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Register a new userEvidence for a 304-day Orbital Period for GX 1+4
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In this paper we report strong evidence for a ~304-day periodicity in the spin history of the accretion-powered pulsar GX 1+4 that is very likely to be a signature of the orbital period of the system. Using BATSE public-domain data, we show a highly-significant periodic modulation of the pulsar frequency from 1991 to date which is in excellent agreement with the ephemeris proposed by Cutler, Dennis & Dolan in 1986, which were based on a few events of enhanced spin-up that occurred during the pulsars spin-up era in the 1970s. Our results indicate that the orbital period of GX 1+4 is 303.8+-1.1 days, making it by far the widest low-mass X-ray binary system known. A likely scenario for this system is an elliptical orbit in which the neutron star decreases its spin-down rate (or even exhibits a momentary spin-up behavior) at periastron passages due to the higher torque exerted by the accretion disk onto the magnetosphere of the neutron star.
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We report on the RXTE detection of a sudden increase in the absorption column density, $N_mathrm{H}$, during the 2011 May outburst of GX 304-1. The $N_mathrm{H}$ increased up to ${sim}16times 10^{22}$ atoms cm$^{-2}$, which is a factor of 3-4 larger than what is usually measured during the outbursts of GX 304-1 as covered by RXTE. Additionally, an increase in the variability of the hardness ratio as calculated from the energy resolved RXTE-PCA light curves is measured during this time range. We interpret these facts as an occultation event of the neutron star by material in the line of sight. Using a simple 3D model of an inclined and precessing Be disk around the Be type companion, we are able to qualitatively explain the $N_mathrm{H}$ evolution over time. We are able to constrain the Be-disk density to be on the order of $10^{-11}$ g cm$^{-3}$. Our model strengthens the idea of inclined Be disks as origin of double-peaked outbursts as the derived geometry allows accretion twice per orbit under certain conditions.
We report optical modulation of the companion to the X-ray source U18 in the globular cluster NGC 6397. U18, with combined evidence from radio and X-ray measurements, is a strong candidate as the second redback in this cluster, initially missed in pulsar searches. This object is a bright variable star with an anomalous red color and optical variability (sim 0.2 mag in amplitude) with a periodicity sim 1.96 days that can be interpreted as the orbital period. This value corresponds to the longest orbital period for known redback candidates and confirmed systems in Galactic globular clusters and one of the few with a period longer than 1 day.
Context. About 120 Be/X-ray binaries (BeXBs) are known in the Small Magellanic Cloud (SMC); about half of them are pulsating with periods from a few to hundreds of seconds. SXP 1323 is one of the longest-period pulsars known in this galaxy. Aims. SXP 1323 is in the field of view of a large set of calibration observations that we analyse systematically, focusing on the time analysis, in search of periodic signals. Methods. We analyse all available X-ray observations of SXP 1323 from Suzaku, XMM-Newton, and Chandra, in the time range from 1999 to the end of 2016. We perform a Lomb-Scargle periodogram search in the band 2.5-10 keV on all observations to detect the neutron star spin period and constrain its long-term evolution. We also perform an orbital period search on the long-term light curve, merging all datasets. Results. We report the discovery of a 26.188+-0.045 d period analysing data from Suzaku, XMM-Newton, and Chandra, which confirms the optical period derived from the Optical Gravitational Lensing Experiment (OGLE) data. If this corresponds to the orbital period, this would be very short with respect to what is expected from the spin/orbital period relationship. We furthermore report on the spin period evolution in the last years. The source is spinning-up with an average rate of Pdot/P of 0.018 yr-1, decreasing from 1340 to 1100 s, in the period from 2006 to the end of 2016, which is also extreme with respect to the other Be/X-ray pulsars. From 2010 to the end of 2014, the pulse period is not clearly detectable, although the source was still bright. Conclusions. SXP 1323 is a peculiar BeXB due to its long pulse period, rapid spin-up for several years, and short orbital period. A continuous monitoring of the source in the next years is necessary to establish the long-term behaviour of the spin period.
We investigate variability of optical and near-infrared light curves of the X-ray binary GX 339-4 on a timescale of days. We use the data in four filters from six intervals corresponding to the soft state and from four intervals corresponding to the quiescent state. In the soft state, we find prominent oscillations with the average period P = 1.772 $pm$ 0.003 d, which is offset from the measured orbital period of the system by 0.7 per cent. We suggest that the measured periodicity originates from the superhumps. In line with this interpretation we find no periodicity in the quiescent state. The obtained period excess $epsilon$ is below typical values found for cataclysmic variables for the same mass ratio of the binary. We discuss implications of this finding in the context of the superhump theory.
We report on the discovery of a dramatic change in the energy spectrum of the X-ray pulsar GX 304-1 appearing at low luminosity. Particularly, we found that the cutoff power-law spectrum typical for accreting pulsars, including GX 304-1 at higher luminosities of $L_{rm X}sim 10^{36} - 10^{37}$ erg s$^{-1}$, transformed at lower luminosity of $L_{rm X}sim 10^{34}$ erg s$^{-1}$ to a two-component spectrum peaking around 5 and 40 keV. We suggest that the observed transition corresponds to a change of the dominant mechanism responsible for the deceleration of the accretion flow. We argue that the accretion flow energy at low accretion rates is released in the atmosphere of the neutron star, and the low-energy component in the source spectrum corresponds to the thermal emission of the optically thick, heated atmospheric layers. The most plausible explanations for the high-energy component are either the cyclotron emission reprocessed by the magnetic Compton scattering or the thermal radiation of deep atmospheric layers partly Comptonized in the overheated upper layers. Alternative scenarios are also discussed.
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