UV and X-ray pulse amplitude variability in the transitional millisecond pulsar PSR J1023+0038
Journal
Date Issued
2022
Author(s)
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Coti Zelati, F.
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Illiano, G.
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Baglio, M. C.
Abstract
The transitional millisecond pulsar PSR\,J1023+0038 is the first millisecond
pulsar discovered to emit UV and optical pulses. Here we present the results of
the UV and X-ray phase-resolved timing analysis of observations performed with
the Hubble Space Telescope, \textit{XMM-Newton} and NuSTAR satellites between
2014 and 2021. Ultraviolet pulsations are detected in the high luminosity mode
and disappear during low and flaring modes, similar to what is observed in the
X-ray band. In the high mode, we find variability in both the UV and X-ray
pulse amplitudes. The root mean square pulsed amplitude in the UV band ranges
from $\sim$2.1\% down to $\sim$0.7\%, while it oscillates in the interval
$5.5-12\%$ in the X-ray band. This variability is not correlated with the
orbital phase, like what has been observed in the optical band. Notwithstanding
the rather low statistics, we have marginal evidence that variations in the
pulse amplitude do not occur simultaneously in the UV and X-ray bands. When the
UV pulsed amplitude decreases below the detection threshold, no significant
variation in the X-ray pulsed amplitude is observed. These oscillations in the
pulse amplitude could be caused by small random variations in the mass
accretion rate leading to a variation in the size of the intra-binary shock
region. Finally, we find that the pulsed flux spectral distribution from the
X-ray to the UV band is well fitted using a power-law relation of the form $\nu
F_{\nu}^{pulsed} \sim \nu^{0.4}$. This supports the hypothesis of a common
physical mechanism underlying the X-ray, UV, and optical pulsed emissions in
PSR\,J1023+0038.
Volume
660
Start page
A63
Issn Identifier
0004-6361
Rights
open.access
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