can be derived and have an average value of 3 × 10-5 M⊙ yr-1. The remainder of the sources are likely to be a combination of stellar wind emission and free-free emission from surrounding ionized gas. In three cases, the radio sources have no stellar counterpart, and the radio emission is thought to arise from compact or ultracompact H II regions. If so, these sources would be the first detections of embedded massive stars to be discovered in the Galactic center clusters. The radio nebula associated with the Pistol star resembles the nebula surrounding the luminous blue variable star η Car and may be related to the stellar wind of the Pistol star. Ten compact radio sources are also detected in the Arches cluster and are interpreted to be stellar wind sources, consistent with previous findings. Several of the sources show moderate variability (10%–30%) in their flux density, possibly related to a nonthermal component in the wind emission. A number of radio sources in both clusters have X-ray counterparts, which have been interpreted to be the shocked, colliding winds of massive binary systems.

HST Paschen-alpha Survey of the Galactic Center: Data Reduction and Products

Our Hubble Space Telescope/Near-Infrared Camera and Multi-Object Spectrometer (HST/NICMOS) Paschen α survey of the Galactic Centre, first introduced by Wang et al., provides a uniform, panoramic, high-resolution map of stars and an ionized diffuse gas in the central 416 arcmin^2 of the Galaxy. This survey was carried out with 144 HST orbits using two narrow-band filters at 1.87 and 1.90 μm in NICMOS Camera 3. In this paper, we describe in detail the data reduction and mosaicking procedures followed, including background level matching and astrometric corrections. We have detected ~570 000 near-infrared (near-IR) sources using the ‘starfinder’ software and are able to quantify photometric uncertainties of the detections. The source detection limit varies across the survey field, but the typical 50 per cent completion limit is ~17th magnitude (Vega system) in the 1.90 μm band. A comparison with the expected stellar magnitude distribution shows that these sources are primarily main-sequence massive stars (≳7 M_⊙) and evolved lower mass stars at the distance of the Galactic Centre. In particular, the observed source magnitude distribution exhibits a prominent peak, which could represent the red clump (RC) stars within the Galactic Centre. The observed magnitude and colour of these RC stars support a steep extinction curve in the near-IR towards the Galactic Centre. The flux ratios of our detected sources in the two bands also allow for an adaptive and statistical estimate of extinction across the field. With the subtraction of the extinction-corrected continuum, we construct a net Paschen α emission map and identify a set of Paschen α emitting sources, which should mostly be evolved massive stars with strong stellar winds. The majority of the identified Paschen α point sources are located within the three known massive Galactic Centre stellar clusters. However, a significant fraction of our Paschen α emitting sources are located outside the clusters and may represent a new class of ‘field’ massive stars, many of which may have formed in isolation and/or in small groups. The maps and source catalogues presented here are available electronically.