Kavli Affiliate: Roger W. Romani
| First 5 Authors: Pavan R. Hebbar, Craig O. Heinke, D. Kandel, Roger W. Romani,
| Summary:
Redback millisecond pulsars (MSPs) typically show pronounced orbital
variability in their X-ray emission due to our changing view of the intrabinary
shock (IBS) between the pulsar wind and stellar wind from the companion. Some
redbacks ("transitional" MSPs) have shown dramatic changes in their
multiwavelength properties, indicating a transition from a radio pulsar state
to an accretion-powered state. The redback MSP 47 Tuc W showed clear X-ray
orbital variability in the Chandra ACIS-S observations in 2002, which were not
detectable in the longer Chandra HRC-S observations in 2005-06, suggesting that
it might have undergone a state transition. However, the Chandra observations
of 47 Tuc in 2014-15 show similar X-ray orbital variability as in 2002. We
explain the different X-ray light-curves from these epochs in terms of two
components of the X-ray spectrum (soft X-rays from the pulsar, vs. harder
X-rays from the IBS), and different sensitivities of the X-ray instruments
observing in each epoch. However, when we use our best-fit spectra with HRC
response files to model the HRC light-curve, we expect a more significant and
shorter dip than that observed in the 2005-06 Chandra data. This suggests an
intrinsic change in the IBS of the system. We use the ICARUS stellar modelling
software, including calculations of heating by an IBS, to model the X-ray,
optical, and UV light-curves of 47 Tuc W. Our best-fitting parameters point
towards a high-inclination system (i~60 deg), which is primarily heated by the
pulsar radiation, with an IBS dominated by the companion wind momentum.
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