NuSTAR and TESS Discovery of an Intermediate Polar IGR J19173+0747

Abstract IGR J19173+0747 was discovered as an X-ray source by the INTEGRAL satellite and was initially classified as either a high-mass X-ray binary (HMXB) or a magnetic cataclysmic variable. Here, we report the detection of a coherent X-ray pulsation with a period of 792.9 ± 1.1 s in recent NuSTAR data, which we interpret as the spin period of the central compact object. Based on TESS optical photometry, we have also identified a periodic modulation of 793.38 ± 0.02 s, a value consistent, within uncertainties, with that determined from the X-ray data. This period lies outside the typical orbital period range for polars but falls within the characteristic spin-period range of intermediate polars (IPs). Furthermore, the X-ray spectrum exhibits a clear emission-line feature dominated by a fluorescent Fe K α line at 6.4 keV. We derive an X-ray luminosity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">X</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>1.3</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.56</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.88</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>34</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">erg</mml:mi> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> and an absolute G -band magnitude of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>G</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>5.6</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.56</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.61</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> for IGR J19173+0747, both of which are consistent with typical values for IPs. Based on these findings, we rule out both the HMXB and polar scenarios and classify IGR J19173+0747 as an IP. Spectral fitting yields a white dwarf mass estimate of 0.948 ± 0.044 M ⊙ for this source.