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

Abstract

We discovered Fe $K_{\alpha }$ 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 $\Gamma =0.4\pm 0.2$. 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 $K_{\alpha }$ 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.

arXiv comment9 pages, six figures, accepted for publication in A&A

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