14 research outputs found
Simulating CCDs for the Chandra Advanced CCD Imaging Spectrometer
We have implemented a Monte Carlo algorithm to model and predict the response
of various kinds of CCDs to X-ray photons and minimally-ionizing particles and
have applied this model to the CCDs in the Chandra X-ray Observatory's Advanced
CCD Imaging Spectrometer. This algorithm draws on empirical results and
predicts the response of all basic types of X-ray CCD devices. It relies on new
solutions of the diffusion equation, including recombination, to predict the
radial charge cloud distribution in field-free regions of CCDs. By adjusting
the size of the charge clouds, we can reproduce the event grade distribution
seen in calibration data. Using a model of the channel stops developed here and
an insightful treatment of the insulating layer under the gate structure
developed at MIT, we are able to reproduce all notable features in ACIS
calibration spectra.
The simulator is used to reproduce ground and flight calibration data from
ACIS, thus confirming its fidelity. It can then be used for a variety of
calibration tasks, such as generating spectral response matrices for spectral
fitting of astrophysical sources, quantum efficiency estimation, and modeling
of photon pile-up.Comment: 42 pages, 22 figures; accepted for publication in Nuclear Instruments
and Methods in Physics Research, Section A; paper with high-quality figures
can be found at ftp://ftp.astro.psu.edu/pub/townsley/simulator.p
The 155-day X-ray cycle of the very massive Wolf-Rayet star Melnick 34 in the Large Magellanic Cloud
The Wolf–Rayet star Mk 34 was observed more than 50 times as part of the deep T-ReX Chandra ACIS-I X-ray imaging survey of the Tarantula Nebula in the Large Magellanic Cloud conducted between 2014 May and 2016 January. Its brightness showed one bright maximum and repeated faint minima which help define an X-ray recurrence time of 155.1 ± 0.1  d that is probably the orbital period of an eccentric binary system. The maximum immediately precedes the minimum in the folded X-ray light curve as confirmed by new Swift XRT observations. Notwithstanding its extreme median luminosity of 1.2 × 1035 erg s−1, which makes it over an order of magnitude brighter than comparable stars in the Milky Way, Mk 34 is almost certainly a colliding-wind binary system. Its spectrum shows phase-related changes of luminosity and absorption that are probably related to the orbital dynamics of two of the most massive stars known
X-ray Spectroscopy and Variability of AGN Detected in the 2 Ms Chandra Deep Field-North Survey
We investigate the nature of the faint X-ray source population through X-ray
spectroscopy and variability analyses of 136 AGN detected in the 2 Ms Chandra
Deep Field-North survey with > 200 background-subtracted 0.5-8.0 keV counts
[F(0.5-8.0 keV)=(1.4-200)e-15 erg cm^{-2} s^{-1}]. Our preliminary spectral
analyses yield median spectral parameters of Gamma=1.61 and intrinsic
N_H=6.2e21 cm^{-2} (z=1 assumed when no redshift available) when the AGN
spectra are fitted with a simple absorbed power-law model. However,
considerable spectral complexity is apparent (e.g., reflection, partial
covering) and must be taken into account to model the data accurately.
Moreover, the choice of spectral model (i.e., free vs. fixed photon index) has
a pronounced effect on the derived N_H distribution and, to a lesser extent,
the X-ray luminosity distribution. Ten of the 136 AGN (~7%) show significant Fe
Kalpha emission-line features with equivalent widths in the range 0.1-1.3 keV.
Two of these emission-line AGN could potentially be Compton thick (i.e., Gamma
< 1.0 and large Fe Kalpha equivalent width). Finally, we find that 81 (~60%) of
the 136 AGN show signs of variability, and that this fraction increases
significantly (~80-90%) when better photon statistics are available.Comment: Submitted to Advances in Space Research for New X-ray Results from
Clusters of Galaxies and Black Holes (Oct 2002; Houston, TX), eds. C. Done,
E.M. Puchnarewicz, M.J. Ward. Requires cospar.sty (6 pgs, 10 figs
Multiwavelength Studies of Young OB Associations
We discuss how contemporary multiwavelength observations of young
OB-dominated clusters address long-standing astrophysical questions: Do
clusters form rapidly or slowly with an age spread? When do clusters expand and
disperse to constitute the field star population? Do rich clusters form by
amalgamation of smaller subclusters? What is the pattern and duration of
cluster formation in massive star forming regions (MSFRs)? Past observational
difficulties in obtaining good stellar censuses of MSFRs have been alleviated
in recent studies that combine X-ray and infrared surveys to obtain rich,
though still incomplete, censuses of young stars in MSFRs. We describe here one
of these efforts, the MYStIX project, that produced a catalog of 31,784
probable members of 20 MSFRs. We find that age spread within clusters are real
in the sense that the stars in the core formed after the cluster halo. Cluster
expansion is seen in the ensemble of (sub)clusters, and older dispersing
populations are found across MSFRs. Direct evidence for subcluster merging is
still unconvincing. Long-lived, asynchronous star formation is pervasive across
MSFRs.Comment: 22 pages, 9 figures. To appear in "The Origin of Stellar Clusters",
edited by Steven Stahler, Springer, 2017, in pres
Melnick 33Na: a very massive colliding-wind binary system in 30 Doradus
We present spectroscopic analysis of the luminous X-ray source Melnick 33Na (Mk 33Na, HSH95 16) in the Large Magellanic Cloud (LMC) 30 Doradus region (Tarantula Nebula), utilizing new time-series Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph spectroscopy. We confirm Mk 33Na as a double-lined O-type spectroscopic binary with a mass ratio q = 0.63 ± 0.02, e = 0.33 ± 0.01, and orbital period of 18.3 ± 0.1 d, supporting the favoured period from X-ray observations obtained via the Tarantula – Revealed by X-rays survey. Disentangled spectra of each component provide spectral types of OC2.5 If* and O4 V for the primary and secondary, respectively. Unusually for an O supergiant the primary exhibits strong C  IV 4658 emission and weak N V 4603-20, justifying the OC classification. Spectroscopic analysis favours extreme physical properties for the primary (Teff = 50 kK, log L/L⊙ = 6.15) with system components of M1 = 83 ± 19 M⊙ and M2 = 48 ± 11 M⊙ obtained from evolutionary models, which can be reconciled with results from our orbital analysis (e.g. M1sin 3i = 20.0 ± 1.2 M⊙) if the system inclination is ∼38° and it has an age of 0.9–1.6 Myr. This establishes Mk 33Na as one of the highest mass binary systems in the LMC, alongside other X-ray luminous early-type binaries Mk34 (WN5h+WN5h), R144 (WN5/6h+WN6/7h), and especially R139 (O6.5 Iafc + O6 Iaf)
The Chandra Deep Field North Survey. XIII. 2 Ms point-source catalogs
We present point-source catalogs for the 2 Ms exposure of the Chandra Deep Field North, currently the deepest X-ray observation of the universe in the 0.58.0 keV band. Five hundred and three (503) X-ray sources are detected over an 448 arcmin2 area in up to seven X-ray bands. Twenty (20) of these X-ray sources lie in the central 5.3 arcmin2 Hubble Deep Field North (13,600 sources deg-2). The on-axis sensitivity limits are 2.5 × 10-17 ergs cm-2 s-1 (0.52.0 keV) and 1.4 × 10-16 ergs cm-2 s-1 (28 keV). Source positions are determined using matched-filter and centroiding techniques; the median positional uncertainty is 03. The X-ray colors of the detected sources indicate a broad variety of source types, although absorbed AGNs (including a small number of possible Compton-thick sources) are clearly the dominant type. We also match lower significance X-ray sources to optical counterparts and provide a list of 79 optically bright (R 23) lower significance Chandra sources. The majority of these sources appear to be starburst and normal galaxies. The average backgrounds in the 0.52.0 keV and 28 keV bands are 0.056 and 0.135 counts Ms-1 pixel-1, respectively. The background count distributions are very similar to Poisson distributions. We show that this 2 Ms exposure is approximately photon limited in all seven X-ray bands for regions close to the aim point, and we predict that exposures up to 25 Ms (0.52.0 keV) and 4 Ms (28 keV) should remain nearly photon limited. We demonstrate that this observation does not suffer from source confusion within 6 of the aim point, and future observations are unlikely to be source-confusion limited within 3 of the aim point even for source densities exceeding 100,000 deg-2. These analyses directly show that Chandra can achieve significantly higher sensitivities in an efficient, nearly photon-limited manner and be largely free of source confusion. To allow consistent comparisons, we have also produced point-source catalogs for the 1 Ms Chandra Deep Field South (CDF-S). Three hundred and twenty-six (326) X-ray sources are included in the main Chandra catalog, and an additional 42 optically bright X-ray sources are included in a lower significance Chandra catalog. We find good agreement with the photometry of the previously published CDF-S catalogs; however, we provide significantly improved positional accuracy
The VLT-FLAMES Tarantula survey. XX. The nature of the X-ray bright emission-line star VFTS 399
The Chandra Deep Field-South Survey: 4 Ms Source Catalogs
[abridged] We present point-source catalogs for the 4Ms Chandra Deep
Field-South (CDF-S), which is the deepest Chandra survey to date and covers an
area of 464.5 arcmin^2. We provide a main source catalog, which contains 740
X-ray point sources that are detected with wavdetect at a false-positive
probability threshold of 1E-5 and also satisfy a binomial-probability
source-selection criterion of P<0.004; this approach is designed to maximize
the number of reliable sources detected. A total of 300 main-catalog sources
are new compared to the previous 2Ms CDF-S main-catalog sources. We also
provide a supplementary catalog, which consists of 36 sources that are detected
with wavdetect at 1E-5, satisfy 0.004< P<0.1, and have an optical counterpart
with R<24. Multiwavelength identifications, basic optical/infrared/radio
photometry, and spectroscopic/photometric redshifts are provided for the X-ray
sources. Basic analyses of the X-ray and multiwavelength properties of the
sources indicate that >75% of the main-catalog sources are AGNs; of the 300 new
main-catalog sources, about 35% are likely normal and starburst galaxies,
reflecting the rise of normal and starburst galaxies at the very faint flux
levels uniquely accessible to the 4Ms CDF-S. Near the center of the 4Ms CDF-S,
the observed AGN and galaxy source densities have reached ~9800 and 6900 per
square degree, respectively. The 4 Ms CDF-S reaches on-axis flux limits of
~9.1E-18 and 5.5E-17 erg/cm^2/s for the soft and hard bands, respectively. An
increase in the CDF-S exposure by a factor of ~2-2.5 would provide further
significant gains and probe key unexplored discovery space.Comment: 31 pages, 24 figures, 8 tables, ApJS in press. Data and images
available at http://www.astro.psu.edu/users/niel/cdfs/cdfs-chandra.htm