Superorbital Modulation Research Articles

Firm detection of 7-year X-ray periodicity from X Persei

AbstractWe report on the detection of long-term X-ray periodicity from the Be/X-ray binary pulsar X Persei. Based on over 23 years of X-ray data observed using RXTE/ASM, Swift/BAT and MAXI/GSC, we confirmed that X Persei exhibits quasi-periodic X-ray flares with a period of ∼7 years. The recurrence timescale corresponds to approximately 10 times its binary orbital period of 250 days. Spectral and hardness ratio changes were not detected along with long-term periodic activity. If we interpret the observed 7 year periodicity of X-ray band flux as a superorbital modulation, then this would be the first observation among the Be/X-ray binaries.

The failed state transition of the ATOLL source GRS 1724–308

The 2004-2012 X-ray time history of the NS LMXB GRS 1724-308 shows, along with the episodic brightenings associated to the low-high state transitions typical of the ATOLL sources, a peculiar, long lasting (about 300 d) flaring event, observed in 2008. This rare episode, characterised by a high-flux hard state, has never been observed before for GRS 1724-308 , and in any case is not common among ATOLL sources. We discuss here different hypotheses on the origin of this peculiar event that displayed the spectral signatures of a failed transition, similar in shape and duration to those rarely observed in Black Hole binaries. We also suggest the possibility that the atypical flare occurred in coincidence with a new rising phase of the 12-years super-orbital modulation that has been previously reported by other authors. The analysed data also confirm for GRS 1724-308 the already reported orbital period of about 90 d.

Superorbital Modulation at GeV Energies in the γ-Ray Binary LS I + 61°303

Abstract We report the results from our analysis of eight years of the data for the γ-ray binary LS I +61°303, obtained with the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. We find a significant dip around the binary’s periastron in the superorbital light curves, and by fitting the light curves with a sinusoidal function, clear phase shifts are obtained. The superorbital modulation seen in the binary has been long known and different scenarios have been proposed. Based on our results, we suggest that the circumstellar disk around the Be companion of this binary may have a non-axisymmetric structure, which rotates at the superorbital period of 1667 days. As a result, the density of the ambient material around the compact star of the binary changes along the binary orbit over the superorbital period, causing the phase shifts in the modulation, and around periastron, the compact star probably enters the Be disk or switches the mode of its emission due to the intereaction with the disk, causing the appearance of the dip. We discuss the implications of this possible scenario to the observed superorbital properties at multiple frequencies.

The super-orbital modulation of supergiant high-mass X-ray binaries

The long-term X-ray lightcurves of classical supergiant X-ray binaries and supergiant fast X-ray transients show relatively similar super-orbital modulations, which are still lacking a sound interpretation. We propose that these modulations are related to the presence of corotating interaction regions (CIRs) known to thread the winds of OB supergiants. To test this hypothesis, we couple the outcomes of 3-D hydrodynamic models for the formation of CIRs in stellar winds with a simplified recipe for the accretion onto a neutron star. The results show that the synthetic X-ray light curves are indeed modulated by the presence of the CIRs. The exact period and amplitude of these modulations depend on a number of parameters governing the hydrodynamic wind models and on the binary orbital configuration. To compare our model predictions with the observations, we apply the 3-D wind structure previously shown to well explain the appearance of discrete absorption components in the UV time series of a prototypical B0.5I-type supergiant. Using the orbital parameters of IGRJ16493-4348 which has the same B0.5I donor spectral type, the period and modulations in the simulated X-ray light-curve are similar to the observed ones, thus providing support to our scenario. We propose, that the presence of CIRs in donor star winds should be considered in future theoretical and simulation efforts of wind-fed X-ray binaries.

Evolution of Spin, Orbital, and Superorbital Modulations of 4U 0114+650

Abstract We report a systematic analysis of the spin, orbital, and superorbital modulations of 4U 0114+650, a high-mass X-ray binary that consists of one of the slowest spinning neutron stars. Using the dynamic power spectrum, we found that the spin period varied dramatically and is anticorrelated with the long-term X-ray flux variation that can be observed using the Rossi X-ray Timing Explorer ASM, Swift BAT, and the Monitor of All-sky X-ray Image. The spin-up rate over the entire data set is consistent with previously reported values; however, the local spin-up rate is considerably higher. The corresponding local spin-up timescale is comparable to the local spin-up rate of OAO 1657−415, indicating that 4U 0114+650 could also have a transient disk. Moreover, the spin period evolution shows two ∼1000-day spin-down/random-walk epochs that appeared together with depressions of the superorbital modulation amplitude. This implies that the superorbital modulation was closely related to the presence of the accretion disk, which is not favored in the spin-down/random-walk epochs because the accretion is dominated by the direct wind accretion. The orbital period is stable during the entire time span; however, the orbital profile significantly changes with time. We found that the depth of the dip near the inferior conjunction of the companion is highly variable, which disfavors the eclipsing scenario. Moreover, the dip was less obvious during the spin-down/random-walk epochs, indicating its correlation with the accretion disk. Further monitoring in both X-ray and optical bands could reveal the establishment of the accretion disk in this system.

Swift Detection of a 65 Day X-Ray Period from the Ultraluminous Pulsar NGC 7793 P13

Abstract NGC 7793 P13 is an ultraluminous X-ray source harboring an accreting pulsar. We report on the detection of a ∼65 day period X-ray modulation with Swift observations in this system. The modulation period found in the X-ray band is P = 65.05 ± 0.10 days and the profile is asymmetric with a fast rise and a slower decay. On the other hand, the u-band light curve collected by Swift UVOT confirmed an optical modulation with a period of P = 64.24 ± 0.13 days. We explored the phase evolution of the X-ray and optical periodicities and propose two solutions. A superorbital modulation with a period of ∼2700–4700 days probably caused by the precession of a warped accretion disk is necessary to interpret the phase drift of the optical data. We further discuss the implication if this ∼65 day periodicity is caused by the superorbital modulation. Estimated from the relationship between the spin-orbital and orbital-superorbital periods of known disk-fed high-mass X-ray binaries, the orbital period of P13 is roughly estimated as 3–7 days. In this case, an unknown mechanism with a much longer timescale is needed to interpret the phase drift. Further studies on the stability of these two periodicities with a long-term monitoring could help us to probe their physical origins.

Tidal tearing of circumstellar disks in Be/X-ray and gamma-ray binaries

AbstractAbout one half of high-mass X-ray binaries host a Be star [an OB star with a viscous decretion (slowly outflowing) disk]. These Be/X-ray binaries exhibit two types of X-ray outbursts (Stella et al. 1986), normal X-ray outbursts (LX~1036−37 erg s−1) and occasional giant X-ray outbursts (LX > 1037 erg s−1). The origin of giant X-ray outbursts is unknown. On the other hand, a half of gamma-ray binaries have a Be star as the optical counterpart. One of these systems [LS I +61 303 (Porb = 26.5 d)] shows the superorbital (1,667 d) modulation in radio through X-ray bands. No consensus has been obtained for its origin. In this paper, we study a possibility that both phenomena are caused by a long-term, cyclic evolution of a highly misaligned Be disk under the influence of a compact object, by performing 3D hydrodynamic simulations. We find that the Be disk cyclically evolves in mildly eccentric, short-period systems. Each cycle consists of the following stages: 1)As the Be disk grows with time, the initially circular disk becomes eccentric by the Kozai-Lidov mechanism.2)At some point, the disk is tidally torn off near the base and starts precession.3)Due to precession, a gap opens between the disk base and mass ejection region, which allows the formation of a new disk in the stellar equatorial plane (see Figure 1).4)The newly formed disk finally replaces the precessing old disk. Such a cyclic disk evolution has interesting implications for the long-term behavior of high energy emission in Be/X-ray and gamma-ray binaries.

Orbital and superorbital monitoring of the Be/X-ray binary A0538–66: constraints on the system parameters

We combine the decade long photometry of the Be/X-ray binary system A0538-66 provided by the MACHO and OGLE IV projects with high resolution SALT spectroscopy to provide detailed constraints on the orbital parameters and system properties. The ~420d superorbital modulation is present throughout, but has reduced in amplitude in recent years. The well-defined 16.6409d orbital outbursts, which were a strong function of superorbital phase in the MACHO data (not occurring at all at superorbital maximum), are present throughout the OGLE IV coverage. However, their amplitude reduces during superorbital maximum. We have refined the orbital period and ephemeris of the optical outburst based on ~25 yrs light curves to HJD = 2455674.48 +/- 0.03 + n*16.6409 +/- 0.0003d. Our SALT spectra reveal a B1 III star with vsini of 285 km/s from which we have derived an orbital radial velocity curve which confirms the high eccentricity of e = 0.72 +/- 0.14. Furthermore, the mass function indicates that, unless the neutron star far exceeds the canonical 1.44 Msun, the donor must be significantly undermassive for its spectral type. We discuss the implications of the geometry and our derived orbital solution on the observed behaviour of the system.

Super-orbital variability of LS I +61°303 at TeV energies

The gamma-ray binary LS I +61$^{\circ}$303 is a well established source from centimeter radio up to very high energy (VHE; E$>$100 GeV). Its broadband emission shows a periodicity of $\sim$26.5 days, coincident with the orbital period. A longer (super-orbital) period of 1667 $\pm$ 8 days was discovered in radio and confirmed in optical and high energy (HE; E>100 MeV) gamma-ray observations. We present a four-year campaign performed by MAGIC together with archival data concentrating on a search for a long timescale signature in the VHE emission. We focus on the search for super-orbital modulation of the VHE peak and on the search for correlations between TeV emission and optical determination of the extension of the circumstellar disk. A four-year campaign has been carried out by MAGIC. The source was observed during the orbital phases when the periodic VHE outbursts have occurred ($\phi$=0.55-0.75). Additionally, we included archival MAGIC observations and data published by the VERITAS collaboration in these studies. For the correlation studies, LS I +61$^{\circ}$303 has also been observed during the orbital phases where sporadic VHE emission had been detected in the past ($\phi$=0.75-1.0). These MAGIC observations were simultaneous with optical spectroscopy from the LIVERPOOL telescope. The TeV flux of the periodical outburst in orbital phases $\phi$=0.5--0.75 was found to show yearly variability consistent with the $\sim$4.5 years long-term modulation found in the radio band. This modulation of the TeV flux can be well described by a sine function with the best fit period of $1610\pm 58$ days. The complete dataset span two super-orbital periods. There is no evidence for a correlation between the TeV emission and the mass-loss rate of the Be star but this may be affected by the strong, short timescale (as short as intra-day) variation displayed by the H$\alpha$ fluxes.

Orbital and superorbital variability of LS I +61 303 at low radio frequencies with GMRT and LOFAR

LS I +61 303 is a gamma-ray binary that exhibits an outburst at GHz frequencies each orbital cycle of $\approx$ 26.5 d and a superorbital modulation with a period of $\approx$ 4.6 yr. We have performed a detailed study of the low-frequency radio emission of LS I +61 303 by analysing all the archival GMRT data at 150, 235 and 610 MHz, and conducting regular LOFAR observations within the Radio Sky Monitor (RSM) at 150 MHz. We have detected the source for the first time at 150 MHz, which is also the first detection of a gamma-ray binary at such a low frequency. We have obtained the light-curves of the source at 150, 235 and 610 MHz, all of them showing orbital modulation. The light-curves at 235 and 610 MHz also show the existence of superorbital variability. A comparison with contemporaneous 15-GHz data shows remarkable differences with these light-curves. At 15 GHz we see clear outbursts, whereas at low frequencies we see variability with wide maxima. The light-curve at 235 MHz seems to be anticorrelated with the one at 610 MHz, implying a shift of $\sim$ 0.5 orbital phases in the maxima. We model the shifts between the maxima at different frequencies as due to changes in the physical parameters of the emitting region assuming either free-free absorption or synchrotron self-absorption, obtaining expansion velocities for this region close to the stellar wind velocity with both mechanisms.

ON THE COMPLEX VARIABILITY OF THE SUPERORBITAL MODULATION PERIOD OF LMC X-4

LMC X-4 is an eclipsing high-mass X-ray binary exhibiting a superorbital modulation with a period of ~ 30:5 days. We present a detailed study of the variations of the superorbital modulation period with a time baseline of ~ 18 years. The period determined in the light curve collected by the Monitor of All-sky X-ray Image (MAXI) significantly deviates from that observed by the All Sky Monitor (ASM) onboard the Rossi X-ray Timing Explorer (RXTE). Using the data collected by RXTE/ASM, MAXI, and the Burst Alert Telescope (BAT) onboard Swift, we found a significant period derivative, <TEX>$\dot{P}=(2.08{\pm}0.12){\times}10^{-5}$</TEX>. Furthermore, the O{C residual shows complex short-term variations indicating that the superorbital modulation of LMC X-4 exhibits complicated unstable behaviors. In addition, we used archive data collected by the Proportional Counter Array (PCA) on RXTE to estimate the orbital and spin parameters. The detected pulse frequencies obtained in small time segments were fitted with a circular orbital Doppler shift model. In addition to orbital parameters and spin frequency for each observation, we found a spin frequency derivative of <TEX>$\dot{v}=(6.482{\pm}0.011){\times}10^{-13}Hz{\cdot}s^{-1}$</TEX>. More precise orbital and spin parameters will be evaluated by the pulse arrival time delay technique in the future.

SPECTRAL ANALYSIS IN ORBITAL/SUPERORBITAL PHASE SPACE AND HINTS OF SUPERORBITAL VARIABILITY IN THE HARD X-RAYS OF LS I +61°303

ABSTRACT We present an INTEGRAL spectral analysis in the orbital/superorbital phase space of LS I +61°303. A hard X-ray spectrum with no cutoff is observed at all orbital/superorbital phases. The hard X-ray index is found to be uncorrelated with the radio index (non-simultaneously) measured at the same orbital and superorbital phases. In particular, the absence of an X-ray spectrum softening during periods of negative radio index does not favor a simple interpretation of the radio index variations in terms of a microquasar's changes of state. We uncover hints of superorbital variability in the hard X-ray flux, in phase with the superorbital modulation in soft X-rays. An orbital phase drift of the radio peak flux and index along the superorbital period is observed in the radio data. We explore its influence on a previously reported double-peak structure of a radio orbital light curve, and present it as a plausible explanation.

SUPERORBITAL PERIODIC MODULATION IN WIND-ACCRETION HIGH-MASS X-RAY BINARIES FROMSWIFTBURST ALERT TELESCOPE OBSERVATIONS

We report the discovery using data from the Swift Burst Alert Telescope (BAT) of superorbital modulation in the wind-accretion supergiant high-mass X-ray binaries 4U 1909+07 (= X 1908+075), IGR J16418-4532, and IGR J16479-4514. Together with already known superorbital periodicities in 2S 0114+650 and IGR J16493-4348, the systems exhibit a monotonic relationship between superorbital and orbital periods. These systems include both supergiant fast X-ray transients (SFXTs) and classical supergiant systems, and have a range of inclination angles. This suggests an underlying physical mechanism which is connected to the orbital period. In addition to these sources with clear detections of superorbital periods, IGR J16393-4643 (= AX J16390.4-4642) is identified as a system that may have superorbital modulation due to the coincidence of low-amplitude peaks in power spectra derived from BAT, RXTE PCA, and INTEGRAL light curves. 1E 1145.1-6141 may also be worthy of further attention due to the amount of low-frequency modulation of its light curve. However, we find that the presence of superorbital modulation is not a universal feature of wind-accretion supergiant X-ray binaries.

SUPERORBITAL PHASE-RESOLVED ANALYSIS OF SMC X-1

The high-mass X-ray binary SMC X-1 is an eclipsing binary with an orbital period of 3.89 d. This system exhibits a superorbital modulation with a period varying between ~40 d and ~65 d. The instantaneous frequency and the corresponding phase of the superorbital modulation can be obtained by a recently developed time-frequency analysis technique, the Hilbert-Huang transform (HHT). We present a phase-resolved analysis of both the spectra and the orbital profiles with the superorbital phase derived from the HHT. The X-ray spectra observed by the Proportional Counter Array onboard the Rossi X-ray Timing Explorer are fitted well by a blackbody plus a Comptonized component. The plasma optical depth, which is a good indicator of the distribution of material along the line of sight, is significantly anti-correlated with the flux detected at 2.5-25 keV. However, the relationship between the plasma optical depth and the equivalent width of the iron line is not monotonic: there is no significant correlation for fluxes higher than ~35 mCrab but clear positive correlation when the intensity is lower than ~20 mCrab. This indicates that the iron line production is dominated by different regions of this binary system in different superorbital phases. To study the dependence of the orbital profile on the superorbital phase, we obtained the eclipse profiles by folding the All Sky Monitor light curve with the orbital period for different superorbital states. A dip feature, similar to the pre-eclipse dip in Her X-1, lying at orbital phase ~0.6-0.85, was discovered during the superorbital transition state. This indicates that the accretion disk has a bulge that absorbs considerable X-ray emission in the stream-disk interaction region. The dip width is anti-correlated with the flux, and this relation can be interpreted by the precessing tilted accretion disk scenario.

An Application of Hilbert-Huang Transform on the Non-Stationary Astronomical Time Series: The Superorbital Modulation of SMC X-1

We present the Hilbert-Huang transform (HHT) analysis on the quasi-periodic modulation of SMC X-1. SMC X-1, consisting of a neutron star and a massive companion, exhibits superorbital modulation with a period varying between ~40 d and ~65 d. We applied the HHT on the light curve observed by the All-Sky Monitor onboard Rossi X-ray Timing Explorer (RXTE) to obtain the instantaneous frequency of the superorbital modulation of SMC X-1. The resultant Hilbert spectrum is consistent with the dynamic power spectrum while it shows more detailed information in both the time and frequency domains. According to the instantaneous frequency, we found a correlation between the superorbital period and the modulation amplitude. Combining the spectral observation made by the Proportional Counter Array onboard RXTE and the superorbital phase derived in the HHT, we performed a superorbital phase-resolved spectral analysis of SMC X-1. An analysis of the spectral parameters versus the orbital phase for different superorbital states revealed that the diversity of <TEX>$n_H$</TEX> has an orbital dependence. Furthermore, we obtained the variation in the eclipse profiles by folding the All Sky Monitor light curve with orbital period for different superorbital states. A dip feature, similar to the pre-eclipse dip of Her X-1, can be observed only in the superorbital ascending and descending states, while the width is anti-correlated with the X-ray flux.

Superexpansion as a possible probe of accretion in 4U 1820-30

The ultracompact X-ray binary 4U 1820-30 is well known for its ≈170-d superorbital modulation in X-ray flux and spectrum, and the exclusiveness of bursting behavior to the low hard “island” state. In May−June 2009, there was an exceptionally long 51-d low state. This state was well covered by X-ray observations and 12 bursts were detected, 9 with the high-throughput RXTE. We investigate the character of these X-ray bursts and find an interesting change in their photospheric expansion behavior. At the lowest inferred mass accretion rates, this expansion becomes very large in 4 bursts and reaches the so-called superexpansion regime. We speculate that this is due to the geometry of the inner accretion flow being spherical and a decreasing accretion rate: when the flow geometry nearest to the neutron star is spherical and the accretion rate is low, the ram pressure of the accretion disk may become too low to counteract that of the photospheric expansion. In effect, this may provide a novel means to probe the accretion flow. Additionally, we observe a peculiar effect: the well-known cessation of X-ray bursts in the high state is too quick to be consistent with a transition to stable helium burning. We suggest an alternative explanation, that the cessation is due to the introduction of a non-nuclear heat source in the neutron star ocean.

Unveiling the super-orbital modulation of LS I + 61°303 in X-rays

AbstractWe found evidence for the super–orbital modulation in the X-ray emission of LS I + 61°303 from the longest monitoring date by the RXTE. The time evolution of the modulated fraction in the orbital light curves can be well fitted with a sinusoidal function having a super-orbital period of 1667 days. However, we have found a 281.8±44.6 day shift between the super-orbital variability found at radio frequencies and our X-ray data. We also find a super-orbital modulation in the maximum count rate of the orbital light curves, compatible with the former results, including the shift.

UNVEILING THE SUPER-ORBITAL MODULATION OF LS I +61°303 IN X-RAYS

From the longest monitoring of LS I +61 303 done to date by the Rossi X-ray Timing Explorer (RXTE) we found evidence for the long-sought, years- long modulation in the X-ray emission of the source. The time evolution of the modulated fraction in the orbital lightcurves can be well fitted with a sinusoidal function having a super-orbital period of 1667 days, the same as the one reported in non-contemporaneous radio measurements. However, we have found a 281.8 \pm 44.6 days shift between the super-orbital variability found at radio frequencies extrapolated to the observation time of our campaign and what we found in the super-orbital modulation of the modulated fraction of our X-ray data. We also find a super-orbital modulation in the maximum count rate of the orbital lightcurves, compatible with the former results, including the shift.

TIME-FREQUENCY ANALYSIS OF THE SUPERORBITAL MODULATION OF THE X-RAY BINARY SMC X-1 USING THE HILBERT-HUANG TRANSFORM

The high-mass X-ray binary (HMXB) SMC X-1 exhibits a superorbital modulation with a dramatically varying period ranging between ~40 d and ~60 d. This research studies the time-frequency properties of the superorbital modulation of SMC X-1 based on the observations made by the All-Sky Monitor (ASM) onboard the Rossi X-ray Timing Explorer (RXTE).We analyzed the entire ASM database collected since 1996. The Hilbert-Huang Transform (HHT), developed for non-stationary and nonlinear time series analysis, was adopted to derive the instantaneous superorbital frequency. The resultant Hilbert spectrum is consistent with the dynamic power spectrum while it shows more detailed information in both the time and frequency domains. The RXTE observations manifest that the superorbital modulation period was mostly betweenn ~50 d and ~65 d, whenas it changed to ~45 d around MJD 50,800 and MJD 54,000. Our analysis further indicates that the instantaneous frequency changed in a time scale of hundreds of days between ~MJD 51,500 and ~MJD 53,500. Based on the instantaneous phase defined by HHT, we folded the ASM light curve to derive a superorbital profile, from which an asymmetric feature and a low state with barely any X-ray emissions (lasting for ~0.3 cycles) were observed. We also calculated the correlation between the mean period and the amplitude of the superorbital modulation. The result is similar to the recently discovered relationship between the superorbital cycle length and the mean X-ray flux for Her X-1.

The optical and X-ray light curves of Hercules X-1

The Galactic neutron star X-ray binary Her X-1 displays a well-known 35-day superorbital modulation in its X-ray and optical light curves. Detected across a broad energy range, the modulation is prevalent in X-rays, cycling between low and high states. The 35-day modulation is believed to be the result of the periodic occultation of the neutron star by a warped precessing accretion disc. Using optical observations of Her X-1 during both the anomalous low state (ALS) and the normal high state, it is shown that the orbital light curve of Her X-1 varies systematically over the 35-day precession cycle. The 35-day precessional profile is remarkably consistent between the ALS and normal high state of Her X-1, suggesting only a very slight change in the form of the disc warp between the two states. Comparison of optical and X-ray light curves suggests that a significant component of the X-ray flux during the ALS originates from the companion star.