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Register a new userThe X-ray Pulsar 2A 1822-371 as a Super Eddington source
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Added by
Ann-Sofie Bak Nielsen
Publication date
2017
fields
Physics
and research's language is
English
Authors
A. Bak Nielsen
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The low mass X-ray binary 2A 1822-371 is an eclipsing system with an accretion disc corona and with an orbital period of 5.57 hr. The primary is an 0.59 s X-ray pulsar with a proposed strong magnetic field of 10^10-10^12 G. In this paper we study the spin evolution of the pulsar and constrain the geometry of the system. We find that, contrary to previous claims, a thick corona is not required, and that the system characteristics could be best explained by a thin accretion outflow due to a super-Eddington mass transfer rate and a geometrically thick inner accretion flow. The orbital, spectral and timing observations can be reconciled in this scenario under the assumption that the mass transfer proceeds on a thermal timescale which would make 2A 1822-371, a mildly super-Eddington source viewed at high inclination angles. The timing analysis on 13 years of RXTE data show a remarkably stable spin-up that implies that 2A 1822-371, might quickly turn into a millisecond pulsar in the next few thousand years.
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The X-ray source 4U 1822-371 is an eclipsing low-mass X-ray binary and X-ray pulsar, hosting a NS that shows periodic pulsations in the X-ray band. The inclination angle of the system is so high that in principle, it should be hard to observe both the direct thermal emission of the central object and the reflection component of the spectrum because they are hidden by the outer edge of the accretion disc. Assuming that the source accretes at the Eddington limit, we analysed non-simultaneous XMM-Newton and NuSTAR observations and studied the average broadband spectrum, with the aim to investigate the presence of a reflection component. No such component has been observed before in a high-inclination source such as 4U 1822-371. We modelled the spectral emission of the source using two different reflection models, Diskline plus Pexriv and the self-consistent model RfxConv. In our analysis, we find significant evidence of a reflection component in the spectrum, in addition to two lines associated with neutral or mildly ionised iron. The continuum spectrum is well fitted by a saturated Comptonisation model and a thermal black-body component emitted by the accretion disc at a lower temperature. We updated the ephemeris, adding two new eclipse times to the most recent ephemeris reported in literature. In our proposed scenario, the source is accreting at the Eddington limit with an intrinsic luminosity of $10^{38}$ erg/s, while the observed luminosity is two orders of magnitude lower. Despite the high inclination, we find that a reflection component is required to fit residuals at the Fe line range and the hard excess observed in the spectrum. The best-fit value of the inner disc radius is still uncertain and model dependent. More observations are therefore needed to confirm these results, which can give important information on this enigmatic and peculiar source.
The source X 1822-371 is an eclipsing compact binary system with a period close to 5.57 hr and an orbital period derivative $dot{P}_{rm orb}$ of 1.51(7)$times 10^{-10}$ s s$^{-1}$. The very large value of $dot{P}_{rm orb}$ is compatible with a super-Eddington mass transfer rate from the companion star, as suggested by X-ray and optical data. The XMM-Newton observation taken in 2017 allows us to update the orbital ephemeris and verify whether the orbital period derivative has been stable over the last 40 yr. We added to the X-ray eclipse arrival times from 1977 to 2008 two new values obtained from the RXTE and XMM-Newton observations performed in 2011 and 2017, respectively. We estimated the number of orbital cycles and the delays of our eclipse arrival times spanning 40 yr using as reference time the eclipse arrival time obtained from the Rossi-XTE observation taken in 1996. Fitting the delays with a quadratic model, we found an orbital period $P_{rm orb}=5.57062957(20)$ hr and a $dot{P}_{rm orb}$ value of $1.475(54) times 10^{-10}$ s s$^{-1}$. The addition of a cubic term to the model does not significantly improve the quality of the fit. We also determined a spin-period value of $P_{rm spin}=0.5915669(4)$ s and its first derivative $dot{P}_{rm spin}= -2.595(11) times 10^{-12}$ s s$^{-1}$. The obtained results confirm the scenario of a super-Eddington mass transfer rate; we also exclude a gravitational coupling between the orbit and the change in the oblateness of the companion star triggered by the nuclear luminosity of the companion star.
We report our measurements for orbital and spin parameters of X 1822-371 using its X-ray partial eclipsing profile and pulsar timing from data collected by the Rossi X-ray Timing Explorer (RXTE). Four more X-ray eclipse times obtained by the RXTE 2011 observations were combined with historical records to trace evolution of orbital period. We found that a cubic ephemeris likely better describes evolution of the X-ray eclipse times during a time span of about 34 years with a marginal second order derivative of $ddot{P}_{orb}=(-1.05 pm 0.59) times 10^{-19}$ s$^{-1}$. Using the pulse arrival time delay technique, the orbital and spin parameters were obtained from RXTE observations from 1998 to 2011. The detected pulse periods show that the neutron star in X 1822-371 is continuously spun-up with a rate of $dot{P}_{s}=(-2.6288 pm 0.0095) times 10^{-12}$ s s$^{-1}$. Evolution of the epoch of the mean longitude $l=pi /2$ (i.e. $T_{pi / 2}$) gives an orbital period derivative value consistent with that obtained from the quadratic ephemeris evaluated by the X-ray eclipse but the detected $T_{pi / 2}$ values are significantly and systematically earlier than the corresponding expected X-ray eclipse times by $90 pm 11$ s. This deviation is probably caused by asymmetric X-ray emissions. We also attempted to constrain the mass and radius of the neutron star using the spin period change rate and concluded that the intrinsic luminosity of X 1822-371 is likely more than $10^{38}$ ergs s$^{-1}$.
Studies of X-ray continuum emission and flux variability have not conclusively revealed the nature of ultra-luminous X-ray sources (ULXs) at the high-luminosity end of the distribution (those with Lx > 1e40 erg/s). These are of particular interest because the luminosity requires either super-Eddington accretion onto a black hole of mass ~10 Msun, or more standard accretion onto an intermediate-mass black hole. Super-Eddington accretion models predict strong outflowing winds, making atomic absorption lines a key diagnostic of the nature of extreme ULXs. To search for such features, we have undertaken a long, 500 ks observing campaign on Holmberg IX X-1 with Suzaku. This is the most sensitive dataset in the iron K bandpass for a bright, isolated ULX to date, yet we find no statistically significant atomic features in either emission or absorption; any undetected narrow features must have equivalent widths less than 15-20 eV at 99% confidence. These limits are far below the >150 eV lines expected if observed trends between mass inflow and outflow rates extend into the super-Eddington regime, and in fact rule out the line strengths observed from disk winds in a variety of sub-Eddington black holes. We therefore cannot be viewing the central regions of Holmberg IX X-1 through any substantial column of material, ruling out models of spherical super-Eddington accretion. If Holmberg IX X-1 is a super-Eddington source, any associated outflow must have an anisotropic geometry. Finally, the lack of iron emission suggests that the stellar companion cannot be launching a strong wind, and that Holmberg IX X-1 must primarily accrete via roche-lobe overflow.
It has been known for nearly three decades that the energy spectra of thermonuclear X-ray bursts are often well-fit by Planck functions with temperatures so high that they imply a super-Eddington radiative flux at the emitting surface, even during portions of bursts when there is no evidence of photospheric radius expansion. This apparent inconsistency is usually set aside by assuming that the flux is actually sub-Eddington and that the fitted temperature is so high because the spectrum has been distorted by the energy-dependent opacity of the atmosphere. Here we show that the spectra predicted by currently available conventional atmosphere models appear incompatible with the highest-precision measurements of burst spectra made using the Rossi X-ray Timing Explorer, such as during the 4U 1820-30 superburst and a long burst from GX 17+2. In contrast, these measurements are well-fit by Bose-Einstein spectra with high temperatures and modest chemical potentials. Such spectra are very similar to Planck spectra. They imply surface radiative fluxes more than a factor of three larger than the Eddington flux. We find that segments of many other bursts from many sources are well-fit by similar Bose-Einstein spectra, suggesting that the radiative flux at the emitting surface also exceeds the Eddington flux during these segments. We suggest that burst spectra can closely approximate Bose-Einstein spectra and have fluxes that exceed the Eddington flux because they are formed by Comptonization in an extended, low-density radiating gas supported by the outward radiation force and confined by a tangled magnetic field.
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