Discovery of periodicities in two highly variable intermediate polars towards the Galactic Center

Abstract

We discovered Fe $K_{\hat{I}}\pm$ complex emission and pulsation in two highly variable sources (4XMM J174917.7--283329, 4XMM J174954.6--294336). The equivalent widths of 6.4 and 6.7 keV lines of 4XMM J174917.7--283329 are $99_{-72}^{+84}$ and $220_{-140}^{+160}$ eV, respectively. The continuum is fitted by a partially absorbed apec model with plasma temperature of $kT=13_{-2}^{+10}$ keV. The inferred mass of the white dwarf (WD) is $0.9_{-0.2}^{+0.3}{M}_{\odot }$. We detected pulsations with a period of $1212\pm 3$ s and a pulsed fraction of $26\pm 6\%$. The light curves of 4XMM J174954.6--294336 display asymmetric eclipse and dipping behaviour. To date, this is only the second intermediate polar (IP) that shows a total eclipse in X-rays. The spectrum of the sources is characterized by a power-law model with photon index $\hat{I}“=0.4\pm 0.2$. The equivalent widths of the 6.4 keV and 6.7 keV iron lines are $171_{-79}^{+99}$ and $136_{-81}^{+89}$ eV, respectively. The continuum is described by emission from optically thin plasma with a temperature of $kT\sim 35$ keV. The inferred mass of the WD is $1.1_{-0.3}^{+0.2}{M}_{\odot }$. We discovered coherent pulsations from the source with a period of $1002\pm 2$ s. The pulsed fraction is $66\pm 15\%$. The measured spin period, hard photon index, and equivalent width of the fluorescent Fe $K_{\hat{I}}\pm$ line in both sources are consistent with the values found in IP. While 4XMM J174954.6--294336 was already previously classified as an IP, we also suggest 4XMM J174917.7--283329 as a new IP. The X-ray eclipses in 4XMM J174954.6--294336 are most likely caused by a low-mass companion star obscuring the central X-ray source. The asymmetry in the eclipse is likely caused by a thick bulge that intercepts the line of sight during the ingress phase but not during the egress phase located behind the WD along the line of sight.

9 pages, six figures, accepted for publication in A&A