UHURU4 - Uhuru Fourth (4U) Catalog

Overview

The Fourth Uhuru (4U) Catalog lists 339 X-ray sources that were observed with the Uhuru (SAS A) X-ray observatory. It contains positional information in the form of 90% confidence level error boxes, 2-6 keV intensities, possible optical and radio counterparts, and alternative names for sources observed in earlier compilations. The major classes of identified objects include binary stellar systems, supernova remnants, Seyfert galaxies, clusters of galaxies, and possibly the new class of superclusters of galaxies.

The Uhuru satellite was a scanning X-ray instrument with a narrow (1 by 10 degree FWFM) and a wide (10 by 10 degree FWFM) collimator (cf. Giacconi et al. 1971, ApJ, 165, L27). Typically, the scan rate was 0.5 degree/second, with the spin axis in one position for roughly one day. During the interval for which the spin axis was fixed, repeated scans were made of the same 10 by 360 degrees band of the sky. For this catalog, the individual scans were superposed using aspect data from an orthogonally mounted triad of magnetometers and a Sun sensor onboard the spacecraft, supplemented by observations of well-located X-ray sources. The observations employed in producing this catalog were obtained over a total of 429 days between 1970 December 12 and 1973 March 18, apart from a gap between 1972 July and December when the spacecraft's transmitter was operating improperly.

Catalog Bibcode

1978ApJS...38..357F

References

The Fourth Uhuru Catalog of X-Ray Sources, Forman, W., Jones, C., Cominsky, L., Julien, P., Murray, S., Peters, G., Tananbaum, H., and Giacconi, R. 1978, ApJS, 38, 357.

Provenance

This online catalog was created by the HEASARC in March 2003 based on a table obtained originally from the ADC website (https://heasarc.gsfc.nasa.gov/FTP/heasarc/dbase/misc_files/uhuru4/4u.dat), which was modified by the HEASARC to include some parameters that were in the published catalog (Table 1 in the reference) but were not in the ADC table. The HEASARC made a small number of other changes and corrections to the table which are listed in the HEASARC_Implementation section of this help. Notice that the ADC table itself differs in a number of respects from the published Table 1, e.g., the comments are sometimes abbreviated and/or different, some of the names are slightly different (usually by one digit in the RA minutes part), and three of the entries have differing names and positions.

HEASARC Implementation

The HEASARC modified the ADC version of this catalog in several ways as listed below. (Notice that the ADC table itself differs in a number of respects from the published catalog (given in Table 1 of the Forman et al. paper), e.g., the comments are sometimes abbreviated and/or different, some of the names are slightly different (usually by one digit in the RA minutes part), and three of the entries have differing names and positions).

A parameter (error_area) was added containing the area of the source error box, as well as a parameter (count_rate_error) containing the count rate error, and a parameter (max_to_min_ratio) containing the ratio of maximum to minimum count rates for highly variable sources. The values for these three parameters were obtained from the printed version of the 4U Catalog. The HEASARC also added flux and flux_error parameters, and flux_mcrab and flux_mcrab_error parameters, which were calculated using conversion factors as described in the help descriptions for these parameters.

The HEASARC changed all the source names which differed between the published and ADC table versions back to their as-published versions, except for the 3 sources with differing positions as well as names, for which the names and positions as listed in the ADC file were adopted rather than the published values:

     Published Version                   ADC/HEASARC Version
  Name       RA       Dec              Name       RA       Dec
            (1950 degrees)                       (1950 degrees)

4U0954+70   148.630   70.200          4U1004+70  151.200   70.900
4U1037-60   159.250  -60.900          4U1053-58  163.320  -58.080
4U1743-19   265.980  -19.990          4U1757-17  269.490  -17.490

Parameters

Name
The 4U Catalog source name, derived from the right ascension and declination (in 1950 coodinates).

RA
The position of the source maximum probability density in right ascension.

Dec
The position of the source maximum probability density in declination.

LII
The position of the source maximum probability density in galactic longitude.

BII
The position of the source maximum probability density in galactic latitude.

Error_Area
The area of the 90% confidence level error box, in square degrees.

Count_Rate
The average intensity of the lines of position used in the source position determination (for sources that were constant in intensity within the statistics of the measurement), or the maximum intensity (for sources that were observed to be variable), in Uhuru counts per second. In the latter cases, the max_to_min_ratio parameter contains the ratio of the maximum to minimum observed values. The intensities reported were derived only from positive source detections, and days on which a source was not detected are not included in the calculation of the average intensity; thus, the quoted values can substantially overestimate the true average intensities, particularly for weak sources.

Count_Rate_Error
The uncertainty in the average intensity of the lines of position used in the source position determination, in Uhuru counts per second. This is quoted only for sources that were constant in intensity within the statistics of the measurement

Max_To_Min_Ratio
The ratio of the maximum to minimum observed count rates. This is quoted only for sources that were observed to be variable.

Flux
The X-ray flux in the 2 - 6 keV band, in erg/cm²/s. This was calculated by the HEASARC using the conversion formula given in the published paper: 1.7 x 10^-11 erg/cm²/s = 1 Uhuru count/s. These values were computed for a Crab-like spectrum, but vary by only 10 - 20% for all but the most extreme source spectra. The intensities reported were derived only from positive source detections, and days on which a source was not detected are not included in the calculation of the average intensity; thus, the quoted values can substantially overestimate the true average intensities, particularly for weak sources.

Flux_Error
The error in the X-ray flux in the 2 - 6 keV band, in erg/cm²/s.

Flux_mCrab
The X-ray flux in the 2 - 6 keV band, in milliCrabs. This was calculated by the HEASARC using the conversion formula: 1.0559 milliCrab = 1 Uhuru count/s. The conversion factor was chosen so that the flux of the source 4U0531+21 associated with the Crab was 1000 milliCrabs.

Flux_mCrab_Error
The error in the X-ray flux in the 2 - 6 keV band, in milliCrabs.

Comments
This parameter contains the then-available information on possible source identifications, including optical, radio, and previous X-ray identifications. Information on the source type and (usually X-ray) periods is also sometimes given.

Contact Person

Questions regarding the UHURU4 database table can be addressed to the HEASARC Help Desk.

Page Author: Browse Software Development Team
Last Modified: Monday, 16-Sep-2024 17:35:55 EDT