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New outburst of the accreting millisecond X-ray pulsar NGC 6440 X-2 and discovery of a strong 1 Hz modulation in the light-curve

ATEL # 2672; A. Patruno, Y. Yang, D. Altamirano, M. Armas-Padilla, Y. Cavecchi, N. Degenaar, M. Kalamkar, R. Kaur, M. van der Klis, A. Watts, R. Wijnands (Amsterdam), M. Linares (MIT), P. Casella (Southampton), N. Rea (CSIC-IEEC), P. Soleri (Groningen), C. Markwardt (U. Maryland and NASA/GSFC), T. Strohmayer (NASA/GSFC), C. Heinke (U. Alberta)
on 12 Jun 2010; 22:44 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Alessandro Patruno (a.patruno@uva.nl)

Subjects: X-ray, Binaries, Neutron Stars, Pulsars, Transients

On June 11th, 2010, RXTE/PCA galactic bulge scan observations showed an increase in activity from the globular cluster NGC 6440. Two accreting millisecond X-ray pulsars (AMXPs) and 22 other X-ray binaries are known in NGC 6440 (see Pooley et al. 2002, ApJ 573, 184, Altarmirano et al. 2010, ApJL 712, 58, Altamirano et al. 2008, ApJL 674, 45 and Gavriil et al. 2007, ApJ 669, 22). On June 12th, 2010 (10h 13m 00s UT), a ~10 ks long pointed RXTE observation was performed. A preliminary analysis of the X-ray data reveals a strong pulsation at an unbarycentered frequency of 205.891 Hz which is consistent with the spin frequency of the AMXP NGC 6440 X-2, making the identification secure. NGC 6440 X-2 is the AMXP (and also the low mass X-ray binary) with the shortest recurrence time (~1 month, Altamirano et al. 2010, ApJL 712, 58, Heinke et al. 2010, ApJ 714, 894) and with very short duration outbursts which last for less than a week. The power density spectrum in the 2-60 keV energy band reveals a broad feature with a peak frequency of ~1 Hz, and an rms amplitude of (46+-8)% rms. Although this feature is too broad to be defined as a QPO, it resembles in many aspects the 1 Hz modulation previously observed in the AMXP SAX J1808.4-3658 (Patruno et al. 2009, ApJ 707, 1296). In SAX J1808.4-3658 the appearance of the 1 Hz modulation was associated with the onset of the propeller regime. A pointed Swift observation was also made on June 12th, and confirms the identification of NGC 6440 X-2 as the source in outburst. Swift XRT operated in Photon-Counting mode and gives a position of the source: RA (2000) = 17 48 52.4497 DEC (2000) = -20 21 27.4491 with an error radius of 3.5 arcseconds. The observed flux is 2.73(+0.25, -0.28)x10^-11 erg/s/cm2 in the 0.3-10 keV energy band, corresponding to an unabsorbed flux of 4.3x10^-10 erg/s/cm2 for an absorption column NH of 6.2(+1.3, -1.2)x10^21 cm^-2. Pile-up corrections were applied. The spectrum is well fitted with an absorbed power law with index 1.77(+0.20,-0.19). The 0.3-10 keV luminosity is ~3.7x10^36 ergs/s assuming a distance of 8.5 kpc. Further observations are encouraged. We would like to thank the RXTE and Swift teams for promptly scheduling the observations of the source.


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