Q. D. Wang

A CHANDRA/HETGS CENSUS OF X-RAY VARIABILITY FROM Sgr A* DURING 2012

We present the first systematic analysis of the X-ray variability of Sgr A ∗ during the Chandra X-ray Observatory’s 2012 Sgr A ∗ X-ray Visionary Project. With 38 High Energy Transmission Grating Spectrometer observations spaced an average of 7 days apart, this unprecedented campaign enables detailed study of the X-ray emission from this supermassive black hole at high spatial, spectral and timing resolution. In 3 Ms of observations, we detect 39 X-ray flares from Sgr A ∗ , lasting from a few hundred seconds to approximately 8 ks, and ranging in 2–10 keV luminosity from ∼10 34 erg s −1 to 2 × 10 35 erg s −1 . Despite tentative evidence for a gap in the distribution of flare peak count rates, there is no evidence for X-ray color differences between faint and bright flares. Our preliminary X-ray flare luminosity distribution dN/dL is consistent with a power law with index −1.9 +0.3 −0.4 ; this is similar to some estimates of Sgr A ∗ ’s near-IR flux distribution. The observed flares contribute one-third of the total X-ray output of Sgr A ∗ during the campaign, and as much as 10% of the quiescent X-ray emission could be comprised of weak, undetected flares, which may also contribute high-frequency variability. We argue that flares may be the only source of X-ray emission from the inner accretion flow.

CHANDRA/HETGS OBSERVATIONS OF THE BRIGHTEST FLARE SEEN FROM Sgr A*

Starting in 2012, we began an unprecedented observational program focused on the supermassive black hole in the center of our Galaxy, Sgr A*, utilizing the High Energy Transmission Grating Spectrometer (HETGS) instrument on the Chandra X-Ray Observatory. These observations will allow us to measure the quiescent X-ray spectra of Sgr A* for the first time at both high spatial and spectral resolution. The X-ray emission of Sgr A*, however, is known to flare roughly daily by factors of a few to ten times over quiescent emission levels, with rarer flares extending to factors of greater than 100 times quiescence. Here we report an observation performed on 2012 February 9 wherein we detected what are the highest peak flux and fluence flare ever observed from Sgr A*. The flare, which lasted for 5.6 ks and had a decidedly asymmetric profile with a faster decline than rise, achieved a mean absorbed 2-8 keV flux of (8.5 +/- 0.9) x 10(-12) erg cm(-2) s(-1). The peak flux was 2.5 times higher, and the total 2-10 keV emission of the event was approximately 10(39) erg. Only one other flare of comparable magnitude, but shorter duration, has been observed in Sgr A* by XMM-Newton in 2002 October. We perform spectral fits of this Chandra-observed flare and compare our results to the two brightest flares ever observed with XMM-Newton. We find good agreement among the fitted spectral slopes (Gamma similar to 2) and X-ray absorbing columns (N-H similar to 15 x 10(22) cm(-2)) for all three of these events, resolving prior differences (which are most likely due to the combined effects of pileup and spectral modeling) among Chandra and XMM-Newton observations of Sgr A* flares. We also discuss fits to the quiescent spectra of Sgr A*.

Warm-hot gas in and around the Milky Way: Detection and implications of OVII absorption toward LMC X-3

X-ray absorption lines of highly ionized species such as O VII at about zero redshift have been firmly detected in the spectra of several active galactic nuclei. However, the location of the absorbing gas remains a subject of debate. To separate the Galactic and extragalactic contributions to the absorption, we have obtained Chandra LETG-HRC and Far Ultraviolet Spectroscopic Explorer observations of the black hole X-ray binary LMC X-3. We clearly detect the O VII K? absorption line with an equivalent width of 20(14, 26) m??(90% confidence range). The Ne IX K? absorption line is also detected, albeit marginally. A joint analysis of these lines, together with the nondetection of the O VII K? and O VIII K? lines, gives the temperature, velocity dispersion, and hot oxygen column density as 1.3(0.7,1.8) ? 106 K, 79(62,132)?km?s-1, and 1.9(1.2, 3.2) ?1016?cm-2, assuming a collisional ionization equilibrium of the X-ray-absorbing gas and a Galactic interstellar Ne/O number ratio of 0.18. The X-ray data allow us to place a 95% confidence lower limit to the Ne/O ratio as 0.14, but the upper limit is not meaningfully constrained. The O VII line centroid and its relative shift from the Galactic O I K? absorption line, detected in the same observations, are inconsistent with the systemic velocity of LMC X-3 (+310?km?s-1). The far-UV spectrum shows O VI absorption at Galactic velocities, but no O VI absorption is detected at the LMC velocity at greater than 3 ? significance. The measured Galactic O VI column density is higher than the value predicted from the O VII-bearing gas, indicating multiphase absorption. Both the nonthermal broadening and the decreasing scale height with the increasing ionization state further suggest an origin of the highly ionized gas in a supernova-driven galactic fountain. In addition, we estimate the warm and hot electron column densities from our detected O II K? line in the LMC X-3 X-ray spectra and from the dispersion measure of a pulsar in the LMC vicinity. We then infer the O/H ratio of the gas to be 8 ? 10-5, consistent with the chemically enriched galactic fountain scenario. We conclude that the Galactic hot interstellar medium should in general substantially contribute to zero-redshift X-ray absorption lines in extragalactic sources.

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.

X-Ray and Ultraviolet Spectroscopy of Galactic Diffuse Hot Gas Along the Large Magellanic Cloud X-3 Sight Line

We present Suzaku spectra of X-ray emission in the fields just off the LMC X-3 sight line. O VII, O VIII, and Ne IX emission lines are clearly detected, suggesting the presence of an optically thin thermal plasma with an average temperature of 2.4 ? 106 K. This temperature is significantly higher than that inferred from existing X-ray absorption line data obtained with Chandra grating observations of LMC X-3, strongly suggesting that the gas is not isothermal. We then jointly analyze these data to characterize the spatial and temperature distributions of the gas. Assuming a vertical exponential Galactic disk model, we estimate the gas temperature and density at the Galactic plane and their scale heights as 3.6(2.9, 4.7) ? 106 K and 1.4(0.3, 3.4) ? 10?3 cm?3 and 1.4(0.2, 5.2) kpc and 2.8(1.0, 6.4) kpc, respectively. This characterization can account for all the O VI line absorption, as observed in a Far Ultraviolet Spectroscopy Explorer spectrum of LMC X-3, but only predicts less than one-tenth of the O VI line emission intensity typically detected at high Galactic latitudes. The bulk of the O VI emission most likely arises at interfaces between cool and hot gases.

Unusual X-ray transients in the Galactic Centre

We report the discovery in the Galactic Centre region of two hard X-ray sources, designated as XMM J174457 2850.3 and XMM J174544 2913.0,which exhibited flux variations in the 2–10 keV band in excess of a factor of 100 in observations spanning roughly a year. In both cases the observed hydrogen column density is consistent with a location near to the Galactic Centre, implying peak X-ray luminosities of � 5×10 34 erg s 1 . These objects may represent a new population of transient source with very different properties to the much more luminous Galactic Centre transients associated with neutron star and black-hole binary systems. Spectral analysis shows that XMM J174457 2850.3 has relatively weak iron-line emission set against a very hard continuum. XMM J174544 2913.0, on the other hand, has an extremely strong K-line from helium-like iron with an equivalent width of �2.4 keV. The nature of the latter source is of particular interest. Does it represent an entirely new class of object or does it correspond to a known class of source in a very extreme configuration?

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.

Limits on Hot Galactic Halo Gas from X-Ray Absorption Lines

United States. National Aeronautics and Space Administration. (Smithsonian Astrophysical Observatory) (Contract Number SV3- 73016)

A Phase-connected Braking Index Measurement for the Large Magellanic Cloud Pulsar PSR B0540–69

We report the results of an extensive monitoring campaign of PSR B0540-69, the 50 ms pulsar in the Large Magellanic Cloud, using data acquired with the Rossi X-Ray Timing Explorer. A set of 45 pointed X-ray observations spanning 1.2 yr reveals a steady spin-down interrupted by a single glitch event, the first recorded for this young, rotation-powered pulsar. The quality of the data allowed us to perform a pulse-timing analysis that successfully maintains cycle count between observations, after allowing for the observed glitch. The glitch event occurred on MJD 51,325 ± 45 (90% CL error) and is characterized by a change in frequency of Δν/ν = (1.90 ± 0.04) × 10-9 and in frequency derivative of Δ/ = (8.5 ± 0.5) × 10-5. By phase-linking the data set we provide a definitive measurement of the spin frequency of the pulsar and its first and second derivatives. These translate into a braking index of n = 1.81 ± 0.07, significantly lower than reported previously for this source using measurements that could not be phase-linked unambiguously.