Pixel area variations in sensors: a novel framework for predicting pixel fidelity and distortion in flat field response

We describe the drift field in thick depleted silicon sensors as a superposition of a one-dimensional backdrop field and various three-dimensional perturbative contributions that are physically motivated. We compute trajectories for the conversions along the field lines toward the channel and into volumes where conversions are confined by the perturbative fields. We validate this approach by comparing predictions against measured response distributions seen in five types of fixed pattern distortion features. We derive a quantitative connection between "tree ring" flat field distortions to astrometric and shape transfer errors with connections to measurable wavelength dependence - as ancillary pixel data that may be used in pipeline analysis for catalog population. Such corrections may be tested on DECam data, where correlations between tree ring flat field distortions and astrometric errors - together with their band dependence - are already under study. Dynamic effects, including the brighter-fatter phenomenon for point sources and the flux dependence of flat field fixed pattern features are approached using perturbations similar in form to those giving rise to the fixed pattern features. These in turn provide drift coefficient predictions that can be validated in a straightforward manner. Once the three parameters of the model are constrained using available data, the model is readily used to provide predictions for arbitrary photo-distributions with internally consistent wavelength dependence provided for free.Comment: 17 pages, 7 figures, submitted to "Precision Astronomy with Fully Depleted CCDs" - conference proceedings to be published by JINS

X-Ray IGM in the Local Group

Recent observations with the dispersive X-ray spectrometers aboard Chandra and Newton Observatory have begun to probe the properties of the X-ray intergalactic medium (IGM) at small redshifts. Using large quantities (~950 ksec) of spectroscopic data acquired using the RGS aboard Newton Observatory, we investigated the intervening material toward three low redshift, high Galactic latitude AGNs with nominally featureless spectra: Mrk421, PKS2155-304 and 3C273. Each spectrum provides clear evidence for what appears to be a local (z~0), highly ionized absorbing medium betrayed by the OVII 1s-2p resonance transition feature seen at 21.6A (N[OVII] ~ 1E16 cm-2). Measurements are also made for the Lyman alpha transition of the adjacent ionization state, (OVIII; 18.97A), which potentially constrains the absorber's temperature. Finally, in a collisional equilibrium approximation, upper limits to diffuse emission intensities place upper limits on the electron density (ne < 2E-4 cm-3), lower limits on the scale length of the absorber (L > 140 kpc) and lower limits on its mass (M > 5E10 M[sun]). Limits on the absorber's scale length and its velocity distribution lead us to identify it with the Local Group.Comment: 6 pages, 1 table and 2 figs. Latex. To appear in "The IGM/Galaxy Connection: The Distribution of Baryons at z=0" to be published by Kluwer Academic Publishing. Resubmitted with 2 typos corrected, page 5 and figure 2. Thanks to Masao Sako for pointing these ou

Evidence for Widespread AGN Activity among Massive Quiescent Galaxies at z ~ 2

We quantify the presence of Active Galactic nuclei (AGN) in a mass-complete (M_* >5e10 M_sun) sample of 123 star-forming and quiescent galaxies at 1.5 < z < 2.5, using X-ray data from the 4 Ms Chandra Deep Field-South (CDF-S) survey. 41+/-7% of the galaxies are detected directly in X-rays, 22+/-5% with rest-frame 0.5-8 keV luminosities consistent with hosting luminous AGN (L_0.5-8keV > 3e42 ergs/s). The latter fraction is similar for star-forming and quiescent galaxies, and does not depend on galaxy stellar mass, suggesting that perhaps luminous AGN are triggered by external effects such as mergers. We detect significant mean X-ray signals in stacked images for both the individually non-detected star-forming and quiescent galaxies, with spectra consistent with star formation only and/or a low luminosity AGN in both cases. Comparing star formation rates inferred from the 2-10 keV luminosities to those from rest-frame IR+UV emission, we find evidence for an X-ray excess indicative of low-luminosity AGN. Among the quiescent galaxies, the excess suggests that as many as 70-100% of these contain low- or high-luminosity AGN, while the corresponding fraction is lower among star-forming galaxies (43-65%). The ubiquitous presence of AGN in massive, quiescent z ~ 2 galaxies that we find provides observational support for the importance of AGN in impeding star formation during galaxy evolution.Comment: 9 pages, 3 figures, 3 tables; Accepted for publication in ApJ. Minor editing changes and a few references added. Matches published versio

IceCube Neutrinos from Hadronically Powered Gamma-Ray Galaxies

In this work we use a multi-messenger approach to determine if the high energy diffuse neutrino flux observed by the IceCube Observatory can originate from $\gamma$-ray sources powered by Cosmic Rays interactions with gas. Typical representatives of such sources are Starburst and Ultra-Luminous Infrared Galaxies. Using the three most recent calculations of the non-blazar contribution to the extragalactic $\gamma$-ray background measured by the Fermi-LAT collaboration, we find that a hard power-law spectrum with spectral index $\alpha \leq 2.12$ is compatible with all the estimations for the allowed contribution from non-blazar sources, within 1$\sigma$. Using such a spectrum we are able to interpret the IceCube results, showing that various classes of hadronically powered $\gamma$-ray galaxies can provide the dominant contribution to the astrophysical signal above 100 TeV and about half of the contribution to the energy flux between 10-100 TeV. With the addition of neutrinos from the Galactic plane, it is possible to saturate the IceCube signal at high energy. Our result shows that these sources are still well motivated candidates.Comment: Accepted for publication on JCA

Structural Insights into the Regulation of Electron Transfer in Nitrogenase, and Modulating the Reactivity of the Isolated Iron Molybdenum Cofactor

Nitrogenase, EC: 1.18.6.1 is the enzyme that catalyzes the reduction of dinitrogen to ammonia; this is known as biological nitrogen fixation. Nitrogen fixation is so important to our daily lives, that we utilize approximately 2% of the annual energy produced worldwide to fix nitrogen industrially via the Haber-Bosch process. The industrial process requires a high input of energy in the form of heat (\u3e450°C) and pressure (\u3e200 atm\u3e), while the enzymatic system is performed under ambient conditions. Research invested into understanding the mechanism of this biological catalyst could eventually lead to understanding how nature performs difficult chemical reductions, which could allow researchers to develop catalysts that mimic this enzyme to perform many important reactions, such as nitrogen fixation, much more efficiently than today. Electron transfer in the nitrogenase is only partially understood, and is one of the key elements of understanding the mechanism of nitrogenase. Nitrogenase is composed of two proteins, the Fe protein delivers electrons to the MoFe protein, where N2 binds and is subsequently reduced. The conformational changes that take place upon Fe protein binding were investigated in order to better understand electron transfer within the enzyme. Further, studies were performed which probed the P-cluster, an iron sulfur cluster in the MoFe protein that acts as an intermediate in the electron transfer event, and successfully identified the biologically relevant redox state of the P-cluster, P+1. Other studies were performed which identified several variants of the MoFe protein which were able to accept electrons from a chemical reductant. These variants are the first examples of nitrogenase enzymes able to accept electrons from any source other than Fe protein and shown substrate reduction. These variants pinpoint where nitrogenase is likely to undergo conformational changes to allow electron transfer to the active site of the enzyme. Finally, studies were done on the isolated active site of the protein, the iron molybdenum cofactor to better understand how the active site of nitrogenase works The goal of this thesis is to better understand how electrons travel through nitrogenase, and how they are utilized at the active site, FeMo-cofactor, when they arrive

On the Putative Detection of z>0 X-ray Absorption Features in the Spectrum of Markarian 421

In a series of papers, Nicastro et al. have reported the detection of z>0 OVII absorption features in the spectrum of Mrk421 obtained with the Chandra Low Energy Transmission Grating Spectrometer (LETGS). We evaluate this result in the context of a high quality spectrum of the same source obtained with the Reflection Grating Spectrometer (RGS) on XMM-Newton. The data comprise over 955ks of usable exposure time and more than 26000 counts per 50 milliAngstrom at 21.6 Angstroms. We concentrate on the spectrally clean region (21.3 < lambda < 22.5 Angstroms) where sharp features due to the astrophysically abundant OVII may reveal an intervening, warm--hot intergalactic medium (WHIM). We do not confirm detection of any of the intervening systems claimed to date. Rather, we detect only three unsurprising, astrophysically expected features down to the Log(N_i)~14.6 (3 sigma) sensitivity level. Each of the two purported WHIM features is rejected with a statistical confidence that exceeds that reported for its initial detection. While we can not rule out the existence of fainter, WHIM related features in these spectra, we suggest that previous discovery claims were premature. A more recent paper by Williams et al. claims to have demonstrated that the RGS data we analyze here do not have the resolution or statistical quality required to confirm or deny the LETGS detections. We show that our careful analysis resolves the issues encountered by Williams et al. and recovers the full resolution and statistical quality of the RGS data. We highlight the differences between our analysis and those published by Williams et al. as this may explain our disparate conclusions.Comment: 19 pages/7 figures/4 tables. 060424 submitted to ApJ 060522 re-submitted following ApJ reques

High Resolution X-ray Spectroscopy of G292.0+1.8/MSH 11-54

We present a preliminary analysis of XMM-Newton observations of the oxygen-rich supernova remnant G292.0+1.8 (MSH 11-54). Although the spatial extent of the remnant is 8 arcmin the bright central bar is narrow (1'-2') resulting in RGS spectra of a high spectral quality. This allows us to spectroscopically identify a cool, Te = 0.3 keV, and underionized component, resolve details of the Fe-L complex, and resolve the forbidden and resonant lines of the O VII triplet. We are also able to constrain the kinematics of the remnant using Ne IX as observed in the second order spectrum, and O VIII in the first order spectrum. We do not find evidence for O VII line shifts or Doppler broadening (sigma_v < 731 km/s), but line broadening of the Ne X Ly-alpha line seems to be present, corresponding to sigma_v ~ 1500 km/s.Comment: To appear in Proc. of the BeppoSAX Symposium: "The Restless High-Energy Universe", E.P.J. van den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers (Eds

Discovery of a very extended X-ray halo around a quiescent spiral galaxy - the "missing link" of galaxy formation

Hot gaseous haloes surrounding galaxies and extending well beyond the distribution of stars are a ubiquitous prediction of galaxy formation scenarios. The haloes are believed to consist of gravitationally trapped gas with a temperature of millions of Kelvin. The existence of such hot haloes around massive elliptical galaxies has been established through their X-ray emission. While gas out-flowing from starburst spiral galaxies has been detected, searches for hot haloes around normal, quiescent spiral galaxies have so far failed, casting doubts on the fundamental physics in galaxy formation models. Here we present the first detection of a hot, large-scale gaseous halo surrounding a normal, quiescent spiral galaxy, NGC 5746, alleviating a long-standing problem for galaxy formation models. In contrast to starburst galaxies, where the X-ray halo can be powered by the supernova energy, there is no such power source in NGC 5746. The only compelling explanation is that we are here witnessing a galaxy forming from gradually in-flowing hot and dilute halo gas.Comment: New Astronomy, in pres