Is superorbital modulation in SMC X-1 caused by absorption in warped precessing accretion disc?. (arXiv:2004.00664v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Pradhan_P/0/1/0/all/0/1">Pragati Pradhan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Maitra_C/0/1/0/all/0/1">Chandreyee Maitra</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Paul_B/0/1/0/all/0/1">Biswajit Paul</a>
We present a broadband spectral-timing analysis of SMC X-1 at different
intensity states of its super-orbital variation using 10 Suzaku and 6 NuSTAR
observations. The spectrum in all the states can be described by an absorbed
powerlaw with a high energy cutoff and a black-body component along with an
iron emission line. Compared to other supergiant HMXBs, the Fe K$alpha$ line
equivalent width is low in SMC X-1 – from less than 10 eV in high state to up
to $sim$ 270 eV in the low states. The spectral shape is dependent on flux
with the hard X-ray spectrum steepening with increasing flux. We also report a
highly variable normalisation of the power-law component across these 16
super-orbital states. Pulsations in the hard X-rays for both the instruments
were detected in all but two observations. The pulse profiles are near
sinusoidal with two peaks and the relative intensity of the second peak
decreasing with decreasing luminosity. These findings suggest that the
super-orbital modulation in SMC X-1 is not caused by absorption in precessing
warped accretion disc alone and there are intrinsic changes in X-rays emanating
from the neutron star at different super-orbital states. We also note a
putative cyclotron line at $sim$ 50 keV in the nustar~spectra of three bright
states indicating a possible magnetic field of $sim$ 4.2 $times$ 10$^{12}$ G.
Finally, with the new pulse period measurements reported here, the time base
for the secular spin-up of SMC X-1 is increased by thirteen years and the
complete pulse period history shows a sudden change in the spin-up trend around
1995.
We present a broadband spectral-timing analysis of SMC X-1 at different
intensity states of its super-orbital variation using 10 Suzaku and 6 NuSTAR
observations. The spectrum in all the states can be described by an absorbed
powerlaw with a high energy cutoff and a black-body component along with an
iron emission line. Compared to other supergiant HMXBs, the Fe K$alpha$ line
equivalent width is low in SMC X-1 – from less than 10 eV in high state to up
to $sim$ 270 eV in the low states. The spectral shape is dependent on flux
with the hard X-ray spectrum steepening with increasing flux. We also report a
highly variable normalisation of the power-law component across these 16
super-orbital states. Pulsations in the hard X-rays for both the instruments
were detected in all but two observations. The pulse profiles are near
sinusoidal with two peaks and the relative intensity of the second peak
decreasing with decreasing luminosity. These findings suggest that the
super-orbital modulation in SMC X-1 is not caused by absorption in precessing
warped accretion disc alone and there are intrinsic changes in X-rays emanating
from the neutron star at different super-orbital states. We also note a
putative cyclotron line at $sim$ 50 keV in the nustar~spectra of three bright
states indicating a possible magnetic field of $sim$ 4.2 $times$ 10$^{12}$ G.
Finally, with the new pulse period measurements reported here, the time base
for the secular spin-up of SMC X-1 is increased by thirteen years and the
complete pulse period history shows a sudden change in the spin-up trend around
1995.
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