Spectral transition of 4U 1608-522 during the undergoing 2010 outburst

ATel #2467; E. Del Monte, R. Campana, I. Donnarumma, Y. Evangelista, M. Feroci (INAF-IASF Roma), D. Altamirano, D. M. Russell (Univ. of Amsterdam), P. Casella (Univ. of Southampton), E. Kuulkers, C. Sanchez-Fernandez (ESA/ESAC), F. Lewis (Faulkes Telescope Project, University of Glamorgan, Open University) and J. J. M. in 't Zand (SRON)
on 9 Mar 2010; 09:46 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Ettore Del Monte (ettore.delmonte@iasf-roma.inaf.it)

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Following the SuperAGILE detection of an X-ray burst from the neutron star transient and atoll source 4U 1608-522 (ATel #2461), on 3 March 2010 at 06:33:25 UT, marking the beginning of a new outburst, as confirmed by the MAXI (ATel #2462) and INTEGRAL (ATel #2464) detection of increased persistent emission, we obtained Swift and RXTE target of opportunity observations.

We find that the source switched to a soft spectral state between the two Swift observations. In the first one, which started about 53 ks after the SuperAGILE X-ray burst, the counting rate of RXTE/ASM was about 2 cts/s (roughly 30 mCrab; 2 - 12 keV) and the spectrum is described by an absorbed power-law with photon index of 2.13 +/- 0.07 and unabsorbed flux (2 - 10 keV) of 6*10^-10 erg/cm^2/s. In the second observation, which started about 359 ks after the X-ray burst, the RXTE/ASM counting rate was about 30 cts/s (about 400 mCrab; 2 - 12 keV). At that time the spectrum is described by a blackbody (with kT = 0.96 +/- 0.05 keV) plus a power-law of 1.82 +/- 0.06 photon index, with unabsorbed flux of 9*10^-9 erg/cm^2/s (2 - 10 keV). In both spectra we assume a Galactic column density of 1.2*10^22 cm^-2 (e.g., Tarana et al., 2008, ApJ, 688, 1295), compatible with the range found in the literature (1 - 2 *10^22 cm^-2, Penninx et al., 1989, A&A 208, 146). Assuming a distance of 5.8 kpc (Guver et al. 2010, arXiv:0811.3979), between the two XRT observations the unabsorbed source luminosity increased from about 2*10^36 erg/s up to about 4*10^37 erg/s. No bursts were detected during the Swift observations.

The RXTE/PCA ToO campaign started about 130 ks after the SuperAGILE X-ray burst and at that time the source was already in the soft spectral state. The flux in the 2 - 16 keV energy range increased from about 50 mCrab up to more than 500 mCrab in about four days. With an observing strategy of daily frequency, only four X-ray bursts were detected, all within a time range of 12 ks. From a preliminary analysis, only in one of the bursts oscillations were detected, at a frequency of 619 Hz, consistent with other episodes reported in literature for the same source.

We would like to thank the Swift and RXTE Teams for making these observations possible, in particular the duty scientists as well as the science planners.