Spin period evolution of GX 1+4

Abstract

Aims. We aim both to complement the existing data on the spin history of the peculiar accreting X-ray pulsar GX 1+4 with more past and current data from BeppoSAX, INTEGRAL, and Fermi and to interpret the evolution in the framework of accretion theory. Methods. We used source light curves obtained from BeppoSAX/WFC and INTEGRAL/ISGRI to derive pulse periods using an epoch-folding analysis. Fermi/GBM data were analysed by fitting a constant plus a Fourier expansion to background-subtracted rates, and maximizing the Y 2 statistic. We completed the sample with hard X-ray light curves from Swift/BAT. The data were checked for correlations between flux and changes of the pulsar spin on different timescales. Results. The spin-down of the pulsar continues with a constant change in frequency, i.e., an apparently accelerating change in the period. Over the past three decades, the pulse period has increased by about ∼50%. Short-term fluctuations on top of this long-term trend do show anti-correlation with the source flux. Possible explanations of the observed long-term frequency and its dependence on flux are discussed.

Properties and observability of glitches and anti-glitches in accreting pulsars

“…The timescale t ∞ between two torque reversals varies from source to source. Some pulsars show numerous torque reversals with moderate spin period derivative (e.g., Vela X−1, Her X−1; Bildsten et al 1997), others show quasi-periodic torque reversals superimposed on a longterm spin-up trend (e.g., OAO 1657−415, Barnstedt et al 2008; EXO 2030+375, Fermi/GBM 6 ; SAX J2103.5+4545, Camero et al 2014), while others show steady spin-up or spin-down sporadically interrupted by a torque reversal (e.g., GX 1+4, González-Galán et al 2012;4U 16264U −67, Beri et al 20144U 1907+09, Inam et al 2009. It is interesting to compare, for each pulsar, t ∞ with Δt gl , the time needed to reach the critical rotational lag for depinning.…”

The protoMIRAX hard X-ray imaging balloon experiment

“…Its peculiar spin history has been the object of intensive study (Nagase 1989), and Makishima et al (1988) have shown a clear transition from spin-up to spin-down behavior in the equilibrium period, characterizing a torque-reversal episode. More recent studies (González-Galán et al 2012) have used data from BeppoSAX, INTEGRAL, Fermi and Swift/BAT to show that the source continues its spin-down trend with a constant change in frequency, and the pulse period has increased by ∼50% over the past three decades. Since the source is highly variable, our spectral and timing observations can possibly contribute to the flux and period histories for this peculiar object.…”

“…The mask is made out of 1 mm-thick lead sheets and the basic cells are blue 20 mm × 20 mm squares. The pattern of closed and open elements are a cyclic extension (2 × 2) of a 13 × 13 Modified Uniformly Redundant Array (MURA) pattern (Gottesman & Fenimore 1989) with 20 mm-side square Fig. 4.…”

“…(Actually, to be precise, the delta function is the result of the cross-correlation between the mask pattern and the decoding function, which is almost exactly equal to the mask pattern with the exception of one single element.) For a point source, the reconstructed image is "perfect" in the sense that it produces a peak in an otherwise completely flat image (see Gottesman &Fenimore 1989 andBraga et al 2002 for details about imaging with MURAs). With this configuration, the protoMIRAX X-ray camera will have a total FOV of 20.…”

“…Taking the expected background levels at balloon altitudes as a function of energy, one can calculate the sensitivity of the experiment in terms of the minimum detectable flux at a particular statistical significance. As shown by Gottesman & Fenimore (1989), the signal-to-noise ratio (S /N ≡ N σ ) of a coded mask instrument that uses URAs or MURAs, for a single point source, is given by…”