Rossi X-ray Timing Explorer observations of the black-hole candidate 4U 1630-47 during its 2002 outburst

ATel #109; Jeroen Homan (INAF-OAB) & Rudy Wijnands (University of St. Andrews)
on 2 Oct 2002; 17:57 UT
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

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After the detection of renewed X-ray activity from the recurrent X-ray transient and black-hole candidate 4U 1630-47 (ATEL #106), the source has been monitored frequently with the Rossi X-ray Timing Explorer (RXTE). We have analysed the publicly available RXTE observations performed during 12 and 26 September 2002 UT, resulting in ~19 ksec of on source time spread out over 13 observations. The X-ray flux (3-100 keV; unabsorbed) varied between 1.4 and 2.5E-08 erg/s/cm^2. The spectrum could be fitted with a disk black-body (with temperature of 1.4-1.6 keV) plus power-law model (with photon index of 2.3-3.2; contributing between 20 and 50% to the total flux). A weak trend was present suggesting that the temperature decreased and the photon index increased when the 3-100 keV flux decreased. The column density obtained (~ 1E23 cm^-2) from the fits was consistent with being constant during the observations.

Light curves in the 3-60 keV energy range were characterized by strong flaring (increases/decreases by factors up to 2) on a time scale of tens to hundreds of seconds. The power density spectra are dominated by power law noise with a strength of 3-11 % rms (0.01-100 Hz) and an index of ~1.3. No QPOs were found.

The spectral shape of 4U 1630-47 during our observations resembles those of other black-hole transients during their very high states (VHS). The strength and shape of the band-limited noise in the power density spectra is consistent with this interpretation. However, the black-hole VHS is usually identified also by the presence of QPOs which have not been detected during the described observations.

Interestingly, during the 1998 outburst 4U 1630-47 entered a true VHS state during which QPOs were detected (Dieters et al. 2000, ApJ 538, 307; Trudolyubov et al. 2001, MNRAS 322, 309) *and* the source was detected at radio wavelengths (Hjellming et al. 1999, ApJ 514, 383). The absence of QPOs during the present candidate VHS in combination with the absence of radio emission during this outburst (ATEL #108) might suggest a possible connection between the QPO and the radio emission mechanisms. Further monitoring by RXTE and radio observations are needed to confirm this possible relation between radio emission and quasi-periodic variability during the VHS in 4U 1630-47.