2013A&A...551A...1R


Query : 2013A&A...551A...1R

2013A&A...551A...1R - Astronomy and Astrophysics, volume 551A, 1-1 (2013/3-1)

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

REIG P. and NESPOLI E.

Abstract (from CDS):

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. Accretion-powered pulsars with Be companions represent the most numerous group of high-mass X-ray binaries. When active, they are amongst the brightest extra-solar objects in the X-ray sky and are characterised by dramatic variability in brightness on timescales of days. 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. Energy spectra were fitted with an absorbed power-law modified by an exponential cutoff. Discrete components such as iron emission lines and cyclotron lines were represented by Gaussian and pseudo-Lorentzian profiles, respectively. Power spectra were fitted by a combination of Lorentzian functions. 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 quasi-periodic oscillation 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. In some sources, a significant correlation between spectral and timing parameters is seen, implying and interplay between the accretion column and the inner accretion disc. 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. In this work, the systems that display the two branches have critical luminosities in the range (1-4)x1037erg/s.

Abstract Copyright:

Journal keyword(s): X-rays: binaries - stars: neutron - stars: emission-line, Be - pulsars: general

CDS comments: Swift J1656.6-5156 is a misprint for Swift J1626.6-5156. In caption, 1A 1118-612 is a misprint for 1A 1118-616.

Simbad objects: 11

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Number of rows : 11
N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2022
#notes
1 V* V635 Cas HXB 01 18 31.9704955824 +63 44 33.072603144 17.05 16.64 15.19 14.34 13.22 B0.2Ve 688 0
2 V* BQ Cam HXB 03 34 59.9116110096 +53 10 23.296879776   17.16 15.42 14.26 13.04 O8.5Ve 425 1
3 HD 245770 HXB 05 38 54.5748918624 +26 18 56.836952784 9.30 9.84 9.39 8.77 8.30 O9/B0III/Ve 920 0
4 2E 1752 HXB 06 58 17.2878153600 -07 12 35.204283324   12.38 10.9 11.40   O9.7Ve 80 0
5 GRO J1008-571 HXB 10 09 46.9642314696 -58 17 35.603851848       14.63   B0e 200 0
6 WRAY 15-793 HXB 11 20 57.1736884968 -61 55 00.166078020 12.81 13.06 12.12     O9.5III/Ve 182 0
7 SWIFT J1626.5-5156 HXB 16 26 36.5197704024 -51 56 30.600051936       16.03 14.59 B0V 74 0
8 V* HZ Her LXB 16 57 49.8110126616 +35 20 32.486555472 11.90 14.89 13.63 13.88   B3/6ep 1988 0
9 4U 1901+03 HXB 19 03 39.3926409480 +03 12 15.758070228           OB 89 0
10 2MASS J19493548+3012317 HXB 19 49 35.4841147968 +30 12 31.775979852   15.16 14.22 13.53 12.88 B0Ve 148 1
11 V* V2246 Cyg HXB 20 32 15.2749858128 +37 38 14.843024844   22.16 19.41 17.32 15.18 B0Ve 398 0

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2022.05.24-12:13:13

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