RXTE observations of the low/hard state X-ray outburst of the new X-ray transient SWIFT J1753.5-0127

Abstract

We present the results of the analysis of RXTE (Rossi X-ray Timing Explorer) observations of the new x-ray transient, SWIFT J1753.5-0127 during its outburst in July, 2005. The source was caught at the peak of the burst with a flux of 7.19e-09 ergs-s$^{-1}$-cm$^{-2}$ in the 3-25 keV energy range and observed until it decreased by about a factor of 10. The photon index of the power law component, which is dominant during the entire outburst, decreases from $\sim$1.76 to 1.6. However, towards the end of the observations the photon index is found to increase, indicating a softening of the spectra. The presence of an ultrasoft thermal component, during the bright phases of the burst, is clear from the fits to the data. The temperature associated with this thermal component is 0.4 keV. We believe that this thermal component could be due to the presence of an accretion disk. Assuming a distance of 8.5 kpc, $L_{X}/L_{Edd} \simeq 0.05$ at the peak of the burst, for a black hole of mass $10 {\rm M}_{\odot}$. The source is found to be locked in the low/hard state during the entire outburst and likely falls in the category of the x-ray transients that are observed in the low/hard state throughout the outburst. We discuss the physical scenario of the low/hard state outburst for this source.