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% (3-sigma
errors) with a polarization angle of PA = -41 +/- 16 degree (3-sigma errors).
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 reflected off the disk, consistent with the
other NS atoll sources.
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