Discovery of mHz QPOs and burst rate evolution in the active Terzan 5 neutron star transient

ATel #2958; M. Linares (MIT), D. Altamirano, A. Watts, M. van der Klis, R. Wijnands (Amsterdam), J. Homan (MIT), P. Casella (Southampton), A. Patruno, M. Armas-Padilla, Y. Cavecchi, N. Degenaar, M. Kalamkar, R. Kaur, Y. Yang (Amsterdam), N. Rea (CSIC-IEEC)
on 20 Oct 2010; 10:32 UT
Credential Certification: Manuel Linares (linares@mit.edu)

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We report on RXTE observations of the neutron star transient currently active in Terzan 5 (also known as IGR J17480-2446; ATels #2919, #2920, #2922, #2924, #2929, #2932, #2933, #2937, #2939, #2940, #2946), which has recently turned into a Z source (Altamirano et al., ATel #2952).

The X-ray burst rate has increased gradually (see also ATels #2939, #2952) from October 13th (recurrence time of at least 55 min) to October 16th (recurrence time of 8 to 6 min), accompanied by a gradual increase in the persistent flux (2-16 keV intensity increasing from ~0.1 Crab to ~0.5 Crab). The net peak burst to persistent flux ratio has decreased smoothly from ~375% to ~15-20% along the same period. In summary, the bursts have become fainter and more frequent as the mass accretion rate increased.

On October 18th and 19th we detect quasi-periodic oscillations (QPOs) at a frequency of about 4.5 mHz. The mHz QPO is visible with naked eye in the lightcurve, in the form of short (~20s) and faint bursts or flares every ~220-250s (3.7-4.2 min). A preliminary Lorentzian fit to the average power spectrum (using the full ~2--60 keV energy band) yields a centroid frequency of 4.3+/-0.1 mHz, a fractional rms amplitude of 1.5+/-0.8 % and a FWHM of 0.6+/-0.3 mHz. The mHz QPO is present mainly on the horizontal branch of the Z-source track in the hardness-intensity diagram (ATel #2952).

The burst recurrence times observed between October 13th and 16th converge asymptotically towards the mHz QPO timescale measured in October 18th and 19th, strongly suggesting that they are part of the same phenomenon. Following previous work (Revnivtsev et al. 2001, A&A, 372, 138; Altamirano et al. 2008, ApJL 673, 35; Heger et al. 2007, ApJ, 665, 1311), we interpret these mHz QPOs as a signature of marginally stable nuclear burning near the transition between unstable and stable burning. In this system, however, the persistent luminosity (Atel #2952) when mHz QPOs are present is about an order of magnitude higher than previously measured, in accordance with theoretical expectations (Bildsten 1998, mfns, 419; Heger et al. 2007). We note that the same stability boundary was recently probed by Cir X-1 (Linares et al. 2010, ApJL, 719, 84), although no mHz QPOs were detected.