Chandra Large and Very Large Projects

Proposal Number: 03400837

Title: COMPLETE COMPACT OBJECT AND BINARY SURVEY OF 47TUC

PI Name: Jonathan Grindlay

Our cycle 1 observation of 47Tuc has shown its promise as the ultimate laboratory for study of the production and evolution of compact binaries and compact objects in globular clusters. We propose a deep (4 x 75ksec) ACIS-S observation to measure the complete populations of millisecond pulsars (MSPs) and cataclysmic variables (CVs) by their variability and spectra and to constrain the main sequence binary population. Coordinated HST (ACS) data would provide unique identifications of CVs and many MSPs and allow the full population of quiescent low mass x-ray binaries to be distinguished from CVs. This dataset would provide fundamental constraints for the origins of neutron stars, millisecond pulsars, magnetic white dwarfs and compact binaries in globular clusters.

Proposal Number: 05400594

Title: High Resolution Photospheric Spectroscopy of the Neutron Star in EXO0748-676

PI Name: FREDERIK PAERELS

Recently, we discovered narrow absorption features in the spectrum of 28 X-ray bursts from the neutron star in the LMXB EXO0748-676, whose properties point to a photospheric origin. The transition wavelengths are consistent with the n=2-3 lines in H- and He-like Fe, exhibiting a gravitational redshift of z=0.35. The measured line strengths indicate that the lines are significantly broadened by the Stark effect, and this will ultimately provide an opportunity to determine both the mass and the radius of this neutron star. Here we propose a very deep spectroscopic observation with Chandra HETGS, to detect the Fe Lyman spectrum and obtain crucial independent spectroscopic confirmation for the Balmer line identification, and to measure the spin period of the neutron star.

Proposal Number: 07400594

Title: The lowest-luminosity quiescent neutron star transient: implications for strong field gravity and the neutron star EoS

PI Name: Peter Jonker

We propose to obtain a 300 ksec ACIS-S observation of the quiescent neutron star (NS) SXT 1H1905+000. Because of a non-detection during a previous 25 ksec Chandra ACIS-S observation, the NS quiescent luminosity is the lowest of all NS SXTs. It also has a low N_H, a well-known source distance, and an accurately known source position. These observations will eliminate the gap in the derived quiescent luminosities between NS and BH SXTs which is used as being evidence for a BH event horizon. Furthermore, this extremely low NS luminosity allows us to derive that 1H1905+000 harbors a cold, massive NS ruling out all equations of state (EoSs) except the stiffest EoSs for NSs with a nucleonic core. Constraining the NS EoS is one of the ultimate goals of NS studies.

Proposal Number: 07400810

Title: CHAZSS: The Chandra HETGS Atoll/Z Spectroscopic Survey

PI Name: Jon Miller

High-res. X-ray spectroscopy of accretion-powered sources in the Chandra era has focused on black holes. Neutron stars with low magnetic fields - "Z" and "atoll" binaries - have largely been ignored and/or poorly observed at high resolution. We propose a survey of 6 "Z" and "atoll" neutron star binaries. In a total of only 300 ksec, we can observe each of the 6 sources in our sample twice, obtaining consistent, robust, and constraining spectra free from photon pile-up. High-res. spectroscopy will break degeneracies in X-ray timing-based models for the accretion flows in these systems, and provide new insights into both the origin of the "kHz" QPOs seen in these sources and the nature of the "Z" and "atoll" tracks they trace in color-color diagrams.

Proposal Number: 10400425

Title: Photospheric radius-expansion bursts at high spectral resolution

PI Name: Duncan Galloway

It has been predicted that strong (a few hundred~eV EW) photoionisation edges might be present in the X-ray spectra at the peak of photospheric radius-expansion thermonuclear bursts. We propose to search for such features in bursts from sources uniquely known to exhibit frequent radius-expansion bursts. These observations will result in detection of up to 30 thermonuclear bursts, also providing a high signal-to-noise low-flux state spectrum, as well as allowing a detailed comparison of recurrence time and energetics with numerical ignition models. Detection of discrete features from the burst spectra may lead to highly-sought constraints on the compactness and hence the neutron-star equation of state.

Proposal Number: 10400827

Title: The cooling neutron star in the super-Eddington accretor XTE J1701-462

PI Name: Jeroen Homan

Observing the cooling of neutron stars that are reheated by accretion provides new insights into neutron star structure. A detailed and efficient study of this cooling requires the use of Chandra. Here we propose to continue our ongoing monitoring campaign of the neutron star binary XTE J1701-462, a unique system that accreted at super- and near Eddington luminosities for more than 1.5 years before recently returning to quiescence. Our goal is to study the cooling of the surprisingly hot neutron star and also the evolution of the enigmatic non-thermal spectral component in unprecedented detail. Our ongoing strategy of frequent observations (every ~2.5 months) is necessary to test the most recent cooling models. We therefore request five ACIS-S observations for a total of 323 ks.