arXiv subjects

G. P. Garmire

Publications and source records attributed to G. P. Garmire.

At least 55 records · Page 3Linked to original sources

arXiv2006

A Chandra/ACIS Study of 30 Doradus II. X-ray Point Sources in the Massive Star Cluster R136 and Beyond

We have studied the X-ray point source population of the 30 Doradus star-forming complex in the Large Magellanic Cloud using high-spatial-resolution X-ray images and spatially-resolved spectra obtained with the Advanced CCD Imaging Spectrometer (ACIS) aboard the Chandra X-ray Observatory. Here we describe the X-ray sources in a 17' x 17' field centered on R136, the massive star cluster at the center of the main 30 Dor nebula. We detect 20 of the 32 Wolf-Rayet stars in the ACIS field. R136 is resolved at the subarcsecond level into almost 100 X-ray sources, including many typical O3--O5 stars as well as a few bright X-ray sources previously reported. Over two orders of magnitude of scatter in L_X is seen among R136 O stars, suggesting that X-ray emission in the most massive stars depends critically on the details of wind properties and binarity of each system, rather than reflecting the widely-reported characteristic value L_X/L_bol ~ 10^{-7}. Such a canonical ratio may exist for single massive stars in R136, but our data are too shallow to confirm this relationship. Through this and future X-ray studies of 30 Doradus, the complete life cycle of a massive stellar cluster can be revealed.

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arXiv2006

A Chandra Snapshot Survey of Representative High-Redshift Radio-Loud Quasars from the Parkes-MIT-NRAO Sample

We present the results of Chandra ACIS-S snapshot observations of six radio-loud quasars (RLQs) at z=3.5-4.7. These observations sample luminous RLQs with moderate-to-high radio-loudness (R=200-9600) and aim to connect the X-ray properties of radio-quiet quasars (R<10) and highly radio-loud blazars (R>1000) at high redshift. This work extends a study by Bassett et al. (2004) which used similar methods to examine z > 4 RLQs with moderate radio-loudness (R=40-400). All of our targets are clearly detected. A search for extended X-ray emission associated with kpc-scale radio jets revealed only limited evidence for X-ray extension in our sample: three sources showed no evidence of X-ray extension, and the other three had 3-30% of their total X-ray fluxes extended >1 arcsec away from their X-ray cores. Additionally, we do not observe any systematic flattening of the optical-to-X-ray spectral index (alpha_{ox}) compared to low-redshift quasars. These results suggest that kpc-scale X-ray jet emission is not dominated by inverse-Compton scattering of CMB-seed photons off jet electrons. We measured X-ray continuum shapes and performed individual and joint spectral fits of our data combined with eight archival RLQs. A single power-law model acceptably fit the data. We added an intrinsic absorption component to our model, and neither the moderate-R nor the high-R fits set a lower bound on N_H. Our spectral results suggest that intrinsic absorption does not strongly depend on radio-loudness, and high-R sources have flatter power laws than moderate-R sources. Overall, our high-redshift RLQs have basic X-ray properties consistent with similar RLQs in the local universe.

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arXiv2005

The Flare Activity of SgrA*; New Coordinated mm to X-Ray Observations

We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the SgrA* counterpart associated with the massive 3-4x10**6 solar mass black hole at the Galactic Center. The main aim is to investigate the physical processes responsible for the variable emission from SgrA*. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA on Mauna Kea, Hawaii, and the Very Large Array in New Mexico. We detected one moderately bright flare event in the X-ray domain and 5 events at infrared wavelengths.

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arXiv2005

51 Eri and GJ 3305: A 10-15 Myr old binary star system at 30 parsecs

Following the suggestion of Zuckerman et al. (2001, ApJ, 562, L87), we consider the evidence that 51 Eri (spectral type F0) and GJ 3305 (M0), historically classified as unrelated main sequence stars in the solar neighborhood, are instead a wide physical binary system and members of the young beta Pic moving group (BPMG). The BPMG is the nearest (d < 50 pc) of several groups of young stars with ages around 10 Myr that are kinematically convergent with the Oph-Sco-Cen Association (OSCA), the nearest OB star association. Combining SAAO optical photometry, Hobby-Eberly Telescope high-resolution spectroscopy, Chandra X-ray data, and UCAC2 catalog kinematics, we confirm with high confidence that the system is indeed extremely young. GJ 3305 itself exhibits very strong magnetic activity but has rapidly depleted most of its lithium. The 51 Eri/GJ 3305 system is the westernmost known member of the OSCA, lying 110 pc from the main subgroups. The system is similar to the BPMG wide binary HD 172555/CD -64d1208 and the HD 104237 quintet, suggesting that dynamically fragile multiple systems can survive the turbulent environments of their natal giant molecular cloud complexes, while still being imparted high dispersion velocities. Nearby young systems such as these are excellent targets for evolved circumstellar disk and planetary studies, having stellar ages comparable to that of the late phases of planet formation.

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arXiv2005

Multi-wavelength analysis of the field of the dark burst GRB 031220

We have collected and analyzed data taken in different spectral bands (from X-ray to optical and infrared) of the field of GRB031220 and we present results of such multiband observations. Comparison between images taken at different epochs in the same filters did not reveal any strong variable source in the field of this burst. X-ray analysis shows that only two of the seven Chandra sources have a significant flux decrease and seem to be the most likely afterglow candidates. Both sources do not show the typical values of the R-K colour but they appear to be redder. However, only one source has an X-ray decay index (1.3 +/- 0.1) that is typical for observed afterglows. We assume that this source is the best afterglow candidate and we estimate a redshift of 1.90 +/- 0.30. Photometric analysis and redshift estimation for this object suggest that this GRB can be classified as a Dark Burst and that the obscuration is the result of dust extinction in the circum burst medium or inside the host galaxy.

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arXiv2005

X-ray Emission from the nearby PSR B1133+16

We detected a nearby (d=360 pc), old (5 Myr) pulsar B1133+16 with Chandra. The observed pulsar's flux is $(0.8\pm 0.2)\times 10^{-14}$ ergs cm$^{-2}$ s$^{-1}$ in the 0.5-8 keV band. Because of the small number of counts detected, the spectrum can be described by various models. A power-law fit of the spectrum gives a photon index $Γ= 2.5$ and an isotropic luminosity of $1.4\times 10^{29}$ ergs s$^{-1}$ in the 0.5-8 keV band, which is about $1.6\times 10^{-3}$ of the spin-down power $\dot{E}$. The spectrum can also be fitted by a blackbody model with a temperature of $\approx 2.8$ MK and a projected emitting area of $\sim 500$ m$^2$, possibly a hot polar cap. The X-ray properties of PSR B1133+16 are similar to those of other old pulsars observed in X-rays, particularly the drifting pulsar B0943+10.

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arXiv2005

The Extended Chandra Deep Field-South Survey. Chandra Point-Source Catalogs

We present Chandra point-source catalogs for the Extended Chandra Deep Field-South (E-CDF-S) survey. The E-CDF-S consists of four contiguous 250 ks Chandra observations covering an approximately square region of total solid angle ~0.3 deg^2, which flank the existing ~1 Ms Chandra Deep Field-South (CDF-S). The survey reaches sensitivity limits of 1.1 X 10^-16 erg/cm^2/s and 6.7 X 10^-16 erg/cm^2/s for the 0.5-2.0 keV and 2-8 keV bands, respectively. We detect 762 distinct X-ray point sources within the E-CDF-S exposure; 589 of these sources are new (i.e., not previously detected in the ~1 Ms CDF-S). This brings the total number of X-ray point sources detected in the E-CDF-S region to 915 (via the E-CDF-S and ~1 Ms CDF-S observations). Source positions are determined using matched-filter and centroiding techniques; the median positional uncertainty is ~0.35". The basic X-ray and optical properties of these sources indicate a variety of source types, although absorbed active galactic nuclei (AGNs) seem to dominate. In addition to our main Chandra catalog, we constructed a supplementary source catalog containing 33 lower significance X-ray point sources that have bright optical counterparts (R<23). These sources generally have X-ray-to-optical flux ratios expected for normal and starburst galaxies, which lack a strong AGN component. We present basic number-count results for our main Chandra catalog and find good agreement with the ~1 Ms CDF-S for sources with 0.5-2.0 keV and 2-8 keV fluxes greater than 3 X 10^-16 erg/cm^2/s and 1 X 10^-15 erg/cm^2/s, respectively. Furthermore, three extended sources are detected in the 0.5-2.0 keV band, which are found to be likely associated with galaxy groups or poor clusters at z ~ 0.1-0.7; these have typical rest-frame 0.5-2.0 keV luminosities of (1-5) X 10^42 erg/s.

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arXiv2005

Parsec-scale X-ray Flows in High-mass Star-forming Regions

The Chandra X-ray Observatory is providing remarkable new views of massive star-forming regions, revealing all stages in the life cycle of high-mass stars and their effects on their surroundings. We present a Chandra tour of several high-mass star-forming regions, highlighting physical processes that characterize the life of a cluster of high-mass stars, from deeply-embedded cores too young to have established an HII region to superbubbles so large that they shape our views of galaxies. Along the way we see that X-ray observations reveal hundreds of stellar sources powering great HII region complexes, suffused by both hard and soft diffuse X-ray structures caused by fast O-star winds thermalized in wind-wind collisions or by termination shocks against the surrounding media. Finally, we examine the effects of the deaths of high-mass stars that remained close to their birthplaces, exploding as supernovae within the superbubbles that these clusters created. We present new X-ray results on W51 IRS2E and 30 Doradus and we introduce new data on Trumpler 14 in Carina and the W3 HII region complexes W3 Main and W3(OH).

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arXiv2005

COUP census of X-ray stars in BN-KL and OMC-1S

We present a study of the X-ray sources detected in the vicinity of two density peaks in the Orion Molecular Cloud 1 (OMC-1) behind the Orion Nebula Cluster (ONC), as seen in the exceptionally deep (~10 days) exposure of the Chandra Orion Ultradeep Project (COUP). We focus on a 40"*50" region around the Becklin-Neugebauer object and Kleinmann-Low nebula (collectively BN-KL) and a 60"*75" region around OMC-1S, a secondary star-forming peak some 90" south of BN-KL. Forty-three and sixty X-ray sources were detected in BN-KL and OMC-1S, respectively, of which half and one-third, respectively, were found to be foreground members of the ONC, while the remaining sources are identified as obscured X-ray sources with column densities 22<log(NH)/cm^{-2})<24. All but 5 and 18 of these sources have near-infrared stellar counterparts, and 22 of these appear to be young stellar objects (YSOs). X-ray sources are found close to four luminous mid-IR sources namely BN, IRc3-i2, IRc2-C, and Source n; their X-ray variability and spectral properties are typical of coronal activity in low-mass stars rather than wind emission from massive stars, suggesting that the X-ray emission may be arising from companions. The X-ray light curve of the X-ray source close to BN shows a periodicity of ~8.3 days and from an X-ray image deconvolution of the region around BN, we conclude that either BN itself or a low mass companion with a projected separation of ~200 AU was detected. On the other hand, no emission is seen from the bright radio Source I, held by some to be the main source of luminosity in BN-KL. In OMC-1S, Chandra unveils a new subcluster of seven YSOs without infrared counterparts (abridged).

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arXiv2005

A Remarkable Low-Mass X-ray Binary within 0.1 pc of the Galactic Center

Recent X-ray and radio observations have identified a transient low-mass X-ray binary (LMXB) located only 0.1 pc in projection from the Galactic center, CXOGC J174540.0-290031. In this paper, we report the detailed analysis of X-ray and infrared observations of the transient and its surroundings. Chandra bservations detect the source at a flux of F_X = 2e-12 erg cm^-2 s^-1 (2-8 keV). After accounting for absorption both in the interstellar medium and in material local to the source, the implied luminosity of the source is only L_X = 4e34 erg/s (2-8 keV; D=8 kpc). However, the diffuse X-ray emission near the source also brightened by a factor of 2. The enhanced diffuse X-ray emission lies on top of a known ridge of dust and ionized gas that is visible infrared images. We interpret the X-ray emission as scattered flux from the outburst, and determine that the peak luminosity of CXOGC J174540.0-290031 was >2e36 erg/s. We suggest that the relatively small observed flux results from the fact that the system is observed nearly edge-on, so that the accretion disk intercepts most of the flux emitted along our line of sight. We compare the inferred peak X-ray luminosity to that of the radio jet. The ratio of the X-ray to radio luminosities, L_X/L_R<1e4, is considerably smaller than in other known LMXBs (> 1e5). This is probably because the jets are radiating with unusually high efficiency at the point where they impact the surrounding interstellar medium. This hypothesis is supported by a comparison with mid-infrared images of the surrounding dust. Finally, we find that the minimum power required to produce the jet, L_jet~1e37 erg/s, is comparable to the inferred peak X-ray luminosity. This is the most direct evidence yet obtained that LMXBs accreting at low rates release about half of their energy as jets.

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arXiv2005

High Resolution Gratings Spectroscopy of GRBs 030382 and 041006 with Chandra LETGS

We present high resolution X-ray spectroscopy of two recent GRB afterglows observed with the Low Energy Transmission Gratings on Chandra. The afterglows to GRBs 030328 and 041006 are detected beginning 15.33 and 16.8 hours after each burst, respectively, and are observed to fade in time during each ~90 ksec observation. We fit for the continuum emission in each full data set and for the data sliced into half and quarter time sections. For both afterglows, the continuum emission is well described by an absorbed power-law model, and the model parameters describing the absorption and spectral slope do not appear to evolve in time. We perform a careful search for deviations from the model continua for the full and time-sliced data and find no evidence for significant (>~3sigma) narrow emission/absorption lines or edges. The lack of detections implies that line emission--if it is a general feature in GRB X-ray afterglows--occurs early (t~0.3 days in the source frame) and/or is short-lived (dt<~10 ksec). We also comment on synchrotron fireball models for the X-ray and optical data.

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arXiv2004

Chandra Orion Ultradeep Project: Observations and Source Lists

We present a description of the data reduction methods and the derived catalog of more than 1600 X-ray point sources from the exceptionally deep January 2003 Chandra X-ray Observatory observation of the Orion Nebula Cluster and embedded populations around OMC-1. The observation was obtained with Chandra's Advanced CCD Imaging Spectrometer (ACIS) and has been nicknamed the Chandra Orion Ultradeep Project (COUP). With an 838 ks exposure made over a continuous period of 13.2 days, the COUP observation provides the most uniform and comprehensive dataset on the X-ray emission of normal stars ever obtained in the history of X-ray astronomy.

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arXiv2004

The Fall of AGN and the Rise of Star-Forming Galaxies: A Close Look at the Chandra Deep Field X-ray Number Counts

We investigate the X-ray number counts in the 1-2 Ms Chandra Deep Fields (CDFs) to determine the contributions of faint X-ray source populations to the extragalactic X-ray background (XRB). X-ray sources were separated into Active Galactic Nuclei (AGN), star-forming galaxies, and Galactic stars based on several criteria. We find that AGN continue to dominate the number counts in the 0.5-2.0 keV and 2-8 keV bands. At flux limits of ~2.5e-17 erg cm-2 s-1 (0.5-2.0 keV) and ~1.4e-16 erg cm-2 s-1 (2-8 keV), the overall AGN source densities are 7166 (+304/-292) and 4558 (+216/-207) sources deg-2, respectively; these are factors of ~10-20 higher than found in the deepest optical spectroscopic surveys. While still a minority, the number counts of star-forming galaxies climb steeply such that they eventually achieve source densities of 1727 (+187/-169) and 711 (+270/-202) sources deg-2 at the CDF 0.5-2.0 keV and 2-8 keV flux limits, respectively. Adopting recent XRB flux densities measurements, the CDFs resolve a total of 89.5% (+5.9%/-5.7%) and 86.9% (+6.6%/-6.3%) of the extragalactic 0.5-2.0 keV and 2-8 keV XRBs, respectively. Extrapolation of the number-count slopes can easily account for the entire 0.5-2.0 keV and 2-8 keV XRBs to within statistical errors. We also revisit the reported differences between the CDF-North and CDF-South number counts, finding that the two fields are consistent except for sources in the 2-8 keV band below F(2-8 keV)~1e-15 erg cm-2 s-1, where deviations gradually increase to ~3.9 sigma.

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arXiv2004

The Spectra and Variability of X-ray Sources in a Deep Chandra Observation of the Galactic Center

We examine the X-ray spectra and variability of the sample of X-ray sources with L_X = 10^{31}-10^{33} erg s^{-1} identified within the inner 9' of the Galaxy. Very few of the sources exhibit intra-day or inter-month variations. We find that the spectra of the point sources near the Galactic center are very hard between 2--8 keV, even after accounting for absorption. When modeled as power laws the median photon index is Gamma=0.7, while when modeled as thermal plasma we can only obtain lower limits to the temperature of kT>8 keV. The combined spectra of the point sources is similarly hard, with a photon index of Gamma=0.8. Strong line emission is observed from low-ionization, He-like, and H-like Fe, both in the average spectra and in the brightest individual sources. The line ratios of the highly-ionized Fe in the average spectra are consistent with emission from a plasma in thermal equilibrium. This line emission is observed whether average spectra are examined as a function of the count rate from the source, or as a function of the hardness ratios of individual sources. This suggests that the hardness of the spectra may in fact to due local absorption that partially-covers the X-ray emitting regions in the Galactic center systems. We suggest that most of these sources are intermediate polars, which (1) often exhibit hard spectra with prominent Fe lines, (2) rarely exhibit either flares on short time scales or changes in their mean X-ray flux on long time scales, and (3) are the most numerous hard X-ray sources with comparable luminosities in the Galaxy.

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arXiv2004

First Simultaneous NIR/X-ray Detection of a Flare from SgrA*

We report on the first simultaneous near-infrared/X-ray detection of the Sgr A* counterpart which is associated with the massive black hole at the center of the Milky Way. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the ACIS-I instrument aboard the Chandra X-ray Observatory. We also report on quasi-simultaneous observations at a wavelength of 3.4 mm using the Berkeley-Illinois-Maryland Association (BIMA) array. A flare was detected in the X-domain with an excess 2-8 keV luminosity of about 6$\times10^{33}$ erg/s. A fading flare of Sgr A* with $>$2 times the interim-quiescent flux was also detected at the beginning of the NIR observations, that overlapped with the fading part of the X-ray flare. Compared to 8-9 hours before the NIR/X-ray flare we detected a marginally significant increase in the millimeter flux density of Sgr A* during measurements about 7-9 hours afterwards. We find that the flaring state can be conveniently explained with a synchrotron self-Compton model involving up-scattered sub-millimeter photons from a compact source component, possibly with modest bulk relativistic motion. The size of that component is assumed to be of the order of a few times the Schwarzschild radius. The overall spectral indices $α_{NIR/X-ray}$ ($S_ν$$\propto$$ν^{-α}$) of both states are quite comparable with a value of $\sim$1.3. Since the interim-quiescent X-ray emission is spatially extended, the spectral index for the interim-quiescent state is probably only a lower limit for the compact source Sgr A*. A conservative estimate of the upper limit of the time lag between the ends of the NIR and X-ray flare is of the order of 15 minutes.

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arXiv2004

Diffuse X-ray Emission in a Deep Chandra Image of the Galactic Center

We examine the spectrum of diffuse emission detected in the 17' by 17' field around Sgr A* during 625 ks of Chandra observations. The spectrum exhibits He-like and H-like lines from Si, S, Ar, Ca, and Fe, that are consistent with originating in a two-temperature plasma, as well as a prominent low-ionization Fe line. The cooler, kT=0.8 keV plasma differs in surface brightness across the image by a factor of 9. This soft plasma is probably heated by supernovae. The radiative cooling rate of the plasma within the inner 20 pc of the Galaxy could be balanced by 1% of the kinetic energy of one supernova every 300,000 y. The hotter, kT=8 keV component is more spatially uniform, ranging over a factor of 2 in surface brightness. The intensity of the hard plasma is correlated with that of the soft, but they are probably only indirectly related, because supernova remnants are not observed to produce thermal plasma hotter than kT=3 keV. Moreover, a kT=8 keV plasma is too hot to be bound to the Galactic center, and therefore would form a slow wind or fountain of plasma. The energy required to sustain such a freely-expanding plasma within the inner 20 pc of the Galaxy is ~10^40 erg/s, which corresponds to the entire kinetic energy of one supernova every 3000 y. This rate is unreasonably high. However, alternative explanations for the kT=8 keV diffuse emission are equally unsatisfying. We are left to conclude that either the diffuse emission is heated by an unanticipated source of energy, or that a population of faint (< 10^31 erg/s), hard X-ray sources that are a factor of 10 more numerous than CVs remains to be discovered. (Abridged)

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arXiv2004

AC 114: a cluster with a soft X-ray tail

We present Chandra observations of the galaxy cluster AC114, which shows a strongly irregular morphology, with signs of multiple merging activity. We report the discovery of a soft X-ray filament originating close to the core of the cluster. We confirm that X-ray emission is associated with two of three mass concentrations identified in previous gravitational lensing studies of this object. These two mass concentrations are located at opposite ends of the soft filament, evidence for interaction between them. In the northern part, the cluster shows two sharp discontinuities, both in surface brightness and in temperature, evincing another, more recent merger event which took place in, or close, to the cluster core. In spite of the merger activity, a combined mass and lensing analysis shows remarkably good agreement between lensing and X-ray masses. We therefore advocate for the robustness of the X-ray mass estimates, and conclude that the assumption of hydrostatic equilibrium can yield accurate mass estimates even in clusters as dynamically active as AC 114, once the gas density distribution is properly mapped.

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arXiv2004

The Chandra Deep Field-North Survey: XVII. Evolution of magnetic activity in old late-type stars

The extremely sensitive Chandra Deep Field-North (CDF-N) pencil-beam X-ray survey is used to identify and characterize the X-ray emission from old high-latitude main sequence Galactic stars. Our principal goal is to investigate the expected long-term decay of magnetic activity of late-type stars due to the gradual spindown of stellar rotation from a magnetized stellar wind. Eleven X-ray sources constitute a well-defined sample of 2 G, 2 K0-K4, and 7 M2-M5 stars with median distance around 300 pc. X-ray luminosities are typically log Lx ~ 27 erg/s and is dominated by flares rather than quiescent coronal emission. Models of the population indicates that the CDF-N stars are the most magnetically active old disk stars. A substantial decline in X-ray luminosities over the 1<t<11 Gyr age interval is required. This is the first demonstration that the coronal and flaring components of stellar magnetic activity -- and presumably the interior magnetic dynamos responsible for the reconnecting fields at the stellar surface -- exhibit long-term decay over the age of the Galactic disk. The model that best fits the magnitudes, spectral types and X-ray luminosities of the sample has Lx ~ 1/t^2 erg/s which is faster than the 1/t decay rate predicted from widely accepted rotational spindown rates and X-ray-activity relations.

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