Simultaneous NICER and NuSTAR Observations of the Ultra-compact X-ray Binary 4U 1543-624. (arXiv:2012.10461v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Ludlam_R/0/1/0/all/0/1">R. M. Ludlam</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jaodand_A/0/1/0/all/0/1">A. D. Jaodand</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Garcia_J/0/1/0/all/0/1">J. A. García</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Degenaar_N/0/1/0/all/0/1">N. Degenaar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tomsick_J/0/1/0/all/0/1">J. A. Tomsick</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cackett_E/0/1/0/all/0/1">E. M. Cackett</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fabian_A/0/1/0/all/0/1">A. C. Fabian</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gandhi_P/0/1/0/all/0/1">P. Gandhi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Buisson_D/0/1/0/all/0/1">D. J. K. Buisson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shaw_A/0/1/0/all/0/1">A. W. Shaw</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chakrabarty_D/0/1/0/all/0/1">D. Chakrabarty</a>
We present the first joint NuSTAR and NICER observations of the ultra-compact
X-ray binary (UCXB) 4U 1543$-$624 obtained in 2020 April. The source was at a
luminosity of $L_{0.5-50 mathrm{keV}} = 4.9 (D/7
mathrm{kpc})^{2}times10^{36}$ ergs s$^{-1}$ and showed evidence of reflected
emission in the form of an O VIII line, Fe K line, and Compton hump within the
spectrum. We used a full reflection model, known as xillverCO, that is tailored
for the atypical abundances found in UCXBs, to account for the reflected
emission. We tested the emission radii of the O and Fe line components and
conclude that they originate from a common disk radius in the innermost region
of the accretion disk ($R_{rm in} leq1.07 R_{mathrm{ISCO}}$). By assuming
that the position of the inner disk is the Alfv'{e}n radius, we placed an
upper limit on the magnetic field strength to be $Bleq0.7(D/7 mathrm
{kpc})times10^{8}$ G at the poles. Given the lack of pulsations detected and
position of $R_{rm in}$, it was likely that a boundary layer region had formed
between the neutron star (NS) surface and inner edge of the accretion disk with
an extent of 1.2 km. This implies a maximum radius of the neutron star accretor
of $R_{mathrm{NS}}leq 12.1$ km when assuming a canonical NS mass of 1.4
$M_{odot}$.
We present the first joint NuSTAR and NICER observations of the ultra-compact
X-ray binary (UCXB) 4U 1543$-$624 obtained in 2020 April. The source was at a
luminosity of $L_{0.5-50 mathrm{keV}} = 4.9 (D/7
mathrm{kpc})^{2}times10^{36}$ ergs s$^{-1}$ and showed evidence of reflected
emission in the form of an O VIII line, Fe K line, and Compton hump within the
spectrum. We used a full reflection model, known as xillverCO, that is tailored
for the atypical abundances found in UCXBs, to account for the reflected
emission. We tested the emission radii of the O and Fe line components and
conclude that they originate from a common disk radius in the innermost region
of the accretion disk ($R_{rm in} leq1.07 R_{mathrm{ISCO}}$). By assuming
that the position of the inner disk is the Alfv'{e}n radius, we placed an
upper limit on the magnetic field strength to be $Bleq0.7(D/7 mathrm
{kpc})times10^{8}$ G at the poles. Given the lack of pulsations detected and
position of $R_{rm in}$, it was likely that a boundary layer region had formed
between the neutron star (NS) surface and inner edge of the accretion disk with
an extent of 1.2 km. This implies a maximum radius of the neutron star accretor
of $R_{mathrm{NS}}leq 12.1$ km when assuming a canonical NS mass of 1.4
$M_{odot}$.
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