MAXI J0556-332 is a transient neutron-star Z source

ATel #3650; Jeroen Homan (MIT), Manu Linares (MIT), Maureen van den Berg (Utrecht, Harvard/CfA), Joel Fridriksson (MIT)
on 17 Sep 2011; 17:52 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)

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We report on ongoing RXTE observations of the X-ray transient MAXI J0556-332. This source was discovered on 2011 January 11 (ATel #3102) and has since been monitored on an almost daily basis with RXTE.

A preliminary X-ray color analysis of ~470 ks of RXTE/PCA data reveals a somewhat chaotic pattern without a clear structure in the color-color (CD) and hardness-intensity (HID) diagrams. However, analysis of shorter time intervals (~5-10 days) reveals tracks that strongly resemble those of the Z sources, the brightest neutron-star low-mass X-ray binaries (LMXBs). In particular, near the peak of its current outburst (around 2011 February 9) the CD/HID tracks of MAXI J0556-332 resemble those of the Cyg-like Z sources, with large count rate changes along the so-called horizontal, normal, and (dipping) flaring branches of the Z track. A rapid variability analysis of some of the Cyg-like Z tracks reveals low-frequency quasi-periodic oscillations (QPOs) between ~1.5 Hz and ~35 Hz on the horizontal branch; QPOs from MAXI J0556-332 were already reported in ATel #3112. The frequency evolution of these QPOs is in line with what is seen in other Cyg-like Z sources, i.e., increasing from the horizontal branch upturn towards the horizontal/normal branch vertex, and their frequency range is similar to that observed in Cir X-1 (Shirey et al. 1996, ApJ, 469, L21). We note that the strong dipping and eclipse-like features reported in ATels #3106, #3110, #3349 are common in the light curves of Cyg-like Z sources.

Currently, at count rates of 40-100 cts/s/PCU (~15%-40% of the peak count rate), the CD/HID tracks are more similar to those of the Sco-like Z sources. We were not able to detect QPOs in the Sco-like phase of the outburst (probably as a result of lower count rates and intrinsically weaker QPOs).

MAXI J0556-332 is the third transient Z source discovered in recent years, with XTE J1701-462 (Homan et al. 2010, ApJ, 719, 201) and IGR J17480-2446 (ATel #2952) being the other two. We note that earlier suggestions about the neutron star nature of MAXI J0556-332 were already made based on optical (ATel #3116) and radio observations (ATel #3119) of the source.

The count rates of MAXI J0556-332 are a factor of ~10 lower than the transient Z source XTE J1701-462 for similarly shaped CD/HID tracks. Using the 8.8 kpc distance estimate of XTE J1701-462 (Lin et al. 2009, ApJ, 699, 60), and assuming that similarly shaped tracks occur at similar luminosities, we derive a distance of ~20-35 kpc, which should be regarded as a very rough estimate only (as our assumption is uncertain). This translates into a distance from the Galactic center of 25-39 kpc, with a distance below the plane of 8-15 kpc. This would make MAXI J0556-332 a very distant halo source, which is obviously a very unusual location for a LMXB, although other distant Galactic LMXBs have been identified before (Casares et al. 2009, ApJS, 181, 238). No known globular clusters are present within a few degrees on the sky of MAXI J0556-332 (Harris 1996, AJ, 112, 1487).

Detection of type-I X-ray bursts, which are expected to occur once the source makes a transition to atoll source behavior, could provide additional information on the source distance and the origin/nature of MAXI J0556-332. Continuing X-ray observations are therefore strongly encouraged.