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تسجيل مستخدم جديدEXO 2030+375 Restarts in Reverse
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The Be X-ray binary pulsar EXO 2030+375, first detected in 1985, has shown a significant detected X-ray outburst at nearly every periastron passage of its 46-day orbit for the past ~25 years, with one low state accompanied by a torque reversal in the 1990s. In early 2015 the outbursts progressively became fainter and less regular while the monotonic spin-up flattened. At the same time a decrease in the H$alpha$ line equivalent width was reported, indicating a change in the disk surrounding the mass donor. In order to explore the source behaviour in the poorly explored low-flux state with a possible transition to a state of centrifugal inhibition of accretion we have undertaken an observing campaign with Swift/XRT, NuSTAR and the Nordic Optical Telescope (NOT). This conference contribution reports the preliminary results obtained from our campaign.
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Context: Episodic flaring activity is a common feature of X-ray pulsars in HMXBs. In some Be/X-ray binaries flares were observed in quiescence or prior to outbursts. EXO 2030+375 is a Be/X-ray binary showing normal outbursts almost every ~46 days, ne
ar periastron passage of the orbital revolution. Some of these outbursts were occasionally monitored with the INTEGRAL observatory. Aims: The INTEGRAL data revealed strong quasi-periodic flaring activity during the rising part of one of the systems outburst. Such activity has previously been observed in EXO 2030+375 only once, in 1985 with EXOSAT. (Some indications of single flares have also been observed with other satellites.) Methods: We present the analysis of the flaring behavior of the source based on INTEGRAL data and compare it with the flares observed in EXO 2030+375 in 1985. Results: Based on the observational properties of the flares, we argue that the instability at the inner edge of the accretion disk is the most probable cause of the flaring activity.
We present a comprehensive timing and spectral studies of Be/X-ray binary pulsar EXO 2030+375 using extensive Rossi X-ray Timing Explorer observations from 1995 till 2011, covering numerous Type I and 2006 Type II outbursts. Pulse profiles of the pul
sar were found to be strongly luminosity dependent. At low luminosity, the pulse profile consisted of a main peak and a minor peak that evolved into a broad structure at high luminosity with a significant phase shift. A narrow and sharp absorption dip, also dependent on energy and luminosity, was detected in the pulse profile. Comparison of pulse profiles showed that the features at a particular luminosity are independent of type of X-ray outbursts. This indicates that the emission geometry is solely a function of mass accretion rate. The broadband energy spectrum was described with a partial covering high energy cutoff model as well as a physical model based on thermal and bulk Comptonization in accretion column. We did not find any signature of cyclotron resonance scattering feature in the spectra obtained from all the observations. A detailed analysis of spectral parameters showed that, depending on source luminosity, the power-law photon index was distributed in three distinct regions. It suggests the phases of spectral transition from sub-critical to super-critical regimes in the pulsar as proposed theoretically. A region with constant photon index was also observed in ~(2-4) x 10^37 erg/s range, indicating critical luminosity regime in EXO 2030+375.
In this paper we study the timing and spectral properties of Be/X-ray binary pulsar EXO 2030+375 using a $Suzaku$ observation on 2012 May 23, during a less intense Type I outburst. Pulsations were clearly detected in the X-ray light curves at a baryc
entric period of 41.2852 s which suggests that the pulsar is spinning-up. The pulse profiles were found to be peculiar e.g. unlike that obtained from the earlier Suzaku observation on 2007 May 14. A single-peaked narrow profile at soft X-rays (0.5-10 keV range) changed to a double-peaked broad profile in 12-55 keV energy range and again reverted back to a smooth single-peaked profile at hard X-rays (55-70 keV range). The 1.0-100.0 keV broad-band spectrum of the pulsar was found to be well described by three continuum models such as (i) a partial covering high energy cut-off power-law model, (ii) a partially absorbed power-law with high-energy exponential rolloff and (iii) a partial covering Negative and Positive power law with EXponential (NPEX) continuum model. Unlike earlier Suzaku observation during which several low energy emission lines were detected, a weak and narrow Iron K_alpha emission line at 6.4 keV was only present in the pulsar spectrum during the 2012 May outburst. Non-detection of any absorption like feature in 1-100 keV energy range supports the claim of absence of cyclotron resonance scattering feature in EXO 2030+375 from earlier Suzaku observation. Pulse-phase resolved spectroscopy revealed the presence of additional dense matter causing the absence of second peak from the soft X-ray pulse profiles. The details of the results are described in the paper.
The Be X-ray binary EXO 2030+375 was in an extended low luminosity state during most of 2016. We observed this state with NuSTAR and Swift, supported by INTEGRAL observations as well as optical spectroscopy with the NOT. We present a comprehensive sp
ectral and timing analysis of these data here to study the accretion geometry and investigate a possible onset of the propeller effect. The H-alpha data show that the circumstellar disk of the Be-star is still present. We measure equivalent widths similar to values found during more active phases in the past, indicating that the low-luminosity state is not simply triggered by a smaller Be disk. The NuSTAR data, taken at a 3-78 keV luminosity of ~6.8e35 erg/s (for a distance of 7.1 kpc), are well described by standard accreting pulsar models, such as an absorbed power-law with a high-energy cutoff. We find that pulsations are still clearly visible at these luminosities, indicating that accretion is continuing despite the very low mass transfer rate. In phase-resolved spectroscopy we find a peculiar variation of the photon index from ~1.5 to ~2.5 over only about 3% of the rotational period. This variation is similar to that observed with XMM-Newton at much higher luminosities. It may be connected to the accretion column passing through our line of sight. With Swift/XRT we observe luminosities as low as 1e34 erg/s during which the data quality did not allow us to search for pulsations, but the spectrum is much softer and well described by either a blackbody or soft power-law continuum. This softer spectrum might be due to the fact that accretion has been stopped by the propeller effect and we only observe the neutron star surface cooling.
Using the archival RXTE/ASM and SWIFT/BAT observations, the new orbital phases of Type I outbursts of EXO 2030+375 are estimated. A possible correlation between the Type II outburst and optical brightness variations is investigated. In order to estim
ate the phases of Type I outbursts, we fitted Gaussian profiles to the RXTE/ASM and SWIFT/BAT light curves. The time corresponding to the maximum value of the profiles is treated as the arrival time of Type I outburst. We used differential magnitudes in the time-series analysis of the optical light curve. MIDAS and its suitable packages were used to reduce and analyze the spectra. Prior to the Type II outburst, orbital phases of Type I outbursts were delayed for 6 days after the periastron passage, which is consistent with findings of Wilson et al., (2002, 2005). After the giant Type II outburst, the phase of Type I outbursts underwent a sudden shift of 13 days after the periastron passage. The amplitudes of Type I outbursts were increased between MJD 52500 and 53500. These amplitudes then decreased for 10 orbital cycles until the Type II outburst was triggered. If the change of outburst amplitudes correlated with the mass accretion, then during the decrease of these amplitudes mass should be deposited in a disk around neutron star temporarily. The release of this stored mass may ignite the Type II outburst. We report that the optical light curve became fainter by 0.4 mag during the decrease of amplitude of the Type I outbursts. The observed H$alpha$ profiles and their equivalent widths during the decay and after the giant outburst are consistent with previous observations of the system.
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