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Register a new userSpectral analysis of SMC X-2 during its 2015 outburst
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Added by
Nicola La Palombara
Publication date
2016
fields
Physics
and research's language is
English
Authors
N. La Palombara
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We report on the results of Swift and XMM-Newton observations of SMC X-2 during its last outburst in 2015 October, the first one since 2000. The source reached a very high luminosity ($L sim 10^{38}$ erg s$^{-1}$), which allowed us to perform a detailed analysis of its timing and spectral properties. We obtained a pulse period $P_{rm spin}$ = 2.372267(5) s and a characterization of the pulse profile also at low energies. The main spectral component is a hard ($Gamma simeq 0$) power-law model with an exponential cut-off, but at low energies we detected also a soft (with kT $simeq$ 0.15 keV) thermal component. Several emission lines can be observed at various energies. The identification of these features with the transition lines of highly ionized N, O, Ne, Si, and Fe suggests the presence of photoionized matter around the accreting source.
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We report the Chandra/HRC-S and Swift/XRT observations for the 2015 outburst of the high-mass X-ray binary (HMXB) pulsar in the Small Magellanic Cloud, SMC X-2. While previous studies suggested that either an O star or a Be star in the field is the high-mass companion of SMC X-2, our Chandra/HRC-S image unambiguously confirms the O-type star as the true optical counterpart. Using the Swift/XRT observations, we extracted accurate orbital parameters of the pulsar binary through a time of arrivals (TOAs) analysis. In addition, there were two X-ray dips near the inferior conjunction, which are possibly caused by eclipses or an ionized high-density shadow wind near the companions surface. Finally, we propose that an outflow driven by the radiation pressure from day ~10 played an important role in the X-ray/optical evolution of the outburst.
We report on the spectral and timing properties of the accreting millisecond X-ray pulsar IGR J00291+5934 observed by XMM-Newton and NuSTAR during its 2015 outburst. The source is in a hard state dominated at high energies by a comptonization of soft photons ($sim0.9$ keV) by an electron population with kT$_esim30$ keV, and at lower energies by a blackbody component with kT$sim0.5$ keV. A moderately broad, neutral Fe emission line and four narrow absorption lines are also found. By investigating the pulse phase evolution, we derived the best-fitting orbital solution for the 2015 outburst. Comparing the updated ephemeris with those of the previous outbursts, we set a $3sigma$ confidence level interval $-6.6times 10^{-13}$ s/s $< dot{P}_{orb} < 6.5 times 10^{-13}$ s/s on the orbital period derivative. Moreover, we investigated the pulse profile dependence on energy finding a peculiar behaviour of the pulse fractional amplitude and lags as a function of energy. We performed a phase-resolved spectroscopy showing that the blackbody component tracks remarkably well the pulse-profile, indicating that this component resides at the neutron star surface (hot-spot).
We report on the results of the $XMM-Newton$ observation of IGR J01572-7259 during its most recent outburst in 2016 May, the first since 2008. The source reached a flux $f sim 10^{-10}$ erg cm$^{-2}$ s$^{-1}$, which allowed us to perform a detailed analysis of its timing and spectral properties. We obtained a pulse period $P_{rm spin}$ = 11.58208(2) s. The pulse profile is double peaked and strongly energy dependent, as the second peak is prominent only at low energies and the pulsed fraction increases with energy. The main spectral component is a power-law model, but at low energies we also detected a soft thermal component, which can be described with either a blackbody or a hot plasma model. Both the EPIC and RGS spectra show several emission lines, which can be identified with the transition lines of ionized N, O, Ne, and Fe and cannot be described with a thermal emission model. The phase-resolved spectral analysis showed that the flux of both the soft excess and the emission lines vary with the pulse phase: the soft excess disappears in the first pulse and becomes significant only in the second, where also the Fe line is stronger. This variability is difficult to explain with emission from a hot plasma, while the reprocessing of the primary X-ray emission at the inner edge of the accretion disk provides a realiable scenario. On the other hand, the narrow emission lines can be due to the presence of photoionized matter around the accreting source.
The high mass X-ray binary 4U 1901+03 was reported to have the pulse profile evolving with the X-ray luminosity and energy during its outburst in February-July 2003: the pulse peak changed from double to single along with the decreasing luminosity. We have carried out a detailed analysis on the contemporary phase-resolved energy spectrum of 4U 1901+03 as observed by Rossi X-ray Timing Explorer (RXTE). We find that, both the continuum and the pulse spectra are phase dependent. The optical depth derived from the pulse spectrum is in general larger than that from the continuum. Fe Ka emission line is only detected in the spectrum of the continuum and is missing in the pulse spectrum. This suggests an origin of Fe emission from the accretion disk but not the surface of the neutron star.
We investigate timing and spectral characteristics of the transient X-ray pulsar 2S 1417$-$624 during its 2018 outburst with emph{NICER} follow up observations. We describe the spectra with high-energy cut-off and partial covering fraction absortion (PCFA) model and present flux-dependent spectral changes of the source during the 2018 outburst. Utilizing the correlation-mode switching of the spectral model parameters, we confirm the previously reported sub-critical to critical regime transitions and we argue that secondary transition from the gas-dominated to the radiation pressure-dominated disc do not lead to significant spectral changes below 12 keV. Using the existing accretion theories, we model the spin frequency evolution of 2S 1417$-$624 and investigate the noise processes of a transient X-ray pulsar for the first time using both polynomial and luminosity-dependent models for the spin frequency evolution. For the first model, the power density spectrum of the torque fluctuations indicate that the source exhibits red noise component ($Gamma sim -2$) within the timescales of outburst duration which is typical for disc-fed systems. On the other hand, the noise spectrum tends to be white on longer timescales with high timing noise level that indicates an ongoing accretion process in between outburst episodes. For the second model, most of the red noise component is eliminated and the noise spectrum is found to be consistent with a white noise structure observed in wind-fed systems.
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