Proposal Details - 0920009
Hercules X-1: Variability of the cyclotron line energy Ecyc and the question of NS free precession (Staubert)

Proposal Abstract

It is proposed to observe Her X-1 during INTEGRAL's AO-9 for a total observing time of 600 ksec: covering one Main-On with two INTEGRAL orbits, the first near the maximum of the Main-On, the second during its decay, plus one INTEGRAL orbit during the following Short-On. The general objective is to continue the long-term covering of this enigmatic source by INTEGRAL, in order to investigate two (intimately related) aspects: A. the physics of generating the cyclotron line and the reasons behind the variability of Ecyc as a function of time, luminosity, 35-day precession-phase and 1.24 s pulse-phase, and B. the question whether the neutron star (NS) does really precess with a period of 35 days in strict synchronization with the accretion disk. If the observed systematic variations of the shape of the pulse profiles are indeed due to the NS's precession, then one would have to conclude that the precessional period is changing by a few percent every few years, which most likely requires some changes in the structure of matter and/or magnetic field of the NS's interior. This may open a new channel to the understanding of the properties of matter at supra-nuclear densities. On the other hand, some observational results seriously challenge the idea of NS precession in Her X-1.

Observation Strategy (Recommended by TAC)

Total of 600 ks split into three observations of 200 ks each in the standard 5x5 dithering mode centered on RA, Dec = 254.458, 35.342 (l, b = 58.149, 37.523). The observations will take place at specific times to cover distinct pulse phases with three INTEGRAL orbits: the first near the early maximum of the "Main-On" (phases 0.05 to 0.15 of the 35-day cycle), the next one during the decay of the same "Main-On" period (phases 0.2 to 0.3 of the 35-day cycle) and, finally, one INTEGRAL orbit during the following "Short-On" (phases 0.6 to 0.7 of the 35-day cycle). The exact observing times are not defined yet since "Main-Ons" appear with a somewhat irregular clock. The timing of the observations will be predicted on the basis of RXTE/ASM, SWIFT/BAT and MAXI observations (with a confidence of ±1 day about two months before), and more reliable one month before the actual INTEGRAL observations (between January-April, June-October, and December 2012).

Proposal grade: B

Data Rights (Recommended by TAC)

The TAC granted data rights for Her X-1

Exposure Map

The exposure maps are in galactic coordinates using the Aitoff projection and units of seconds, and the colour scale is indicated at the bottom. The green lines provide the 100, 300 and 500 ksec contour.