Spectral characteristics of GX 1+4 throughout a low flux episode

GX 1+4 was observed using the Rossi X-ray Timing Explorer (RXTE) satellite (Giles et al. 1995) over 1996 July 19-21 during a period of unusually low X-ray brightness for the source. For a detailed report see Galloway et al (1999) and Giles et al (1999). The countrate from the Proportional Counter Array (PCA) aboard RXTE indicates that the mean flux decreased smoothly from an initial level of $\approx 6\times 10^{36}\,{\rm erg\,s^{-1}}$ to a minimum of $\approx 4\times 10^{35}\,{\rm erg\,s^{-1}}$ (20-60 keV, assuming a source distance of 10 kpc) before partially recovering towards the initial level at the end of the observation.

The pulse profiles (folded at the best-fit constant period) and the mean photon spectra before and after the flux minimum show significant variation. The observation is divided up into three distinct intervals based on the mean flux. Interval 1 includes the start of the observation to just before the flux minimum. Interval 2 spans $\sim 6$ hours including the flux minimum, while during interval 3 the mean flux is rising steadily towards the end of the observation.

The pulse profile is asymmetric and characterised by a narrow, deep primary minimum (Fig. 2). During interval 1, the flux reaches a maximum closely following the primary minimum; this is referred to as a `leading-edge bright' profile. Pulsations all but cease during interval 2, and in interval 3 the asymmetry is reversed, with the flux reaching a maximum just before the primary minimum (`trailing-edge bright' profile). This is the first observation of such dramatic pulse profile variations over timescales of < 1 day.

**Figure 2:** Pulse profiles for GX 1+4 during July 1996 folded on the best-fit constant barycentre corrected period of $P=124.36568 \pm 0.00020$ s. The data is taken from the PCA on board *RXTE* and spans the energy range 2-60 keV. Typical $1\sigma$ errorbars for each interval are shown at the left.
$\begin{figure} \begin{center} \psfig{file=figure.ps,height=7cm} \end{center}\end{figure}$

Leading-edge bright profiles are generally associated with phases of spin-down in GX 1+4, while trailing-edge bright profiles are mostly observed during phases of spin-up (Greenhill, Galloway & Storey, 1998). Re-analysed data from the regular monitoring of the source by the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma-Ray Observatory (CGRO) indicate that the source switched from spin-down to spin-up $\approx 12$ days after the RXTE observation. This suggests that the mechanism for the pulse profile variations may be related to that causing the poorly-understood spin period evolution in this source.

The best fitting spectral model (Galloway et al, 1999) is based on the work of Titarchuk (1994). The principal component is generated by Comptonisation of a thermal input spectrum at $\approx 1$ keV by hot ( $kT \approx 8$ keV) plasma close to the source with scattering optical depth $\tau_P \approx 3$ . Additional components include a gaussian to fit Fe K $\alpha$ line emission from the source and a multiplicative component representing photoelectric absorption by cold material in the line-of-sight. Variations in the mean spectrum over the course of the observation are associated with a dramatic increase in the column density n_H from 13 x 10²² to $28\times 10^{22}\,{\rm cm}^{-2}$ between intervals 1 and 3, and also with significant energy-independent variations in the flux.

Similar spectral variations were seen in 4U 1626-67 before and after the spin rate transition in that source (Yi & Vishniac, 1999). This strengthens the argument that the pulse profile and spectral changes reported here were associated with the torque reversal in GX1+4 reported by Giles et al. (1999).

Pulse-phase spectral fitting indicates that variations in flux with phase can be accounted for by changes in the Comptonised model component, with in particular variations in the fitted optical depth $\tau_P$ and the component normalisation $A_{\rm C}$ accounting for the phase dependence. The spectral fits suggest that the soft input photons originate from the neutron star poles, and are subsequently Comptonised by matter in the accretion columns. The sharp dip in the pulse profiles is then tentatively identified with the closest passage of one of the magnetic axes to the line of sight. More details of the spectral analysis can be found in Galloway et al. (1999).