Salvatore Serio

A survey of soft X-ray limb flare images - The relation between their structure in the corona and other physical parameters

A survey of soft X-ray limb flare images obtained by the S-054 experiment on board Skylab is presented. From a morphological point of view, limb flares have been subdivided into three groups: (A) flares characterized by compact loop structures; (B) flares with a pointlike appearance; (C)flares with large and diffuse systems of loops. The significance of this subdivision is investigated with the aid of plasma parameters determined by combining the S-054 spatially resolved observations with full-disk data obtained by the Solrad 9 satellite. From a comparison of the spatial structure with physical parameters such as height, volume, energy density, and characteristic times, and from the correlation with white-light coronal transients and H..cap alpha.. active prominences, the existence of two physically distinct classes of flares is established: class I, which consists of both morphological groups A and B, and class II, which comprises only events of group C.

Coronal closed structures. IV - Hydrodynamical stability and response to heating perturbations

The response of magnetically confined atmospheres to perturbations in the temperature and density distribution, and the local heating rate by means of a one-dimensional time-dependent hydrodynamical code, which incorporates the full energy, momentum and mass conservation equations is studied. These studies extend the linear instability analysis of Habbal and Rosner (1979) into the finite-amplitude regime, and generalize the confined atmosphere models of Serio et al., to the time-dependent domain. The results show that closed coronal atmospheres are stable against finite-amplitude perturbations if the chromospheric response is taken into account; and observed correlated increases in coronal density and temperature can only be achieved under quiescent conditions by increasing the heat deposition rate relatively more in the chromosphere than in the corona.

Magnetic fields and coronal heating

General considerations concerning the scaling properties of magnetic-field--related colonal heating mechanisms are used to build a two-parameter model for the heating of closed coronal regions. The model perdicts the way in which coronal temperature and electron density are related to photospheric magnetic field strength and the size of the region, using the additional constraint provided by the scaling law of rosner, tucker, and Viaiana. The model successfully duplicates the observed scaling of total thermal energy content with total longitudinal flux; it also predict a relation between the coronal energy density (or pressure) and the longitudinal field strength modified by the region scale size. The observational data yield a similar relation, pproportional/sup 1.6/. A parameter of the theory, which is evaluated by fitting to the data, is the product ..cap alpha..upsilon/sub phi/, where ..cap alpha.. is the ratio of azimuthal to longitudinal magnetic field and upsilon/sub phi/ is the effective twisting velocity of the loop footpoints, which supplies the energy for coronal heating.

Einstein X-ray survey of the Pleiades - The dependence of X-ray emission on stellar age

The data obtained with two pointed observations of 1 deg by 1 deg fields of the Pleiades region have been analyzed, and the results are presented. The maximum-likelihood X-ray luminosity functions for the Pleiades G and K stars in the cluster are derived, and it is shown that, for the G stars, the Pleiades X-ray luminosity function is significantly brighter than the corresponding function for Hyades G dwarf stars. This finding indicates a dependence of X-ray luminosity on stellar age, which is confirmed by comparison of the same data with median X-ray luminosities of pre-main sequence and local disk population dwarf G stars. It is suggested that the significantly larger number of bright X-ray sources associated with G stars than with K stars, the lack of detection of M stars, and the relatively rapid rotation of the Pleiades K stars can be explained in terms of the onset of internal differential rotation near the convective envelope-radidative core interface after the spin-up phase during evolution to the main sequence. 48 references.

AXAF High-Resolution Camera (HRC): calibration and recalibration at XRCF and beyond

The high resolution camera (HRC) is a microchannel plate based imaging detector for the Advanced X-Ray Astrophysics Facility (AXAF) that will be placed in a high earth orbit scheduled for launch in August, 1998. An end-to-end calibration of the HRC and the AXAF high resolution mirror assembly (HRMA) was carried out at the Marshall Space Flight Centers X-Ray Calibration Facility (XRCF). This activity was followed by several modifications to the HRC to improve its performance, and a series of flat field calibrations. In this paper, and the following companion papers, we discuss the calibration plans, sequences, and results of these tests. At the time of this conference, the HRC has been fully flight qualified and is being integrated into the science instrument module (SIM) in preparation for integration into the AXAF spacecraft.

In-flight performance of the Chandra high-resolution camera

The High Resolution Camera (HRC) is one of the two focal plane instruments on NASAs Chandra X-ray Observatory which was successfully launched July 23, 1999. The Chandra Observatory will perform high resolution spectroscopy and imaging in the X-ray band of 0.1 to 10 keV. The HRC instrument consists of two detectors, the HRC-I for imaging and the HRC-S for spectroscopy. In this paper we present an overview of the in-flight performance of the High Resolution Camera and discuss some of the initial scientific results.

Hydrodynamic modeling of an X-ray flare on Proxima Centauri observed by the Einstein telescope

Hydrodynamic numerical calculations of a flare which occurred on Proxima Centauri and was observed by the Einstein satellite on August 20, 1980 at 12:50 UT are presented. The highlights of the hydrodynamic code are reviewed, and the physical and geometrical parameters necessary for the calculations are derived and compared with observations. The results are consistent with the stellar flare being caused by the rapid dissipation of 5.9 x 10 to the 31st ergs, within a magnetic loop structure whose semilength is 7 x 10 to the 9th cm and cross-sectional radius is 7.3 x 10 to the 8th cm. The results provide evidence that flares on late-type stars can be described by a hydrodynamic model with a relatively simple geometry, similar to solar compact flares. 39 references.

Simulations of the Ca XIX spectral emission from a flaring solar coronal loop. I. Thermal case

Spectral profiles synthesized from the results of numerical simulations of solar coronal loop plasma are compared with the observed line profiles, and the sensitivity of this comparison to variations in the parameter values is tested. The comparison allows the procedures commonly adopted for obtaining temperatures and velocities directly from the bent crystal spectrometer observations, as well as the interpretation of parameters obtained from hydrodynamical calculations, to be independently validated. The characteristic spectral signatures of different models for the deposition of impulsive thermal energy in the loop are derived assuming two distinct spatial distributions of thermal heating: in one case, the heating function peaks near the apex of the loop, while in the second, energy is deposited directly in the lower atmosphere. 37 references.

Variability analysis in low count rate sources

A method, based on the chi-square statistics, is described for detecting pulselike time variability in low count rate sources observed with photon-counting instruments. This method can be used even in the presence of observational gaps, takes full advantage of the filtering effect due to binning with different bin sizes, and takes into account the arbitrariness introduced by the binning phase. The procedure developed to limit the dependence of the results on the binning phase and ensure statistically correct results is described along with the application of the proposed procedure to a model of a variable source. Monte Carlo simulations are used to show how the method can be used to derive the characteristic variability time scales and that the method is more sensitive than the nonparametric Kolmogorov-Smirnov test in detecting variability to a given confidence level.

High-resolution camera (HRC) on the Advanced X-Ray Astrophysics Facility (AXAF)

The Advanced X-ray Astrophysics Facility (AXAF) is a major NASA space observatory and is scheduled for launch in 1998. AXAF will perform high spatial and spectral resolution observations of celestial sources in the soft x-ray band 0.1 - 10 keV. The high resolution camera (HRC) is one of two focal plane instruments being developed for the AXAF. The HRC will be capable of observing point and extended sources with high sensitivity and high spatial resolution and will be used to record the high resolution spectra produced by an objective transmission grating. The HRC is based on microchannel plates (MCPs). We describe the design and development of the HRC, its expected performance, and some of its observational goals. The HRC consists of two separate detectors, HRC-I (imaging) and HRC-S (spectroscopy). HRC-I is used for imaging and has a field of view of 31 arc min by 31 arc min and a spatial resolution of less than 25 micrometers (equivalent to less than 0.5 arc sec). HRC- S is optimized to readout the spectrum of AXAFs low energy transmission grating (LETG) and this combination will achieve resolving powers in excess of 1000 at low energies and cover a wavelength range of 4 to 140 angstroms.