A ROSAT observation of the dipping, flaring and eclipsing polar MN Hya (RX J0929.1-2404

Abstract

We present the results of a 22.5 ks pointed ROSAT PSPC observation of the 3.4-h period eclipsing polar MN Hya (RX J0929.112404). The X-ray light curve exhibits a ‘doublehumped’ shape, with a secondary minimum occuring at f ,0.45, a morphology consistent with two-pole accretion. Strong aperiodic flaring activity, with flux enhancements of ,6× the quiescent level, is also observed. A pre-eclipse ‘dip’ occurs in the phase interval f = 0.87–0.95 with the X-rays becoming harder, indicative of photoelectric absorption by the pre-shock flow. There is also evidence of a secondary spectrally hard ‘dip’ near f 1⁄4 0:45–0:55, which might be associated with a second accretion stream flowing to the other magnetic pole. The X-ray spectrum is best represented by a combination of a ,50 eV blackbody and a thermal bremsstrahlung component of kT *1.6 keV, with a total absorption column of NH 1⁄4 2:9 × 10cm. The primary maximum (f ,0.65) has a slightly larger column and normalization compared to the secondary maximum. Although there are few photons, the dip spectrum is very flat in comparison to other phases, and is best represented by a single bremsstrahlung component. This is indicative of the spectral hardening seen in the light curves attributed to photoabsorption. The ratio of unabsorbed bremsstrahlung and blackbody luminosities is ,0.1 for the bestfitting average spectral models. This implies a magnetic field strength &30 MG on the basis of the empirical Lhard=Lsoft 1 B relationships, although consideration of the cyclotron flux and aspect effects could allow for an even higher field (& 55 MG).