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Astronomical Data Analysis Software and Systems VI ASP Conference Series, Vol. 125, 1997 Gareth Hunt and H. E. Payne, eds.

Simulated AXAF Observations with MARX
Michael W. Wise, David P. Huenemoerder, and John E. Davis MIT Center for Space Research, AXAF Science Center 70 Vassar St. Building 37-644, Cambridge, MA 02139-4307 Abstract. The AXAF Science Center group at MIT has develop ed an end-to-end simulator of the AXAF satellite called MARX. MARX includes models of the AXAF mirrors, the low- and high-energy transmission grating assemblies (LETG and HETG), and the HRC and ACIS focal plane detectors. We discuss the role of MARX within the ASC Data System and present sample simulated images and sp ectra for two typical cosmic X-ray sources.

1.

Introduction

The AXAF Science Center (ASC) Data Analysis System will include the ability to simulate the detailed resp onse of the AXAF satellite to X-ray sources. These simulations will b e used for a variety of purp oses including: development of processing algorithms; ground-based and on-orbit p erformance prediction; testing of the standard processing pip elines; and scientific observation planning and prediction. As part of this modeling effort, the AXAF Science Center group at MIT has develop ed a modular, p ortable, stand-alone simulator MARX (Model of AXAF Resp onse to X-rays). In this pap er, we will briefly discuss some of the motivations which have driven MARX's development as well the simulator's current level of functionality. To demonstrate its capabilities, we present simulated AXAF images and sp ectra for two cosmic sources. 2. Motivations

MARX was originally develop ed to provide sample AXAF data with an exactly known instrument resp onse in order to develop and test sp ectral extraction and analysis algorithms. It has since develop ed into an accurate and flexible model of the in-flight capabilities of AXAF and provides a fast and p ortable alternative to other simulation methods. Here we list some of the uses MARX simulations serve within the ASC Data System: · Development of data analysis software MARX provides realistic data incorp orating a variety of physical effects which characterize AXAF. These data can b e used to develop new algorithms, such as event-based sp ectral extraction of HETG and LETG sp ectra, or deconvolution of overlapping orders in the case of the LETG. MARX simulations provide knowledge of the exact solution for comparison with the reconstructed data. 477

© Copyright 1997 Astronomical Society of the Pacific. All rights reserved.


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Wise, Huenemoerder, and Davis · Fitting and Data Analysis As part of the ASC Data System, a fitting application is b eing designed to allow comparison of models with calibration and flight data (see Doe et al. 1997). This Fit Engine will b e capable of driving MARX to produce high-quality simulations as part of the fitting process. · Calibration Ground calibration of AXAF will utilize detailed predictions to plan the tests and as an aid to the interpretation and digestion of resulting test data. Many of these predictions are b eing compiled using MARX. · Flight Observation planning MARX provides a realistic model of the AXAF resp onse to cosmic Xray sources and can b e used for detailed planning of anticipated science programs. One may investigate issues related to sensitivity, spatial resolution, or sp ectral resolution, for instance. MARX will b e released to the community as part of the prop osal planning software package.

3.

Functionality

MARX provides the capability to simulate the various combinations of scientific instruments on-b oard the AXAF satellite and includes models of the AXAF mirrors, the low- and high-energy transmission grating assemblies (LETG and HETG), and the HRC and ACIS focal plane detectors. For several comp onents of the system, the user may choose b etween several simulation modes dep ending on their needs. The mirrors, for example, can b e simulated using a simple effective area model or via a full ray-trace if desired. A numb er of simple source models (extended or p oint-like) are supp orted and users may extend the capabilities by incorp orating their own models. Sources may have arbitrary sp ectral energy distributions which are sp ecified via an input ASCI I file. MARX also provides supp ort for simulations of ground-based calibration tests at XRCF. The HRMA shutter assembly is modeled as well as sources at a finite distance. 4. Implementation

MARX has b een coded entirely in C and consists of a single executable. Control of the program is accomplished through a parameter file using the IRAF parameter interface library. The simulator is reasonably compact (requiring only 6 MB for the entire distribution) and fast (3,000 photons/sec on a SPARC 20). It has b een successfully compiled under SunOS, Solaris, and Linux. Output from MARX can take several forms: a directory of simple binary vectors for each photon prop erty, a FITS photon event list from which images and sp ectra may b e extracted, an SAOSAC DPDE rayfile, or even ASCI I files. 5. 5.1. Sample Simulations UX Ari Simulation

UX Ari is a bright RS CVn binary system--a prototypical coronal source. A continuous emission measure model was constructed based on the published


Simulated AXAF Observations with MARX

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Figure 1. A simulation of a stellar coronal plasma as detected by the HETG with the ACIS-S detector. In the upp er panel, an expanded view of the detected photon distribution is shown. Sp ectral lines show up as dark, vertical bands. The lower panel gives the total counts p er pixel for the region of the sp ectrum shown in the upp er panel. EUVE data (Dupree 1996); the p eak emission occurred near 107 K. Simulations were run for b oth the HETG and LETG with the same input sp ectrum for an exp osure time of approximately 40 ksec. The flux for the simulations was set at 4в10-11 ergs cm-2 s-1 in the range 0.1­ 10 keV. For the HETGS Simulation, one million input photons gave 124,000 output photons for an equivalent exp osure time of 36 ksec. In the extraction rectangle, there were 14,000 first order HEG photons and 50,000 MEG. The high-order contribution was ab out 4% of the first order, and ab out 50,000 photons went into the zeroth order. With the LETGS, four million input photons resulted in 85,000 detected photons for a 39 ksec equivalent exp osure time. The zeroth order had 33,000 photons, and the first order 46,000, with high order contribution ab out 10% of the first order. Figure 1 shows an expanded view of a p ortion of the detected HETG image as well as the extracted sp ectrum. 5.2. Ab ell 2029 Simulation

The Ab ell 2029 cluster of galaxies is a bright, nearby (z=0.0767) cluster which is bright in X-rays and contains a strong cooling flow. Such cooling plasmas should b e rich in X-ray line emission and make interesting targets for the HETGS. A2029 is, however, an extended ob ject making analysis of its sp ectrum more involved. The sp ectrum for the cluster emission was taken from XSPEC fits to ASCA data for this ob ject (Wise & Sarazin 1997, in preparation). The flux for the simulations was taken to b e 7.5в10-11 ergs cm-2 s-1 in the range 2­10 keV. The sp ectral emission was assumed to consist of two comp onents: isothermal cluster emission from a T=7.45 keV Raymond-Smith thermal plasma with an abundance of 0.4 solar and a central cooling flow with MX =300 M yr-1 .


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Figure 2. A simulated AXAF observation of the A2029 cluster of galaxies. The lower image depicts an observation utilizing the HETG on-b oard AXAF while the upp er image shows the cluster without the HETG in place. The cluster emission was assumed to b e distributed in a standard spherically symmetric, isothermal b eta surface brightness distribution with a core radius of rc =200 kp c (100 ) and =0.73 (Sarazin 1986). The cooling flow emission was modeled spatially as a radially symmetric Gaussian with =100 kp c (50 ) corresp onding to the cooling radius of the flow. A 100 ksec simulated HETG observation yielded 78,000 detected events. The same exp osure without the grating in place yielded 218,000 detected events. Figure 2 shows the resulting images from the two simulations. Acknowledgments. We gratefully acknowledge the aid and supp ort of various memb ers of the AXAF pro ject including the Mission Supp ort Team, the Calibration group, and the Data Systems group. References Doe, S., Siemiginowska, A., Joye, W., & McDowell, J. 1997, this volume, 492 Dupree, A. K. 1996, in ASP Conf. Ser., Vol. 109, Cool Stars 9, ed. R. Pallavicini & A. K. Dupree (San Francisco: ASP), 237