Kavli Affiliate: Herman L. Marshall
| First 5 Authors: Unnati Kashyap, Thomas J. Maccarone, Thomas D. Russell, Mason Ng, Swati Ravi
| Summary:
We report the first X-ray and radio polarimetric results of the neutron star
(NS) low-mass X-ray binary (LMXB) atoll-source 4U 1728-34 using the Imaging
X-ray Polarimetry Explorer (IXPE) and Australia Telescope Compact Array (ATCA).
We discovered that the X-ray source was polarized at PD = 1.9 +/- 1.0% with a
polarization angle of PA = -41 +/- 16 degree. Simultaneous Neutron Star
Interior Composition Explorer (NICER) observations show that the source was in
a relatively hard state, marking it as the first IXPE observation of an NS
atoll source in the hard state. We do not detect any significant linear
polarization (LP) in the radio band, with a 3-sigma upper limit of 2% at 5.5
GHz and 1.8% at 9 GHz. Combining the radio datasets provides the deepest upper
limits on the radio polarization at < 1.5% on the linear and circular
polarization (measured at 7.25 GHz). The X-ray polarimetric results suggest a
source geometry with a Comptonization component possibly attributed to a
boundary layer (BL) emission or BL emission reflected off the disk, consistent
with the other NS atoll sources.
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