Diab Jerius

Ubvri light curves of 44 type ia supernovae

We present UBVRI photometry of 44 Type Ia supernovae (SNe Ia) observed from 1997 to 2001 as part of a continuing monitoring campaign at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics. The data set comprises 2190 observations and is the largest homogeneously observed and reduced sample of SNe Ia to date, nearly doubling the number of well-observed, nearby SNe Ia with published multicolor CCD light curves. The large sample of U-band photometry is a unique addition, with important connections to SNe Ia observed at high redshift. The decline rate of SN Ia U-band light curves correlates well with the decline rate in other bands, as does the U - B color at maximum light. However, the U-band peak magnitudes show an increased dispersion relative to other bands even after accounting for extinction and decline rate, amounting to an additional ~40% intrinsic scatter compared to the B band.

BOW SHOCK AND RADIO HALO IN THE MERGING CLUSTER A520

Chandra observations of the merging galaxy cluster A520 reveal a prominent bow shock with M = 2.1. This is only the second clear example of a substantially supersonic merger shock front in clusters. Comparison of the X-ray image with that of the previously known radio halo reveals a coincidence of the leading edge of the halo with the bow shock, offering an interesting experimental setup for determining the role of shocks in the radio halo generation. The halo in A520 apparently consists of two spatially distinct parts, the main turbulence-driven component and a cap-like forward structure related to the shock, where the latter may provide preenergized electrons for subsequent turbulent reacceleration. The radio edge may be caused by electron acceleration by the shock. If so, the synchrotron spectrum should have a slope of α 1.2 right behind the edge, with quick steepening farther away from the edge. Alternatively, if shocks are inefficient accelerators, the radio edge may be explained by an increase in the magnetic field and density of preexisting relativistic electrons due to gas compression. In the latter model, there should be radio emission in front of the shock with the same spectrum as that behind it, but 10-20 times fainter. If future sensitive radio measurements do not find such preshock emission, then the electrons are indeed accelerated (or reaccelerated) by the shock, and one will be able to determine its acceleration efficiency. We also propose a method to estimate the magnetic field strength behind the shock, based on measuring the dependence of the radio spectral slope upon the distance from the shock. In addition, the radio edge provides a way to constrain the diffusion speed of the relativistic electrons.

A CHANDRA HIGH-RESOLUTION X-RAY IMAGE OF CENTAURUS A

We present first results from a Chandra X-Ray Observatory observation of the radio galaxy Centaurus A with the High-Resolution Camera. All previously reported major sources of X-ray emission including the bright nucleus, the jet, individual point sources, and diffuse emission are resolved or detected. The spatial resolution of this observation is better than 1&arcsec; in the center of the field of view and allows us to resolve X-ray features of this galaxy not previously seen. In particular, we resolve individual knots of emission in the inner jet and diffuse emission between the knots. All of the knots are diffuse at the 1&arcsec; level, and several exhibit complex spatial structure. We find the nucleus to be extended by a few tenths of an arcsecond. Our image also suggests the presence of an X-ray counterjet. Weak X-ray emission from the southwest radio lobe is also seen, and we detect 63 pointlike galactic sources (probably X-ray binaries and supernova remnants) above a luminosity limit of approximately 1.7x1037 ergs s-1.

Performance expectation versus reality

The AXAF (Advanced X-ray Astrophysics Facility) high resolution mirror assembly (HRMA) now is complete and has been tested at the NASA Marshall Space Flight Center (MSFC) X-ray Calibration Facility (XRCF). The surface and alignment properties of the mirror were thoroughly measured before the x-ray test, which allowed accurate performance predictions to be performed. The preliminary analysis of the measured x-ray image distributions for all energies tested show excellent agreement with predictions made before the beginning of the test. There is a discrepancy between the measured and predicted effective areas; this typically is less than 5%, and is less than 13% for all energies measured. We present evidence that this discrepancy is due to uncertainties in the calibration of the test instrumentation, and therefore can be expected to be reduced when results from further instrument calibration tests now in progress are incorporated into the analysis. We predict that 65 - 80% (depending upon energy) of the flux from an imaged point source will be contained on a one arc second diameter aperture in flight. We expect the HRMA to more than fulfill the requirements necessary to achieve the AXAF scientific objectives.

Deep Chandra Observations of Edges and Bubbles in the NGC 5846 Galaxy Group

We use a combined 120 ks Chandra exposure to analyze X-ray edges produced by non-hydrostatic gas motions (sloshing) from galaxy collisions, and cavities formed by active galactic nucleus (AGN) activity. Evidence for gas sloshing is seen in the spiral morphology and multiple cold front edges in NGC 5846s X-ray surface brightness distribution, while the lack of spiral structure in the temperature map suggests that the perturbing interaction was not in the plane of the sky. Density and spectral modeling across the edges indicate that the relative motion of gas in the cold fronts is at most transonic. Evidence for AGN activity is seen in two inner bubbles at 0.6 kpc, filled with 5 GHz and 1.5 GHz radio plasma and coincident with Hα emission, and in a ghost bubble at 5.2 kpc west of NGC 5846s nucleus. The outburst energy and ages for the inner (ghost) bubbles are ~1055 erg and ~2 Myr (~5 × 1055 erg and 12 Myr), respectively, implying an AGN duty cycle of 10 Myr. The inner bubble rims are threaded with nine knots, whose total 0.5-2 keV X-ray luminosity is 0.3 × 1040 erg s–1, a factor ~2-3 less than that of the surrounding rims, and 0.7 keV mean temperature is indistinguishable from that of the rims. We suggest that the knots may be transient clouds heated by the recent passage of a shock from the last AGN outburst. We also observe gas stripping from a cE galaxy, NGC 5846A, in a 0.5 kpc long (~105 M ☉) hot gas tail, as it falls toward NGC 5846.

Constellation-X to Generation-X: evolution of large collecting area moderate resolution grazing incidence x-ray telescopes to larger area high-resolution adjustable optics

Large collecting area x-ray telescopes are designed to study the early Universe, trace the evolution of black holes, stars and galaxies, study the chemical evolution of the Universe, and study matter in extreme environments. The Constellation-X mission (Con-X), planned for launch in 2016, will provide ~ 104 cm2 collecting area with 15 arc-sec resolution, with a goal of 5 arc-sec. Future missions require larger collecting area and finer resolution. Generation-X (Gen-X), a NASA Visions Mission, will achieve 100 m2 effective area at 1 keV and angular resolution of 0.1 arc-sec, half power diameter. We briefly describe the Con-X flowdown of imaging requirements to reflector figure error. To meet requirements beyond Con-X, Gen-X optics will be thinner and more accurately shaped than has ever been accomplished. To meet these challenging goals, we incorporate for the first time active figure control with grazing incidence optics. Piezoelectric material will be deposited in discrete cells directly on the back surface of the optical segments, with the strain directions oriented parallel to the surface. Differential strain between the two layers of the mirror causes localized bending in two directions, enabling local figure control. Adjusting figure on-orbit eases fabrication and metrology. The ability to make changes to mirror figure adds margin by mitigating risk due to launch-induced deformations and/or on-orbit degradation. We flowdown the Gen-X requirements to mirror figure and four telescope designs, and discuss various trades between the designs.

Orbital Measurement and Verification of the Chandra X-Ray Observatory's PSF

We present here results of the on-orbit calibration of the point spread function (PSF), comparing it with our predictions. We discuss how the PSF varies with source location in the telescope field of view, as well as with the spectral energy distribution of the source.

High-Resolution X-Ray Telescopes

High-energy astrophysics is a relatively young scientific field, made possible by space-borne telescopes. During the half-century history of x-ray astronomy, the sensitivity of focusing x-ray telescopes-through finer angular resolution and increased effective area-has improved by a factor of a 100 million. This technological advance has enabled numerous exciting discoveries and increasingly detailed study of the high-energy universe-including accreting (stellarmass and super-massive) black holes, accreting and isolated neutron stars, pulsar-wind nebulae, shocked plasma in supernova remnants, and hot thermal plasma in clusters of galaxies. As the largest structures in the universe, galaxy clusters constitute a unique laboratory for measuring the gravitational effects of dark matter and of dark energy. Here, we review the history of high-resolution x-ray telescopes and highlight some of the scientific results enabled by these telescopes. Next, we describe the planned next-generation x-ray-astronomy facility-the International X-ray Observatory (IXO). We conclude with an overview of a concept for the next next-generation facility-Generation X. The scientific objectives of such a mission will require very large areas (about 10000 m2) of highly-nested lightweight grazing-incidence mirrors with exceptional (about 0.1-arcsecond) angular resolution. Achieving this angular resolution with lightweight mirrors will likely require on-orbit adjustment of alignment and figure.

Parameterization of the Chandra point spread function

The Chandra X-ray Observatory point spread function (PSF) is a complex function of source position and energy. On-orbit calibration observations with sufficient S/N sample only a small fraction of the possible parameter space, and are complicated by detector systematics. Thus, the standard method of analyzing Chandra data uses the standard Chandra optics model as a reference. The optics model accurately simulates the telescopes PSF, but as it is a raytrace based technique, it can be time-consuming to run and is not always appropriate for a given analysis task. A simple parameterization of the PSF would be useful for many analysis purposes, in many cases obviating the need for users to run lengthy raytraces. We present an approach to a simple PSF parameterization of off-axis point sources, discussing its applicability to analysis of Chandra observations in light of the complicated PSF structure. We also present some results of our PSF parameterization and discuss its accuracy.

Chandra X-ray Observatory mirror effective area

Chandra X-ray Observatory (CXO) -- the third of NASAs Great Observatories -- has now been successfully operated for four years and has brought us fruitful scientific results with many exciting discoveries. The major achievement comparing to previous X-ray missions lies in the heart of the CXO -- the High Resolution Mirror Assembly. Its unprecedented spatial resolution and well calibrated performing characteristics are the keys for its success. We discuss the effective area of the CXO mirrors, based on the ground calibration measurements made at the X-Ray Calibration Facility in Marshall Space Flight Center before launch. We present the derivations of both on-axis and off-axis effective areas, which are currently used by Chandra observers.