Giant Outburst Research Articles

V4641 Sgr: a candidate precessing microblazar

Abstract The X-ray spectrum of the Galactic X-ray binary V4641 Sgr in outburst has been found to exhibit a remarkably broad emission feature above 4 keV, with inferred equivalent widths up to 2 keV. Such a feature was first detected during the X-ray flaring activity associated with the giant outburst that the source experienced in 1999 September. The extraordinarily large equivalent width line was then ascribed to reflection/reprocessing of fluorescent Fe emission within an extended optically thick outflow enshrouding the binary system as a result of a short-lived, super-Eddington accretion episode. Making use of new and archival X-ray observations, we show here that a similar feature persists over four orders of magnitude in luminosity, down to Eddington ratios as low as log (LX/LEdd) ≃ −4.5, where the existence of an optically thick envelope appears at odds with any viable accretion flow model. Possible interpretations for this highly unusual X-ray spectrum include a blend of Doppler-shifted/boosted Fe lines from unresolved X-ray jets (a la SS433), or, the first Galactic analogue of a blazar spectrum, where the >4 keV emission would correspond to the onset of the inverse Compton hump. This either requires a low-inclination angle of the jet with respect to the line of sight, in agreement with the estimates for the 1999 superluminal jet (ijet < 10°). The fast variability of the feature, combined with the high orbital axis inclination (60° < iorb < 71°), argue for a rapidly precessing accretion flow around V4641 Sgr, possibly leading to a transient microblazar behaviour.

Precessing Warped Be Disk Triggering the Giant Outbursts in 2009 and 2011 in A 0535 + 262/V725 Tau

Abstract We carried out optical high-dispersion spectroscopic monitoring of the Be disk in a Be/X-ray binary, A 0535$+$ 262/V725 Tau, from 2009 to 2012, covering two giant outbursts and several normal ones. This monitoring was performed in order to investigate any variabilities of the Be disk due to interactions with the neutron star in the recent X-ray active phase from 2008 to 2011. Such variabilities give a clue to any uncleared detailed mechanism for very bright X-ray outbursts, which are unique to some Be/X-ray binaries with a relatively wide and eccentric orbit. In a previous letter (Moritani et al. 2011, PASJ, 63, L25), a brief overview of the line-profile variabilities around the 2009 giant outburst was given, and the possibility of a warped Be disk was discussed. In this paper, a full analysis of the H$\alpha$ line profiles as well as other line profiles is carried out. A bright blue component, or blue “shoulder,” showing up after periastron indicates the presence of a dense gas stream toward the neutron star, which is associated with observed outbursts. We re-analyze the H$\alpha$ line profiles before 2009 (down to 2005) in order to investigate the variability of the disk structure in the innermost region, which seems to have detached from the Be star surface by 2008. A redshifted enhanced component is remarkable in all emission lines observed around the 2009 giant outburst, occasionally forming a triple peak. These features indicate that the Be disk was warped in the X-ray active phase. We estimated the position of the warped region from fitting the radial velocity of the redshifted enhanced component of H$\alpha$, and found that it was very close to the periastron when two giant outbursts in 2009 and 2011 and a bright normal outburst in 2010 March occurred. These facts strongly suggest that the precessing warped Be disk triggered these giant outbursts.

Fermi/LAT detection of extraordinary variability in the gamma-ray emission of the blazar PKS 1510-089

We have reanalyzed the giant outburst of the blazar PKS 1510-089 (z=0.36) that occurred on 2011 October-November. The gamma-ray flux in the 0.1-100 GeV energy range exceeded the value of 10^-5 ph/cm2/s for several days. The peak flux was reached on 2011 October 19, with a value of 4.4 x 10^-5 ph/cm2/s, which in turn corresponds to a luminosity of 2 x 10^49 erg/s. A very short timescale variability was measured. Particularly on 2011 October 18, the flux-doubling time was as short as 20 minutes. This is the shortest variability ever detected in the MeV-GeV energy band. We compared our analysis with two other outbursts observed in 2009 March and 2012 February-March, when the blazar was also detected by HESS and MAGIC to infer information about the emission at hundreds of GeV.

Origin of Two Types of X-Ray Outbursts in Be/X-Ray Binaries. I. Accretion Scenarios

We propose a new scenario for X-ray outbursts in Be/X-ray binaries that normal and giant outbursts are, respectively, caused by radiatively inefficient accretion flows (RIAFs) and Bondi-Hoyle-Lyttleton (BHL) accretion of material transferred from the outermost part of a Be disk misaligned with the binary orbital plane. Based on simulated mass-transfer rates from misaligned Be disks, together with simplified accretion flow models, we show that mass-accretion rates estimated from the luminosity of normal X-ray outbursts are consistent with those obtained with advection-dominated accretion flows, not with the standard, radiative-cooling dominated, accretion. Our RIAF scenario for normal X-ray outbursts resolves problems that have challenged the standard disk picture for these outbursts. When a misaligned Be disk crosses the orbit of a neutron star, e.g., by warping, the neutron star can capture a large amount of mass via BHL-type accretion during the disk transit event. We have numerically shown that such a process can reproduce the X-ray luminosity of giant X-ray outbursts. In the case of a very high Be disk density, the accretion flow associated with the disk transit becomes supercritical, giving rise to a luminosity higher than the Eddington luminosity.

Patterns of variability in Be/X-ray pulsars during giant outbursts

The discovery of source states in the X-ray emission of black-hole binaries and neutron-star low-mass X-ray binaries constituted a major step forward in the understanding of the physics of accretion onto compact objects. While there are numerous studies on the correlated timing and spectral variability of these systems, very little work has been done on high-mass X-ray binaries, the third major type of X-ray binaries. The main goal of this work is to investigate whether Be accreting X-ray pulsars display source states and characterise those states through their spectral and timing properties. We have made a systematic study of the power spectra, energy spectra and X-ray hardness-intensity diagrams of nine Be/X-ray pulsars. The evolution of the timing and spectral parameters were monitored through changes over two orders of magnitude in luminosity. We find that Be/X-ray pulsars trace two different branches in the hardness-intensity diagram: the horizontal branch corresponds to a low-intensity state of the source and it is characterised by fast colour and spectral changes and high X-ray variability. The diagonal branch is a high-intensity state that emerges when the X-ray luminosity exceeds a critical limit. The photon index anticorrelates with X-ray flux in the horizontal branch but correlates with it in the diagonal branch. The correlation between QPO frequency and X-ray flux reported in some pulsars is also observed if the peak frequency of the broad-band noise that accounts for the aperiodic variability is used. The two branches may reflect two different accretion modes, depending on whether the luminosity of the source is above or below a critical value. This critical luminosity is mainly determined by the magnetic field strength, hence it differs for different sources.

New insights into the Be/X-ray binary system MXB 0656-072

The X-ray transient MXB 0656-072 is a poorly studied member of high-mass X-ray binaries. Based on the transient nature of the X-ray emission, the detection of pulsations, and the early-type companion, it has been classified as a Be X-ray binary (Be/XRB). However, the flaring activity covering a large fraction of a giant outburst is somehow peculiar. Our goal is to investigate the multiwavelength variability of the high-mass X-ray binary MXB 0656-072. We carried out optical spectroscopy and analysed all RXTE archive data, performing a detailed X-ray-colour, spectral, and timing analysis of both normal (type-I) and giant (type-II) outbursts from MXB 0656-072 This is the first detailed analysis of the optical counterpart in the classification region (4000-5000 A). From the strength and ratio of the elements and ions, we derive an O9.5Ve spectral type, in agreement with previous classification. This confirms its Be nature. The characterisation of the Be/XRB system relies on Balmer lines in emission in the optical spectra, long-term X-ray variability, and the orbital period vs. spin period and EW(H\alpha) relation. The peculiar feature that distinguishes the type-II outburst is flaring activity, which occurs during the whole outburst peak, before a smoother decay. We interpret it in terms of magneto-hydrodynamic instability. Colour and spectral analysis reveal a hardening of the spectrum as the flux increases. We explored the aperiodic X-ray variability of the system for the first time, finding a correlation of the central frequency and rms of the main timing component with luminosity, which extends up to a "saturation" flux of 1E-8 erg/cm^2/s. A correlation between timing and spectral parameters was also found, pointing to an interconnection between the two physical regions responsible for both phenomenologies.

The energy source of the most energetic giant outbursts in MS 0735 + 7421

AbstractIn this work, we investigate the power source of the most energetic giant outbursts in MS 0735 + 7421, which released ~ 1062 erg of energy. Due to the very high mean jet power in the cavities (Pjet/LEdd ~ 0.02), we produce several jet formation models based on a relativistic thin disk model, i.e., general BP + BZ mechanisms (model A), Livio's (model B) and Meier's (model C) model, to explain the giant outbursts in AGNs. It is found that the energy provided by both model B and model C are inadequate for an initial black hole spin a0 ~ 0.1, only model A can explain the most violent outbursts in MS 0735 + 7421. But if the initial black hole spin a0 ~ 0.95, model B can also blow up the cavity. The final spin of the black hole is found to be very high in spite of the initial spin.

X-RAY AND OPTICAL OBSERVATIONS OF A 0535+26

We present recent contemporaneous X-ray and optical observations of the Be/X-ray binary system A\,0535+26 with the \textit{Fermi}/Gamma-ray Burst Monitor (GBM) and several ground-based observatories. These new observations are put into the context of the rich historical data (since $\sim$1978) and discussed in terms of the neutron star Be-disk interaction. The Be circumstellar disk was exceptionally large just before the 2009 December giant outburst, which may explain the origin of the unusual recent X-ray activity of this source. We found a peculiar evolution of the pulse profile during this giant outburst, with the two main components evolving in opposite ways with energy. A hard 30-70 mHz X-ray QPO was detected with GBM during this 2009 December giant outburst. It becomes stronger with increasing energy and disappears at energies below 25\,keV. In the long-term a strong optical/X-ray correlation was found for this system, however in the medium-term the H$_\alpha$ EW and the V-band brightness showed an anti-correlation after $\sim$2002 Agust. Each giant X-ray outburst occurred during a decline phase of the optical brightness, while the H$_\alpha$ showed a strong emission. In late 2010 and before the 2011 February outburst, rapid V/R variations are observed in the strength of the two peaks of the H$_\alpha$ line. These had a period of $\sim$\,25 days and we suggest the presence of a global one-armed oscillation to explain this scenario. A general pattern might be inferred, where the disk becomes weaker and shows V/R variability beginning $\sim$\,6 months following a giant outburst.

Glacial outburst floods and loess sedimentation documented during Oxygen Isotope Stage 4 on the Columbia Plateau, Washington State

Stratigraphy and age control of late Pleistocene loess, associated glacial outburst flood deposits and flood-cut unconformities in the Channeled Scabland, Washington State, United States, indicate a significant Cordilleran ice sheet advance during marine Oxygen Isotope Stage 4. Glacial outburst flood deposits from stage 2 (classic Bretz flood deposits, ca 21 to 13 ka) and related features in the Channeled Scabland overlie a widespread layer of loess that contains buried soils and the Mount St. Helens set C tephra (ca 46 ka). This loess in turn overlies deposits of the penultimate episode of giant outburst floods and an unconformity cut by those floods. Regional trends in the thickness, texture, and overall composition of the older loess are strikingly similar to those from the youngest loess, known to be derived from stage 2 flood deposits. We conclude that the older loess also is derived from fine-grained flood deposits. Luminescence ages, tephrochronology, and soil development rates indicate that the bulk of deposition of the older loess occurred during stage 3, following glacial outburst flooding marked by a regional flood-cut unconformity. The apparent cyclical pattern of cold-climate buried soils, flood deposits, and thick loess accumulations demonstrate that sediment supply renewed by flood episodes is a major control on accumulation of loess on glacial timescales.

Cyclotron resonance energies and orbital elements of accretion pulsar 4U 0115+63 during the giant outburst in 2008

We present both timing and spectral analysis of the outburst of 4U 0115+63 in April -- May 2008 with INTEGRAL and RXTE observations. We have determined the spin period of the neutron star at $\sim 3.61430 \pm 0.00003$ s, and a spin up rate during the outburst of $\dot{P}=(-7.24 \pm 0.03)\times10^{-6} {\rm s d^{-1}}$, the angle of periapsis $\omega=48.67^\circ \pm 0.04^\circ$ in 2008 and its variation (apsidal motion) $\dot{\omega} = 0.048^\circ \pm 0.003^\circ {\rm yr}^{-1}$. We also confirm the relation of spin-up torque versus luminosity in this source during the giant outburst. The hard X-ray spectral properties of 4U 0115+63 during the outburst are studied with INTEGRAL and RXTE. Four cyclotron absorption lines are detected using the spectra from combined data of IBIS and JEM-X aboard INTEGRAL in the energy range of 3 -- 100 keV. The 5 -- 50 keV luminosities at an assumed distance of 7 kpc are determined to be in the range of $(1.5-12)\times 10^{37} {\rm ergs s^{-1}}$ during the outburst. The fundamental absorption line energy varies during the outburst: around 15 keV during the rising phase, and transiting to $\sim 10$ keV during the peak of the outburst, and further coming back to $\sim 15$ keV during the decreasing phase. The variations of photon index show the correlation with the fundamental line energy changes: the source becomes harder around the peak of the outburst and softer in both rising and decreasing phases. This correlation and transition processes during the outburst need further studies in both observations and theoretical work. The known relation of the fundamental line energy and X-ray luminosity is confirmed by our results, however, our discoveries suggest that some other factors besides luminosity play the important role in fundamental line energy variations and spectral transitions.

Isolated Millimeter Flares of Cyg X-3

Abstract Cygnus X-3 (Cyg X-3) is a well-known microquasar with relativistic jets. It is especially famous for its giant radio outbursts, which have been observed once every few years since their first discovery. Each giant outburst presumably consists of a series of short-duration flares. The physical parameters of the flares in the giant outbursts are difficult to derive because the successive flares overlap. Here, we report on isolated flares in the quiescent phase of Cyg X-3, as observed at 23, 43, and 86 GHz with the 45-m radio telescope at Nobeyama Radio Observatory. The observed flares have small amplitude (0.5–2 Jy) and short duration (1–2 hr). The millimeter fluxes rapidly increase, and then exponentially decay. The lifetime of the decay is shorter at higher frequency. The radio spectrum of Cyg X-3 during the flares is flat or inverted around the peak flux density. After that, the spectrum gradually becomes steeper. The observed characteristics are consistent with those of an adiabatic expanding plasma. The brightness temperature of the plasma at the peak is estimated to be $T_{\rm B}$$\gtrsim$ 1 $\times$ 10$^{11}\ $K. The magnetic field in the plasma is calculated to be 0.2 $\lesssim$$H$$\lesssim$ 30 G.

Near-infrared observations of the Be/X-ray binary pulsar A0535+262

We present the results obtained from extensive near-infrared (IR) spectroscopic and photometric observations of the Be/X-ray binary A0535+262/HDE 245770 at different phases of its ∼111 d orbital period. This observation campaign is part of the monitoring program of selective Be/X-ray binary systems aimed at understanding X-ray and near-IR properties at different orbital phases, especially during the periastron passage of the neutron star. The near-IR observations presented here were carried out using the 1.2 m telescope at the Mt. Abu IR Observatory. Though the source was relatively faint for spectroscopic observations with the 1.2 m telescope, we monitored the source closely during the 2011 February—March giant X-ray outburst to primarily investigate whether any drastic changes in the near-IR J H K spectra took place at the periastron passage. Changes of such a striking nature were expected to be detectable in our spectra. Photometric observations of the Be star show a gradual and systematic fading in the J H K light curves since the onset of the X-ray outburst, which could suggest a mild evacuation/truncation of the circumstellar disk of the Be companion. Near-IR spectroscopy of the object shows that the J H K spectra are dominated by the emission lines of hydrogen Brackett and Paschen series and HeI lines at 1.0830, 1.7002 and 2.0585 μm. The presence of all the hydrogen emission lines in the J H K spectra, along with the absence of any significant change in the continuum of the Be companion during X-ray quiescent and X-ray outburst phases, suggests that the near-IR line emitting regions of the disk are not significantly affected during the X-ray outburst.

Do magnetars really exist?

We perform a comparative analysis of the properties of isolated single neutron stars and show the absence of any single typical feature providing unambiguous evidence that they belong to the classes of AXPs or SGRs. Several objects with features intermediate between AXPs and radio transients (RRATs) have been discovered recently: radio pulsars with high magnetic fields, radio-emitting AXPs, etc. Assuming the existence of fields of 1016 G in the stellar interiors cannot explain the giant gamma-ray outbursts of SGRs. It appears necessary to invoke other energy sources, such as nuclear reactions in the matter that breaks through the crust of the neutron star. For the recently discovered AXP PSR J1642-4950, we find that the angle β between its spin axis and magnetic moment is 15.6°. This agrees with earlier estimates for the AXPs J1810-197 and 1E 1547.0-5408, which have β < 10°. The similarity of these objects to aligned rotators enables a description using the drift model. This model yields a rotational period for PSR J1642-4950 of P = 0.32 s, amagnetic field in the radiation generation region of B = 950 G, and a surfacemagnetic field ofB s = 3.39×1012 G. It is shown that the cyclotron instability in the neighbourhood proximity of the light cylinder, associated with particles in the tail of the secondary-plasma distribution, can explain the generation of the radio emission of PSR J1642-4950, which should be observed predominantly at low frequencies (∼100 MHz).

DISCRETE MASS EJECTIONS FROM THE Be/X-RAY BINARY A0535+26/HD 245770

We present the long-term optical spectroscopic observations on the Be/X-ray binary A0535+26 from 1992 to 2010. Combining with the public V-band photometric data, we find that each giant X-ray outburst occurred in a fading phase of the optical brightness. The anti-correlation between the optical brightness and the H$\alpha$ intensity during our 2009 observations indicates a mass ejection event had taken place before the 2009 giant X-ray outburst, which might cause the formation of a low-density region in the inner part of the disk. The similar anti-correlation observed around 1996 September indicates the occurrence of the mass ejection, which might trigger the subsequent disk loss event in A0535+26.

Drastic Spectroscopic Variability of the Be/X-Ray Binary Ariel 0535+262/V725 Tau during and after the 2009 Giant Outburst

Abstract We report on high-dispersion optical spectroscopic observations of the Be/X-ray binary A 0535$+$262/V725 Tau during a giant outburst in 2009 November/December, and after it. The observed emission line profiles, reflecting the structure of the geometrically thin circumstellar envelope of the Be star (Be disk), show drastic variabilities, and indicate the existence of a warped component. The enhanced blue shoulder seen after periastron passage implies a gas stream from a dense part of the Be disk to the neutron star.

4U 0115 + 63: phase lags and cyclotron resonant scattering

High mass X-ray binaries are among the brightest objects of our Galaxy in the high energy domain (0.1-100 keV). Despite our relatively good knowledge of their basic emission mechanisms, the complex problem of understanding their time and energy dependent X-ray emission is not completely solved. <P> In this paper, we study the energy dependent pulse profiles of the high mass X-ray binary pulsar 4U 0115+63 to investigate how they are affected by cyclotron resonant scattering. <P> We analyze archival BeppoSAX and RXTE observations performed during the giant outburst of the source which occurred in 1999. We exploit a cross correlation technique to compare the pulse profiles in different energy ranges and develop a relativistic ray-tracing model to interpret our findings. We also study the phase dependency of the cyclotron absorption features by performing phase resolved spectroscopy. <P> The pulse profiles of 4U 0115+63 displayed clear "phase-lags" at energies close to those of the cyclotron absorption features that characterize the X-ray emission of the source. We reproduce qualitatively this phenomenon by assuming an energy dependent beaming of the emission from the column surface and verify that our model is also compatible with the results of phase resolved spectral analysis. <P> We showed that cyclotron resonant scattering affects the pulse profile formation mechanisms in a complex way, which necessitates both improvements in the modeling and the study of other sources to be better understood.

GAMMA-RAY OBSERVATIONS OF THE Be/PULSAR BINARY 1A 0535+262 DURING A GIANT X-RAY OUTBURST

Giant X-ray outbursts, with luminosities of about 1037 erg s−1, are observed roughly every five years from the nearby Be/pulsar binary 1A 0535+262. In this article, we present observations of the source with VERITAS at very high energies (VHEs; E >100 GeV) triggered by the X-ray outburst in 2009 December. The observations started shortly after the onset of the outburst and provided comprehensive coverage of the episode, as well as the 111 day binary orbit. No VHE emission is evident at any time. We also examined data from the contemporaneous observations of 1A 0535+262 with the Fermi/Large Area Telescope at high-energy photons (E > 0.1 GeV) and failed to detect the source at GeV energies. The X-ray continua measured with the Swift/X-Ray Telescope and the RXTE/PCA can be well described by the combination of blackbody and Comptonized emission from thermal electrons. Therefore, the gamma-ray and X-ray observations suggest the absence of a significant population of non-thermal particles in the system. This distinguishes 1A 0535+262 from those Be X-ray binaries (such as PSR B1259–63 and LS I +61°303) that have been detected at GeV–TeV energies. We discuss the implications of the results on theoretical models.

THE DIVERSITY OF MASSIVE STAR OUTBURSTS. I. OBSERVATIONS OF SN2009ip, UGC 2773 OT2009-1, AND THEIR PROGENITORS

Despite both being outbursts of luminous blue variables (LBVs), SN 2009ip and UGC 2773 OT2009-1 have very different progenitors, spectra, circumstellar environments, and possibly physical mechanisms that generated the outbursts. From pre-eruption HST images, we determine that SN 2009ip and UGC 2773 OT2009-1 have initial masses of >60 and >25 M_sun, respectively. Optical spectroscopy shows that at peak SN 2009ip had a 10,000 K photosphere and its spectrum was dominated by narrow H Balmer emission, similar to classical LBV giant outbursts, also known as "supernova impostors." The spectra of UGC 2773 OT2009-1, which also have narrow H alpha emission, are dominated by a forest of absorption lines, similar to an F-type supergiant. Blueshifted absorption lines corresponding to ejecta at a velocity of 2000 - 7000 km/s are present in later spectra of SN 2009ip -- an unprecedented observation for LBV outbursts, indicating that the event was the result of a supersonic explosion, rather than a subsonic outburst. The velocity of the absorption lines increases between two epochs, suggesting that there were two explosions in rapid succession. A rapid fading and rebrightening event concurrent with the onset of the high-velocity absorption lines is consistent with the double-explosion model. A near-infrared excess is present in the spectra and photometry of UGC 2773 OT2009-1 that is consistent with ~2100 K dust emission. We compare the properties of these two events and place them in the context of other known massive star outbursts such as eta Car, NGC 300 OT2008-1, and SN 2008S. This qualitative analysis suggests that massive star outbursts have many physical differences which can manifest as the different observables seen in these two interesting objects.

Millimeter Light Curve with Abrupt Jump in Cyg X-3 2008 April–May Outburst

Abstract Cyg X-3 is a well-known microquasar with a bipolar relativistic jet. Its famous giant radio outbursts have been repeated once every several years. However, the behavior of the millimeter-wave emission has remained unclear because of limitations of time resolution in previous observations. We report here on millimeter wave observations of Cyg X-3 experiencing giant outbursts with one of the finest time resolutions. We have found a series of short-lived flares with amplitudes of 1–2 Jy in the millimeter light curve of the 2008 April–May outburst. They have flat spectra at around 100 GHz. We have also found abrupt and large amplitude flux density changes with $e$-folding time of 3.6 min or less. The source size of Cyg X-3 is constrained to within 0.4 AU, and the brightness temperature is estimated to be $T_\mathrm{B}$$\gtrsim$ 1 $\times$ 10$^{11}\ $K.

Discovery of a quasi-periodic oscillation in the X-ray pulsar 1A 1118-615: correlated spectral and aperiodic variability

Our goal is to investigate the X-ray timing and spectral variability of the high-mass X-ray binary 1A 1118-615 during a type-II outburst. We performed a detailed color, spectral and timing analysis of a giant outburst from 1A 1118-615, using RXTE data. Results. We report the discovery of a variable quasi-periodic oscillation (QPO) in the power spectral density of 1A 1118-615, with a centroid frequency of ~0.08 Hz. The centroid frequency of the QPO correlates with the X-ray flux, as expected according to the most accredited models for QPO production. For energies above ~4 keV, the QPO rms variability decreases as the energy increases. Pulse profiles display energy dependence, with a two-peak profile at lower energies, and a single peak at higher energies. From spectral analysis, we confirm the presence of a cyclotron absorption feature at ~60 keV, the highest value measured for an X-ray pulsar. We find that the spectral parameters (photon index, cutoff energy, iron fluorescence line strength) display a marked dependence with flux. We detect two different levels of neutral hydrogen column density, possibly due to the Be companion activity. We report for the first time a correlation between the timing and spectral parameters in an X-ray pulsar. All the correlations found between spectral/timing parameters and X-ray flux are present up to a flux of ~6x10^-9 erg cm^-2 s^-1, when a saturation level is reached. We propose that the saturation observed corresponds to the minimum extent of the neutron star magnetosphere. We estimate the magnetic field of the neutron star from two independent ways, using results from spectral (cyclotron line energy) and timing (QPO frequency) analysis, obtaining consistent values, of ~7-8x10^12 G. Results from the comprehensive spectral and timing analysis are discussed in comparison with other X-ray pulsars.