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تسجيل مستخدم جديدThe low luminosity behaviour of the 4U 0115+63 Be/X-ray transient
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The Be/X-ray transient 4U 0115+63 exhibited a giant, type-II outburst in October 2015. The source did not decay to its quiescent state but settled in a meta-stable plateau state (a factor ~10 brighter than quiescence) in which its luminosity slowly decayed. We used XMM-Newton to observe the system during this phase and we found that its spectrum can be well described using a black-body model with a small emitting radius. This suggests emission from hot spots on the surface, which is confirmed by the detection of pulsations. In addition, we obtained a relatively long (~7.9 ksec) Swift/XRT observation ~35 days after our XMM-Newton one. We found that the source luminosity was significantly higher and, although the spectrum could be fitted with a black-body model the temperature was higher and the emitting radius smaller. Several weeks later the system started a sequence of type-I accretion outbursts. In between those outbursts, the source was marginally detected with a luminosity consistent with its quiescent level. We discuss our results in the context of the three proposed scenarios (accretion down to the magnestospheric boundary, direct accretion onto neutron star magnetic poles or cooling of the neutron star crust) to explain the plateau phase.
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In 2017, the Be/X-ray transient 4U 0115+63 exhibited a new type-II outburst that was two times fainter than its 2015 giant outburst (in the Swift/BAT count rates). Despite this difference between the two bright events, the source displayed similar X-
ray behaviour after these periods. Once the outbursts ceased, the source did not transit towards quiescence directly, but was detected about a factor of 10 above its known quiescent level. It eventually decayed back to quiescence over time scales of months. In this paper we present the results of our Swift monitoring campaign, and an XMM-Newton observation of 4U 0115+63 during the decay of the 2017 type-II outburst, and its subsequent low-luminosity behaviour. We discuss the possible origin of the decaying source emission at this low-level luminosity, which has now been shown as a recurrent phenomenon, in the framework of the two proposed scenarios to explain this faint state: cooling from an accretion-heated neutron-star crust or continuous low-level accretion. In addition, we compare the outcome of our study with the results we obtained from the 2015/2016 monitoring campaign on this source.
4u 0115+63 is one of the most active and best studied Be/X-ray transients. Previous studies of 4u0115+63 have led to the suggestion that it undergoes relatively fast quasi-cyclic activity. However, due to the lack of good coverage of the observations
, the variability time scales are uncertain. Our objective is to investigate the long-term behaviour of 4u 0115+63 to confirm its quasi-cyclic nature and to explain its correlated optical/IR and X-ray variability. We have performed optical/IR photometric observations and optical spectroscopic observations of 4u 0115+63 over the last decade with unprecedented coverage. We have focused on the Halpha line variability and the long-term changes of the photometric magnitudes and colours and investigated these changes in correlation with the X-ray activity of the source. results The optical and infrared emission is characterised by cyclic changes with a period of ~ 5 years. This long-term variability is attributed to the state of the circumstellar disc around the Be star companion. Each cycle involves a low state when the disc is very weak or absent and the associated low amplitude variability is orbitally modulated and a high state when a perturbed disc precesses, giving rise to fast and large amplitude photometric changes. X-ray outbursts in 4u 0115+63 come in pairs, i.e., two in every cycle. However, sometimes the second outburst is missing. Our results can be explained within the framework of the decretion disc model. The neutron star acts as the perturbing body, truncating and distorting the disc. The first outburst would occur before the disc is strongly perturbed. The second outburst leads to the dispersal of the disc and marks the end of the perturbed phase.
We present the results of the monitoring programmes performed with the Swift/XRT telescope and aimed specifically to detect an abrupt decrease of the observed flux associated with a transition to the propeller regime in two well known X-ray pulsars 4
U 0115+63 and V 0332+53 during their giant outbursts in 2015. Such transitions were detected at the threshold luminosities of $(1.4pm0.4)times10^{36}$ erg s$^{-1}$ and $(2.0pm0.4)times10^{36}$ erg s$^{-1}$ for 4U 0115+63 and V 0332+53, respectively. Spectra of the sources are shown to be significantly softer during the low state. In both sources, the accretion at rates close to the aforementioned threshold values briefly resumes during the periastron passage following the transition into propeller regime. The strength of the dipole component of the magnetic field required to inhibit the accretion agrees well with estimates based on the position of the cyclotron lines in their spectra, thus excluding presence of a strong multipole component of the magnetic field in the vicinity of the neutron star.
We report on a BeppoSAX observation of the transient X-ray pulsar 4U 0115+63 close to periastron. This led to the discovery of a dramatic luminosity variation from ~2x10^34 erg/s to ~5x10^36 erg/s (factor ~250) in less than 15 hr. The variation was a
ccompanied by only minor (if any) changes in the emitted spectrum and pulse fraction. On the contrary an observation near apastron detected the source in a nearly constant state at a level of ~2x10^33 erg/s. Direct accretion onto the neutron star surface encounters major difficulties in explaining the source variability properties. When the different regimes expected for a rotating magnetic neutron star subject to a variable inflow of matter from its companion are taken into consideration, the results of BeppoSAX observations of 4U 0115+63 can be explained naturally. In particular close to apastron, the regime of centrifugal inhibition of accretion applies, whereas the dramatic source flux variability observed close to periastron is readily interpreted as the transition regime between direct neutron star accretion and the propeller regime. In this centrifugal transition regime small variations of the mass inflow rate give rise to very large luminosity variations. We present a simple model for this transition, which we successfully apply to the X-ray flux and pulse fraction variations measured by BeppoSAX.
The Be X-ray Binary 4U 0115+63 was observed by Large Area X-ray Proportional Counter (LAXPC) instrument on AstroSat on 2015 October 24 during the peak of a giant Type II outburst. Prominent intensity oscillations at ~ 1 and ~ 2 mHz frequency were det
ected during the outburst. Nuclear Spectroscopic Telescope Array (NuSTAR) observations made during the same outburst also show mHz quasi periodic oscillations (QPOs). Details of the oscillations and their characteristics deduced from LAXPC/AstroSat and NuSTAR observations are reported in this paper. Analysis of the archival Rossi X-ray Timing Explorer (RXTE)/Proportional Counter Array (PCA) data during 2001-11 also show presence of mHz QPOs during some of the outbursts and details of these QPOs are also reported. Possible models to explain the origin of the mHz oscillations are examined. Similar QPOs, albeit at higher frequencies, have been reported from other neutron star and black hole sources and both may have a common origin. Current models to explain the instability in the inner accretion disk causing the intense oscillations are discussed.
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