Abstract for Proposal 087219

MEASURING THE HIGH ENERGY EMISSION OF MILLISECOND X-RAY PULSARS IN OUTBURST

We propose to perform two 170 ks INTEGRAL target of opportunity (ToO) observations of a transient millisecond X-ray pulsar in outburst. To constrain the broadband spectrum we also request one 30 ks simultaneous NuSTAR and two 50 ks XMM-Newton observations. The target can be either one of the nineteen known transient accreting millisecond X-ray pulsars (AMXPs) under going a new outburst, or a “newly” discovered object of this class. These observations will allow us to study the broad band spectrum in detail, from hard to soft X-ray energies, as well as the timing properties or eclipsing features of the source during its outburst. The high signal-to-noise spectral information will make it possible to disentangle the contributions of soft black body, reflection (if any), and hard Comptonized spectral components. Moreover, we may detect type-I X-ray bursts, and/or for the first time also burst oscillations at high-energy (if present). In particular we will be able to study the energy spectrum in a broad energy range (0.1–300 keV) and with unprecedented high sensitivity above 20 keV. The INTEGRAL, NuSTAR, NICER, HXMT and XMM-Newton observations will also allow a timing analysis to study the pulse profile, time lags and pulsed spectrum, and will thus provide important constraints on emission mechanisms.



Details on Observing Strategy and Trigger Criteria
Criteria for ToO triggering are based on ongoing observations with INTEGRAL (ISGRI), Swift (BAT) and MAXI, whose wide field of views scan the sky regions containing known AMXPs. Also, new X-ray transients, detected by these wide field instruments, identified later through dedicated pointed observations with narrow-field instruments, like XMM-Newton, Chandra, NuSTAR, Swift, NICER or HXMT, as AMXP can trigger our proposed program.

The first (INTEGRAL+XMM+NuSTAR) ToO observation will be triggered if one of the following conditions are fulfilled: a) One of the nineteen known transient AMXP sources goes into outburst with an X-ray flux > 30 mCrab. b) A new AMXP has been identified/detected with an X-ray flux > 30 mCrab at hard X-rays with any satellite. The second (INTEGRAL+XMM) observation should be performed 3-5 days after the first one when, based on the typical AMXP outburst profile, the mass accretion rate onto the NS has decreased by a factor of 1.5 -- 2.