Continuing RXTE observations of XTE J1701-462: a new "Z" in town?

ATel #725; Jeroen Homan (MIT), Tomaso Belloni (INAF/OAB), Michiel van der Klis, Piergiorgio Casella (U. Amsterdam), Mariano Mendez (SRON), Walter Lewin (MIT), Rob Fender (U. Southampton), Elena Gallo (UCSB), and Neil Gehrels (NASA/GSFC)
on 8 Feb 2006; 12:50 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)

Tweet

Since January 19 the new X-ray transient XTE J1701-462 (ATels #696, #703, #708, #710) has been observed more than 45 times with the Rossi X-ray Timing Explorer (RXTE). Preliminary analysis of the RXTE data suggest the source is a neutron star LMXB and is currently showing behavior typical of the so-called "Z sources" (i.e. the brightest, persistent neutron star LMXBs). This would make XTE J1701-462 the first new Z source in nearly 35 years.

The prospect of XTE J1701-462 of being a transient Z source opens up interesting possibilities if the source eventually fades away. A major question that can be addressed is whether at lower mass accretion rates Z sources start to behave like atoll sources. Regardless of these future opportunities, XTE J1701-462 already is a unique and interesting source and we strongly urge observers to continue or start monitoring the source at all wavelengths. For those who are interested a detailed description of the RXTE observations is given below.

For figures, visit: http://tahti.mit.edu/Z

RXTE/PCA light curves reveal that the source continues to exhibit large count rate changes (by factors of more than two on a time scale of hours). However, occasionally the count rate can be relatively stable for periods of ~1 day. The count rate changes are often accompanied by large changes in the X-ray colors, especially above 8 keV.

In a color-color diagram (CCD) these spectral changes result in a multi-branched track that resembles those of some of the neutron star Z sources, in particular that of GX 5-1 (see figure 13 in Jonker et al. 2002, MNRAS, 333, 665). In the plot of our CCD we tentatively identify some of the branches as the horizontal branch (with strong upturn), the normal branch, and the flaring branch (with a dipping branch, as also seen in GX 5-1). Over the course of our observing campaign the overall position of the track has shifted to slightly softer colors. Such shifts are not uncommon in Z sources. These shift are even more apparent in color-intensity diagrams (CIDs). The sharp turn that we identify as the horizontal/normal branch vertex has slowly increased in count rate during the past two and a half weeks, suggesting that the source has increased in brightness by more than 20%. We also note that the flaring branch is inverted - this is particularly clear when plotting count rate vs. soft X-ray color. Such behavior has previously been seen before in the Z source Cygnus X-2.

The rapid X-ray variability properties of the source evolve smoothly along the track in the CCD/CID (see link above for example power spectra). At the start of the horizontal branch (upturn) we detect a ~10 Hz QPO that smoothly increases in frequency along the horizontal branch onto the top normal branch where it reaches a maximum of ~55 Hz (see also ATel #703). This QPO is accompanied by band limited noise that also increases in frequency. Both QPO and noise decrease in strength as their frequencies increase. Around the middle of the normal branch we detect a broad feature near 5-6 Hz (see also ATel #703), which is particularly clear in power spectra above 8 keV. The feature disappears near the bottom of the normal branch. On the flaring branch the power spectrum only shows weak red noise (rms variability [0.1-100 Hz] is ~2.5%, compared to 16% on the top of the horizontal branch).

The evolution of the timing features along the tracks in the CCD/CID as well as their typical strength and frequency are very similar to those seen in the Z sources. The relatively weak power-law component in the energy spectra further supports the link with the Z sources. Other neutron star signatures, like X-ray bursts, pulsations, or kHz QPOs have not been detected so far.