I NSTALACIÓN DE PFSENSE

The Rossi X-ray Timing Experiment (RXTE) was launched in late 1995, for the purpose of analyzing the timing properties of sources such as active galaxies, X-ray binaries, and other extreme objects. Although perhaps not producing the flashy results and pretty pictures of an observatory such as HST, RXTE has provided a tremendous amount of useful information, especially to the AGN community. Its high timing resolution, large energy window, and flexible observing schedule naturally lend themselves to the types of data analysis that PEGAns perform on a regular basis.

B.1.1 Detectors and Modes

RXTE carries three detectors on board: the Proportional Counter Array (PCA), the High Energy X-ray Timing Experiment (HEXTE), and the All-Sky Monitor (ASM). The ASM scans roughly 80% of the sky each orbit, looking for transient X-ray sources. The HEXTE consists of two clusters of four scintillation detectors each, with a col- lecting area of 1.6 square meters and a time resolution of 8 microseconds. Neither the ASM nor the HEXTE will be discussed here.

For the vast majority of AGN timing analysis, the PCA instrument is used. The PCA is an array of 5 proportional counter units (PCUs), numbered 0-4. Because of various issues, typically only the first three of the PCUs are kept on; PCUs 3 and 4 are switched on and off occasionally. Also, each PCU consists of three separate layers of detectors. The first layer is the most sensitive, and is typically the only one used.

This also helps to cut down on readout noise from the other 2 layers. Therefore, this document deals with how to reduce PCA data using PCUs 0-2 and layer 1.

Note that beginning in early 2000, PCU1 began to malfunction as well. Therefore, more recent observations may have intermittent data from PCU1. Info on how to deal with this will be dealt with in the FAQs.

The PCA also contains a wide variety of data collection modes, such as Good Xenon, Standard2, Fourier, etc. Some of these modes (such as the Fourier mode) are so complex that even the RXTE Guest Observer Facility (GOF) has yet to provide a suitable method for data analysis. However, for our purposes, the Standard2 data mode is all we will need. In fact, the GOF stresses that Standard2 is the only data mode you should ever need, unless you want time resolution less than 16s (which we don’t).

B.1.2 Retrieving Data

The RXTE data archive is located at the HEASARC website, and the w3Browse interface is used to choose which data sets you want. Enter in the object name or coordinates you want to look for, the coordinate epoch (if necessary), and choose a range of observation dates if you want. Check the ‘RXTE’ box in section 2, and then in section 3 make sure that ‘Archived data and observations’ is the only box checked. You can take a look at proposed observations if you want, but if you are looking for data that information will only clutter up your screen.

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want the public data. So choose ‘More Options’ at the bottom, and uncheck all of the choices except ‘XTE Public Archived Data.’ Now submit your search, and it should return all of the public archived observations of your object (note: the site is often slow). Browse now shows you the ObsID for each one, the start and stop times, and some other info. Check which observations you want to retrieve.

There are a wide range of data products available, ranging from GIF previews of light curves and spectra to full datasets, which include nearly all of the spacecraft telemetry during that time. The intermediate level products contain just the data from the instruments, along with a few bare bones files needed for analysis. Thanks to an update in w3Browse, many intermediate level observations can now be tarred into a single file. Yay! So intermediate level products are now the best choice.

Browse will now create a tar file (or files) from all of the data sets you selected, which can take a while. This tar file can now be downloaded and stored in your data directory. When extracted, it will create the directory hierarchy from the observing cycle (if necessary) on down through the proposer ID and individual ObsIDs. Note that the proposer ID directory will probably NOT have an FITS Master Index (FMI) file, you will have to generate one (explained below).

B.1.3 Data Files and Directory Structures

Previous X-ray observatories such as ROSAT and ASCA stored their data in just a few files. However, because of the numerous instruments on board RXTE and various operating modes, as well as the high time resolution of those instruments, the data

files which come from an observation are often large in both size and complexity. The top level of directories is based on the proposer ID number for each observa- tion. These are labeled as P followed by a number. One of our proposer numbers is 70162, so any data from one of our observations would be in the directory P70162. Below that are the ObsID directories, which are the ID numbers for each individual observation. They have a form such as 70162-01-07-00. So an individual observa- tion directory would be like: P70162/70162-01-07-00. There can be many ObsID’s for each proposal directory. Note that if you have data spanning multiple observing cycles (years), it may be in separate proposer ID directories. These must be analyzed separately, and then you can combine the data later by putting the names of the light curves in a list, and giving that to lcurve. (§B.3.1.2)

The actual data from the observation are stored within each ObsID directory. The ObsID directory contains several subdirectories, such as pca, hexte, orbit, and so on. These directories contain all of the data files for those instruments, as well as the position of the spacecraft during the observation, and other important information. Methods of finding which files you need will be discussed later.

Within each ObsID directory, there should be an FMI file. The FMI is somewhat similar to a traditional FITS header, in that it contains standard information about the observation. However, it also contains pointers to all of the data files. Each directory, including the proposer ID directory, should have an FMI file. Chances are good that if you downloaded your data from the archive, each ObsID will have an

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FMI, but the proposer ID directory won’t. So how do you make an FMI file? You use the recofmiftool.

recofmineeds an FMI file to reconstruct, so if there isn’t one in the directory you want, just copy one from any place (it doesn’t matter where). The format you use is:

]$ recofmi level=N delete=yes dirpath

wheredirpathis the path to the directory whose FMI file you want to make,level=N

is the number of levels you want it to go down (typically N=1), anddelete=yestells it to overwrite the old FMI information. You should have FMI files in each directory in your data hierarchy, down through the proposer ID level, down to the individual ObsID directories.

B.1.4 Background Issues

The PCA is a non-imaging detector with a field of view of approximately 1 degree. It collects all X-rays from a 1 square degree patch of sky, whether they come from your object or not. Because of the non-imaging nature of the PCA, background can’t simply be subtracted from a sky annulus as in optical photometry. Also, there are other sources of noise caused by the South Atlantic Anomaly (SAA), and the decay of various radioactive elements within the spacecraft itself.

Because of this, care must be taken to ensure proper background subtraction. Models are generated using blank-sky and earth pointings, and are distributed at the HEASARC website. For the latest news on background models and subtraction, please refer to the PCA Digest.