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The PI of the Sky is a collaboration among several institutions in Poland. The PI of the Sky project is a collection of instruments and software designed to detect optical transients. An optical transient is a source in the sky that flares up or appears and then fades to eventually become undetectable. While supernovae, novae, and cataclysmic variable star outbursts fall into this category of event, the term "optical transient" is customarily reserved for events that last only for a few seconds at most. While this does include flare stars, the primary motivation for the PI project is to attempt to detect the optical outbursts associated with Gamma Ray Bursts (GRBs). PI should be capable of detecting such events independently without reliance on gamma ray detectors in space.
The proposed system will consist of twin arrays of 16 cameras each. Each camera will have a focal length of 85 mm and a field of view of 22 degrees. The proposal calls for continuously imaging the entire sky using 10 s exposures. This system would generate about 5 TB of image data each night. A real-time data pipeline would search for optical transcients and save cutouts of the full images for interesting objects for subsequent analysis. The full image set will be stored only for 12 hours. During the daytime the system determines positions and brightness and lightcurve data for potential targets and objects of interest. Under optimum conditions, individual 10 s exposures should reach 12th magnitude, and a stack of 10 - 10 s exposures should reach 14th magnitude. Ultimately, it is intended to replicate this system at a number of observing locations around the planet. A prototype system is currently installed at Las Campanas Observatory (LCO) in Chile and has been in operation since July 2004. An upgraded system has been in operation since January 2006.
Las Campanas Observatory
Latitude
-29o 00' 12" (S)
Longitude
70o 42' 00" (W)
Elevation
2282 meters
Time Zones
CST / CDT ( - 4 H, W of Greenwich )
CST = Chilean Standard Time
As presently configured, the prototype system consists of two 50 mm lenses each with a field of view of 33 degrees. The system currently takes two scans of the entire observable sky each night in the early evening after evening twilight and again before morning twilight. The remaining time is devoted to targeted examination of fields of interest. These fields can consist of recent GCN announcements of GRBs, as well as shadowing the fields of view (FOV) for the gamma ray satellites Swift and Integral. PI will ultimately shadow the GLAST LAT fields as well after the successful launch of GLAST. The system is automatically scripted each night, but manual corrections can be added at any time. Under normal conditions, the system will autonomously obtain at least the two sky scans every evening.
The cameras of the prototype system
Beginning 2006 October 13, all objects in the GTN Object Catalog are included in the PI database as objects of interest. Those GTN object fields observable from Las Campanas will now be observed whenever they occur within the sky scans and whenever they occur close to FOV of one of the gamma ray satellites. If an object is detected having the specified coordinates, a magnitude is determined and archived. This data becomes part of the PI data archive.
Of course, most GTN objects are fainter than the 14th magnitude limit for PI, but some are not too faint. And, of course, the whole point of searching for optical transients is that we do not know for sure what objects may produce them, or when transients may occur.
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PI Observations of GTN Objects
This is the link to the data accumulated by PI for the GTN "objects of interest." This includes all objects in the GTN Object Catalog which can be observed from Las Campanas Observatory and which become bright enough to be detected by PI. The listing includes for each object the number of points currently available (magnitude determinations) along with the date for the most recent observation (LAST OBSERV.). Clicking on the object name will load a data summary page for the object.
Clicking on a LIGHT CURVES link will then display the interactive lightcurve tool for the object. Just below the plot are links to display the archived lightcurve data. The "plot's data" link will display HJD and magnitude values. The "detailed plot's data" link will display more extensive data which includes the calendar date and UT time for each point along with the field in which the object was located and a link to the image (when available). This data may be copied and pasted into an analysis tool such as Excel.
Clicking on the ALL FITS link will display a page with links to cutout images centered on the target objects. Images normally include JPEG and FITS versions.
New "interesting objects" can be added for PI to monitor and archive. (A user name and password are required to specify new objects.)
The PI of the Sky Project Website
Merely click on the home page to access the detailed information. This information includes a listing of publications related to the system. There are also examples of GRB followups and optical limits. There are also some possible unidentified optical transcients (OTs), and some variable star data which includes novae and flare star detections. There is also a link to the set of GTN pages where data is archived for GTN objects. Interactive lightcurves can be generated for these objects. The link to the GTN pages can be found on the "results" page.
A detailed description of the PI project is available as the Pi-Guide.
Public access to all data archived by the PI project is available through an online database. Data is released on an annual basis.
The PI project has also compiled all observations of short-period variable stars observed by PI during 2004 and 2005. This includes variable stars detected by PI with periods between 0.1 and 10 days. A total of 725 stars are included in the online database. All objects were re-classified by means of visual examination of the light curves. Periods were determined for 15 objects for which periods were previously unknown.
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PI searches for optical transients by comparing images of the same field and looking for objects which suddenly appear and were not detected on previous images. In order to accomplish this, PI finds all stars on an image and compares these detections with an archived catalog of objects previously detected in that field. All detected objects are assigned a unique PI ID number. If a new object appears, it is flagged as a tentitive transient to be examined by more sophisticated software and ultimately examined manually. A tentitive transient must be detected by more than one camera at the same time.
The raw images are retained only for approximately 12 hours. The detected objects and their derived magnitudes are added to the archived catalogs and become part of the permanent PI database.
The photometric system produces broad band magnitudes based on the detector sensitivity and a filter to minimize the infrared sensitivity. The magnitudes generated are approximate visual magnitudes that are somewhat red sensitive. No color corrections are applied. The photometric system and the techniques used to determine final magnitudes are the same as used for the All Sky Automated Survey (ASAS, US mirror). Both PI and ASAS are located in a shared enclosure at the Las Campanas Observatory
All PI data is based on 10 second exposures. The telescopes and cameras are only pointed to a specific set of field centers (RAs and DECs) that have been established for the sky visible from Las Campanas. These are termed FoV (fields of view). These fields overlap so that any sky location can usually be found in four or more separate FoVs. The fields are named using the approximate RA and DEC of the field center. For example, the native FoV (most centrally located) for the blazar OJ287 is 0840+20.
There are two separate databases. The scan database is derived from the two nightly all-sky scans taken just after evening twilight and just before morning twilight. The aver (for average) database is derived as the average of 10 individual PI exposures for each field observed. It is claimed that the aver database reaches 14th magnitude, while the scan databse likely reaches only 12th magnitude. The aver database is derived from pointed observations of "interesting" objects or shadowing the fields being observed by Swift or Integral (eventually GLAST as well). PI will automatically plan observations for each night including the standard scans and targeted observations of GRB related fields. The PI controlers can manually insert targeted observations for interesting objects when desired. The data is archived separately for the two telescopes (Unit A and Unit B).
Magnitude data, lightcurves, and images for the interesting objects may be examined and downloaded. Clicking on GTN under the interesting objects results listing takes one to the GTN pages. Here one can select either the scan or the aver database. The scan database is selected by default. These pages contain the object name and coordinates, typical magnitude, number of points available, and the date of most recent observations (LAST OBSERV).
Clicking on the object name from these pages takes one to the data details for the object. From here one can select the LIGHTCURVES from either unit or the cutout images from which each of the magnitude values has been derived (ALL FITS).
The lightcurve display may be changed using the boxes under the lightcurve plot. The data values may be displayed using the links immediately under the plot.
Selecting the links after ALL FITS will display a listing of all available cutout images. Images are available in both JPEG and FITS format. The JPEG images will just display in your browser. The FITS images will try to display in your browser, but if your browser is not configured to handle FITS images, it will neither be pretty nor useful. To download individual FITS images on the Macintosh use option-click on the FITS link. Under Windows use right-click.
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