Observation of an outburst of the transient X-ray pulsar A0535+26 in 1980

Santangelo

et al. 2014

A&A

Accretion onto magnetized neutron stars is expected to be centrifugally inhibited at low accretion rates. Several sources, however, are known to pulsate in quiescence at luminosities below the theoretical limit predicted for the onset of the centrifugal barrier. The source 1A 0535+262 is one of them. Here we present the results of an analysis of a ∼50 ks long XMM-Newton observation of 1A 0535+262 in quiescence. At the time of the observation, the neutron star was close to apastron, and the source had remained quiet for two orbital cycles. In spite of this, we detected a pulsed X-ray flux of ∼3 × 10 −11 erg cm −2 s −1 . Several observed properties, including the power spectrum, remained similar to those observed in the outbursts. Particularly, we have found that the frequency of the break detected in the quiescent noise power spectrum follows the same correlation with flux observed when the source is in outburst. This correlation has been associated with the truncation of the accretion disk at the magnetosphere boundary. We argue that our result, along with other arguments previously reported in the literature, suggests that the accretion in quiescence also proceeds from an accretion disk around the neutron star. The proposed scenario consistently explains the energy of the cyclotron line observed in 1A 0535+262, and the timing properties of the source including the spin frequency evolution within and between the outbursts, and the frequency of the break in power spectrum.

Section: Timing Analysismentioning

confidence: 94%

XMM-Newtonobservations of 1A 0535+262 in quiescence

Santangelo

et al. 2014

A&A

“…Among them are Rosenberg et al (1975), Fishman & Watts (1977), Bradt et al (1976), Li et al (1979), Hameury et al (1983), Frontera et al (1985), Nagase et al (1982), Sembay et al (1990), Motch et al (1991), Coe et al (1990), Makino (1989), Sunyaev & Titarchuk (1989), Cusumano et al (1992), and Finger et al (1994and Finger et al ( , 2006and Finger et al ( , 2009a.…”

Section: Long-term Pulse Frequency Evolutionmentioning

confidence: 99%

X-Ray and Optical Observations of a 0535+26

Camero-Arranz

Finger

Wilson-Hodge

et al. 2012

ApJ

We present recent contemporaneous X-ray and optical observations of the Be/X-ray binary system A 0535+26 with the 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 ∼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 quasi-periodic oscillation 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 Hα equivalent width and the V-band brightness showed an anti-correlation after ∼2002 August. Each giant X-ray outburst occurred during a decline phase of the optical brightness, while the Hα 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 Hα line. These had a period of ∼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 ∼6 months following a giant outburst.

“…It allows to determine the Doppler delays and therefore the orbital parameters of the system. A fixed phase of the pulsating flux from a pulsar is observed at times (Nagase et al 1982):…”

Section: Timing Analysismentioning

confidence: 99%

Is there a highly magnetized neutron star in GX 301–2?

Santangelo

Suleimanov

et al. 2010

A&A

We present the results of an in-depth study of the long-period X-ray pulsar GX 301−2. Using archival data of INTEGRAL, RXTE ASM, and CGRO BATSE, we study the spectral and timing properties of the source. Comparison of our timing results with previously published work reveals a secular decay of the orbital period at a rate of −3.25 × 10 −5 d yr −1 , which is an order of magnitude faster than for other known systems. We argue that this is probably result either of the apsidal motion or of gravitational coupling of the matter lost by the optical companion with the neutron star, although current observations do not allow us to distinguish between those possibilities. We also propose a model to explain the observed long pulse period. We find that a very strong magnetic field B ∼ 10 14 G can explain the observed pulse period in the framework of existing models for torques affecting the neutron star. We show that the apparent contradiction with the magnetic field strength B CRSF ∼ 4 × 10 12 G derived from the observed cyclotron line position may be resolved if the line formation region resides in a tall accretion column of height ∼2.5-3 R NS . The color temperature measured from the spectrum suggests that such a column may indeed be present, and our estimates show that its height is sufficient to explain the observed cyclotron line position.