D. Altamirano (Amsterdam), J. Homan (MIT), M. Linares (MIT), A. Patruno (Amsterdam), Y. Yang (Amsterdam), A. Watts (Amsterdam), M. Kalamkar (Amsterdam), P. Casella (Southampton), M. Armas-Padilla (Amsterdam), Y. Cavecchi (Amsterdam), N. Degenaar (Amsterdam), D. Russell (Amsterdam), R. Kaur (Amsterdam), M. van der Klis (Amsterdam), N. Rea (CSIC-IEEC), R. Wijnands (Amsterdam)

ATel #2952; The transient neutron star LMXB in the globular cluster Terzan 5 has turned into a Z-source
on 18 Oct 2010; 21:42 UT
Credential Certification: Diego Altamirano (diego@science.uva.nl)

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We report on the recent RXTE pointed observations of the the ongoing outburst of the neutron star (NS) transient source in the globular cluster Terzan 5 (Atel #2919, #2920, #2922, #2924, #2929, #2932, #2933, #2937, #2939, #2940, #2946).

Papitto et al. (Atel #2939) reported the occurrence of type I X-ray bursts; their recurrence time decreased from ~1000s to ~700 since the first RXTE observation. In following RXTE observations the 2-16 keV increased further and the X-ray burst recurrence time decreased until October 16th, 2010 (MJD 55485.7, 2-16 keV flux of ~0.55 Crab), when we detected one burst every 315-400 seconds.

In following RXTE observations on October 17th and 18th, 2010, the 6-16 keV spectrum of this transient became softer and its intensity started to show large flux variations between 0.45 and 0.75 Crab. A preliminary analysis reveals no evidence of X-ray bursts during this period. From a preliminary spectral fit to the average PCU2 spectrum we estimate a 2-50 keV unabsorbed flux of ~2.2e-8 erg/cm2/s, which corresponds to a luminosity of 2e38 erg/s at 8.7 kpc (Cohn et al 2002; ApJ 571, 818).

In the October 18th observation (ObsID: 95437-01-06-00) we detected for the first time high-frequency (>10 Hz) quasi-periodic oscillations (QPOs) on top of broad-band noise. These QPOs have frequency of ~48 Hz and ~173 Hz and rms amplitudes of ~2.7 and ~1.4, respectively (2-60 keV). We also found a 3.8\sigma (single trial) kHz QPO at ~815 Hz (10-50 keV).

The power and energy spectral changes observed in the last days suggest that the source was in an Atoll phase until October 16-17th, during the period when X-ray burst occurred regularly, but now has transited to a Z-source. Particularly, the color-color and hardness-intensity diagrams suggest a Cyg-like Z source behavior; where we observe what it looks like the horizontal, the normal and the dipping-flaring branches (see for a review: van der Klis 2006, "Compact stellar X-ray sources", astro-ph/0410551). The QPOs only appear in the horizontal branch.

This is the second source in the last 15 years to fully show Atoll and Z-source characteristics (XTE J1701-462, see, e.g. Homan et al. 2010, ApJ, 719, 201), however, unlike XTE J1701-462, this shows strong coherent pulsations (Atel #2929).

Observations at other wavelengths are strongly encouraged.