Spectral and Timing Analysis of the accretion-powered pulsar 4U 1626-67 observed with Suzaku and NuSTAR. (arXiv:1905.05356v1 [astro-ph.HE])

Spectral and Timing Analysis of the accretion-powered pulsar 4U 1626-67 observed with Suzaku and NuSTAR. (arXiv:1905.05356v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Iwakiri_W/0/1/0/all/0/1">Wataru B. Iwakiri</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pottschmidt_K/0/1/0/all/0/1">Katja Pottschmidt</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Falkner_S/0/1/0/all/0/1">Sebastian Falkner</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hemphill_P/0/1/0/all/0/1">Paul B. Hemphill</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fuerst_F/0/1/0/all/0/1">Felix Fuerst</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nishimura_O/0/1/0/all/0/1">Osamu Nishimura</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schwarm_F/0/1/0/all/0/1">Fritz-Walter Schwarm</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wolff_M/0/1/0/all/0/1">Michael T. Wolff</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Marcu_Cheatham_D/0/1/0/all/0/1">Diana M. Marcu-Cheatham</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chakrabarty_D/0/1/0/all/0/1">Deepto Chakrabarty</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tomsick_J/0/1/0/all/0/1">John A. Tomsick</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wilson_Hodge_C/0/1/0/all/0/1">Colleen A. Wilson-Hodge</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kuehnel_M/0/1/0/all/0/1">Matthias Kuehnel</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Terada_Y/0/1/0/all/0/1">Yukikatsu Terada</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Enoto_T/0/1/0/all/0/1">Teruaki Enoto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wilms_J/0/1/0/all/0/1">Joern Wilms</a>

We present an analysis of the spectral shape and pulse profile of the
accretion-powered pulsar 4U 1626-67 observed with Suzaku and NuSTAR during a
spin-up state. The pulsar, which experienced a torque reversal to spin-up in
2008, has a spin period of 7.7 s. Comparing the phase-averaged spectra obtained
with Suzaku in 2010 and with NuSTAR in 2015, we find that the spectral shape
changed between the two observations: the 3-10 keV flux increased by 5% while
the 30-60 keV flux decreased significantly by 35%. Phase-averaged and
phase-resolved spectral analysis shows that the continuum spectrum observed by
NuSTAR is well described by an empirical NPEX continuum with an added broad
Gaussian emission component around the spectral peak at 20 keV. Taken together
with the observed Pdot value obtained from Fermi/GBM, we conclude that the
spectral change between the Suzaku and NuSTAR observations was likely caused by
an increase of the accretion rate. We also report the possible detection of
asymmetry in the profile of the fundamental cyclotron line. Furthermore, we
present a study of the energy-resolved pulse profiles using a new relativistic
ray tracing code, where we perform a simultaneous fit to the pulse profiles
assuming a two-column geometry with a mixed pencil- and fan-beam emission
pattern. The resulting pulse profile decompositions enable us to obtain
geometrical parameters of accretion columns (inclination, azimuthal and polar
angles) and a fiducial set of beam patterns. This information is important to
validate the theoretical predictions from radiation transfer in a strong
magnetic field.

We present an analysis of the spectral shape and pulse profile of the
accretion-powered pulsar 4U 1626-67 observed with Suzaku and NuSTAR during a
spin-up state. The pulsar, which experienced a torque reversal to spin-up in
2008, has a spin period of 7.7 s. Comparing the phase-averaged spectra obtained
with Suzaku in 2010 and with NuSTAR in 2015, we find that the spectral shape
changed between the two observations: the 3-10 keV flux increased by 5% while
the 30-60 keV flux decreased significantly by 35%. Phase-averaged and
phase-resolved spectral analysis shows that the continuum spectrum observed by
NuSTAR is well described by an empirical NPEX continuum with an added broad
Gaussian emission component around the spectral peak at 20 keV. Taken together
with the observed Pdot value obtained from Fermi/GBM, we conclude that the
spectral change between the Suzaku and NuSTAR observations was likely caused by
an increase of the accretion rate. We also report the possible detection of
asymmetry in the profile of the fundamental cyclotron line. Furthermore, we
present a study of the energy-resolved pulse profiles using a new relativistic
ray tracing code, where we perform a simultaneous fit to the pulse profiles
assuming a two-column geometry with a mixed pencil- and fan-beam emission
pattern. The resulting pulse profile decompositions enable us to obtain
geometrical parameters of accretion columns (inclination, azimuthal and polar
angles) and a fiducial set of beam patterns. This information is important to
validate the theoretical predictions from radiation transfer in a strong
magnetic field.

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