16 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
Studies of Neutron Stars at Optical/IR Wavelengths
In the last years, optical studies of Isolated Neutron Stars (INSs) have expanded from the more classical rotation-powered ones to other categories, like the Anomalous X-ray Pulsars (AXPs) and the Soft Gamma-ray Repeaters (SGRs), which make up the class of the magnetars, the radio-quiet INSs with X-ray thermal emission and, more recently, the enigmatic Compact Central Objects (CCOs) in supernova remnants. Apart from 10 rotation-powered pulsars, so far optical/IR counterparts have been found for 5 magnetars and for 4 INSs. In this work we present some of the latest observational results obtained from optical/IR observations of different types of INSs
High-time Resolution Astrophysics and Pulsars
The discovery of pulsars in 1968 heralded an era where the temporal
characteristics of detectors had to be reassessed. Up to this point detector
integration times would normally be measured in minutes rather seconds and
definitely not on sub-second time scales. At the start of the 21st century
pulsar observations are still pushing the limits of detector telescope
capabilities. Flux variations on times scales less than 1 nsec have been
observed during giant radio pulses. Pulsar studies over the next 10 to 20 years
will require instruments with time resolutions down to microseconds and below,
high-quantum quantum efficiency, reasonable energy resolution and sensitive to
circular and linear polarisation of stochastic signals. This chapter is review
of temporally resolved optical observations of pulsars. It concludes with
estimates of the observability of pulsars with both existing telescopes and
into the ELT era.Comment: Review; 21 pages, 5 figures, 86 references. Book chapter to appear
in: D.Phelan, O.Ryan & A.Shearer, eds.: High Time Resolution Astrophysics
(Astrophysics and Space Science Library, Springer, 2007). The original
publication will be available at http://www.springerlink.co
Observations of metals in the intra-cluster medium
Because of their deep gravitational potential wells, clusters of galaxies
retain all the metals produced by the stellar populations of the member
galaxies. Most of these metals reside in the hot plasma which dominates the
baryon content of clusters. This makes them excellent laboratories for the
study of the nucleosynthesis and chemical enrichment history of the Universe.
Here we review the history, current possibilities and limitations of the
abundance studies, and the present observational status of X-ray measurements
of the chemical composition of the intra-cluster medium. We summarise the
latest progress in using the abundance patterns in clusters to put constraints
on theoretical models of supernovae and we show how cluster abundances provide
new insights into the star-formation history of the Universe.Comment: 28 pages, 12 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 16; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
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