White Paper on GEMS Study of Polarized X-rays from Neutron Stars. (arXiv:1301.5514v2 [astro-ph.HE] UPDATED)

White Paper on GEMS Study of Polarized X-rays from Neutron Stars. (arXiv:1301.5514v2 [astro-ph.HE] UPDATED):
We examine the expected X-ray polarization properties of neutron-star X-ray
sources of various types, e.g., accretion and rotation powered pulsars,
magnetars, and low-mass X-ray binaries. We summarize the model calculations
leading to these expected properties. We describe how a comparison of these
with their observed properties, as inferred from GEMS data, will probe the
essential dynamical, electromagnetic, plasma, and emission processes in
neutron-star binaries, discriminate between models of these processes, and
constrain model parameters. An exciting goal is the first observational
demonstration in this context of the existence of vacuum resonance, a
fundamental quantum electrodynamical phenomenon first described in the 1930s.

To differentiate neutron star models by X-ray polarimetry. (arXiv:1302.1328v1 [astro-ph.HE])

To differentiate neutron star models by X-ray polarimetry. (arXiv:1302.1328v1 [astro-ph.HE]):
The nature of pulsar is still unknown because of non-perturbative effects of
the fundamental strong interaction, and different models of pulsar inner
structures are then suggested, either conventional neutron stars or quark
stars. Additionally, a state of quark-cluster matter is conjectured for cold
matter at supranuclear density, as a result pulsars could thus be quark-cluster
stars. Besides understanding different manifestations, the most important issue
is to find an effective way to observationally differentiate those models.
X-ray polarimetry would play an important role here. In this letter, we focus
on the thermal X-ray polarization of quark/quark-cluster stars. While the
thermal X-ray linear polarization percentage is typically higher than ~10% in
normal neutron star models, the percentage of quark/quark-cluster stars is
almost zero. It could then be an effective method to identify
quark/quark-cluster stars by soft X-ray polarimetry. We are therefore expecting
to detect thermal X-ray polarization in the coming decades.

X-ray plateaus followed by sharp drops in GRBs 060413, 060522, 060607A and 080330: Further evidences for central engine afterglow from Gamma-ray Bursts. (arXiv:1301.3975v1 [astro-ph.HE])

X-ray plateaus followed by sharp drops in GRBs 060413, 060522, 060607A and 080330: Further evidences for central engine afterglow from Gamma-ray Bursts. (arXiv:1301.3975v1 [astro-ph.HE]):
The X-ray afterglows of GRBs 060413, 060522, 060607A and 080330 are
characterized by plateaus that are followed by very sharp drops. An X-ray
plateau is interpretable within the framework of the external forward shock
model but the sharp drop is not. In this work we interpret these peculiar X-ray
afterglow data as the central engine afterglows from some magnetized central
engines, plausibly magnetars. In this model, the X-ray afterglows are powered
by the internal magnetic energy dissipation and the sudden drop is caused by
the collapse of the magnetar. Accordingly, the X-ray plateau photons should
have a high linear polarization, which can be tested by the future X-ray
polarimetry.

X-ray Polarization from Black Holes: GEMS Scientific White Paper. (arXiv:1301.1957v1 [astro-ph.HE])

X-ray Polarization from Black Holes: GEMS Scientific White Paper. (arXiv:1301.1957v1 [astro-ph.HE]):
We present here a summary of the scientific goals behind the Gravity and
Extreme Magnetism SMEX (GEMS) X-ray polarimetry mission's black hole (BH)
observing program. The primary targets can be divided into two classes:
stellar-mass galactic BHs in accreting binaries, and super-massive BHs in the
centers of active galactic nuclei (AGN). The stellar-mass BHs can in turn be
divided into various X-ray spectral states: thermal-dominant (disk), hard
(radio jet), and steep power-law (hot corona). These different spectral states
are thought to be generated by different accretion geometries and emission
mechanisms. X-ray polarization is an ideal tool for probing the geometry around
these BHs and revealing the specific properties of the accreting gas.

X-ray polarimetry as a new tool to discriminate reflection from absorption scenarios -- Predictions for MCG-6-30-15. (arXiv:1208.3314v1 [astro-ph.HE])

X-ray polarimetry as a new tool to discriminate reflection from absorption scenarios -- Predictions for MCG-6-30-15. (arXiv:1208.3314v1 [astro-ph.HE]):
We present modelling of X-ray polarisation spectra emerging from the two
competing scenarios that are proposed to explain the broad Fe K{\alpha} line in
the Seyfert 1 galaxy MCG-6-30-15. The polarisation signature of complex
absorption is studied for a partial covering scenario using a clumpy wind and
compared to a reflection model based on the lamp-post geometry. The shape of
the polarisation percentage and angle as a function of photon energy are found
to be distinctly different between the reflection and the absorption case.
Relativistic reflection produces significantly stronger polarisation in the
1-10 keV energy band than absorption. The spectrum of the polarisation angle
adds additional constraints: in the absorption case it shows a constant shape,
whereas the relativistic reflection scenario typically leads to a smooth
rotation of the polarisation angle with photon energy. Based on this work, we
conclude that a soft X-ray polarimeter on-board a small X-ray satellite may
already discriminate between the absorption and the reflection scenario. A
promising opportunity may arise with the X-ray Imaging Polarimetry Explorer
(XIPE) mission, which has been proposed to ESA in response to a small-size
(S-class) mission call due for launch in 2017.