Frequent bursts from the 11 Hz transient pulsar IG J17480-2446

Manoneeta Chakraborty1* ,Arunava Mukherjee2 and S. Bhattacharyya1
1TIFR, Homi Bhabha Road, Navy Nagar, Colaba, Mumbai 400005, India
2IUCAA, Post Bag 4, Ganeshkhind, Pune University Campus, Pune 411007, India

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Accreted matter falling on the surface of the neutron star in a Low Mass X-ray Binary (LMXB) system gives rise to intense X-ray bursts originating from unstable thermonuclear conflagration and these bursts can be used as a tool to constrain the equation of state. A series of such Xray bursts along with millihertz (mHz) quasi-periodic oscillations (QPOs) at the highest source luminosities were observed during the 2010 outburst of the transient LMXB pulsar IGR J17480–2446. The quite diverse burst properties compared to typical type-I bursts suggested them to be the type- II bursts originating from accretion disc instability. We show that the bursts are indeed of thermonuclear origin and thus confirm the quasi-stable burning model for mHz QPOs. Various properties of the bursts such as, peak flux, fluence, periodicity and duration, were highly dependent on the source spectral states and their variation over a large accretion rate range revealed the evolution of the burning process at different accretion rate regimes.

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Keywords : accretion, accretion discs – stars: neutron – stars: individual: IGR J17480-2446