2. X-ray observations
The location of RX J0719.2+6557 was scanned in the RASS during September 22-24, 1990 for a total exposure time of 570 sec. RX J0719.2+6557 is found as a source with a total of 66 photons, which corresponds to a vignetting corrected countrate of 0.16 cts/s in the ROSAT PSPC. No strong variability in the X-ray intensity is seen at this statistics, in particular also no orbital variations. The spectrum as derived from these 66 photons is rather hard, extending up to 2.4 keV (the upper bound of the PSPC). The standard hardness ratio (HR1 is the normalized count difference (N - N )/(N + N ), where N denotes the number of counts in the PSPC between channel a and channel b, and HR2 is similarly defined as (N - N )/N with the count number divided by hundred corresponding roughly to the energy in keV) values are HR1 = 0.48 0.12 and HR2 = 0.50 0.12. A thermal bremsstrahlung model gives a reasonable fit for a rather wide range of temperatures (2-15 keV). For a fixed temperature of kT=10 keV the best fit absorbing column is 1.6 1020 cm-2, and the unabsorbed flux is 2 10-12 erg/cm2 /s in the 0.1-2.4 keV range. This is a lower limit because any, even very weak, soft component as usually found in polars would require a higher absorbing column to be compatible with the observed X-ray spectrum. RX J0719.2+6557 has not been covered by any (serendipitous) ROSAT pointing until the time of this writing, so that the X-ray parameters cannot be improved further.
Given the rather low absorbing column (which varies by only 30% for models with different temperature) as compared to the total galactic column in the direction of RX J0719.2+6557 of 4.6 1020 cm-2 (Dickey & Lockman 1990) the distance of RX J0719.2+6557 is possibly only about 100 pc. Again, this is a lower limit in the sense that any (expected, but due to statistics and background contamination not detectable) soft blackbody-like component would result in an increase of the absorbing column and thus an increased distance. At this distance the (lower limit of the) phase-averaged, unabsorbed X-ray luminosity is 2.5 10 erg/s in the 0.1-2.4 keV range.
© European Southern Observatory (ESO) 1997
Online publication: March 26, 1998