Thermonuclear burst spectroscopy with LOFT

LOFT meeting Oct 2011, UvA

Duncan GallowayHauke WorpelMonash University

Duncan.Galloway@monash.edu http://users.monash.edu.au/~dgallow

• For the vast majority of bursts the net X-ray spectra (after subtraction of the pre-burst/persistent emission) are consistent with a Planck (blackbody) spectrum

• Several (generally successful) past attempts to test/verify this Kuulkers et al. 2002, A&A 382, 947; Boutloukos et al. 2010, ApJL 720. L15; Güver et al. 2011, arXiv:1103.5767

“Conventional” spectral analysis

• This requires an unchanging persistent flux cf. with Walker 1992, ApJ 385, 642

• The spectrum is thought to be distorted slightly so we correct based on results from atmosphere models work of London, Madej, Suleimanov etc.

Thermonuclear burst spectroscopy with LOFT

The neutron star equation of state

• Burst blackbody normalisation has been used to infer the mass and radius e.g. in 4U 1608-52, EXO 1745-248, 3A 1820-30, and KS 1731-26, Özel, Güver &c; see also Steiner et al. 2010, ApJ 722, 33

• Atmosphere models compared to data Suleimanov et al. 2011, arXiv:1004.4871

• These analyses rely on the correctness of the blackbody model (+ various assumptions, model-dependent corrections etc.)

Thermonuclear burst spectroscopy with LOFT

Non-Planckian spectra during bursts

• A study of a very large (>60,000) sample of burst spectra indicate that they are not (en masse) consistent with blackbodies

• We need to better understand the burst spectrum

• There are at least two different sources of deviation

Thermonuclear burst spectroscopy with LOFT

~2000 bursts observed with RXTE

1. Spectral features during bursts

• There has been much observational effort to detect X-ray spectral lines during bursts, with the best candidate to date EXO 0748-676 Cottam et al. 2002, Nature 420, 51

Thermonuclear burst spectroscopy with LOFT

• However, the 552 Hz burst oscillation (Galloway &c

2010, ApJL 711, 148), indicates that the narrow spectral lines could not have arisen at the neutron star surface Lin et al. ApJ 723, 1053, 2010

• Also not seen in repeat observations

incl

inati

on

“hotspot” latitude

Features in radius-expansion bursts

• Low-resolution RXTE/PCA spectra of the most intense radius-expansion bursts show features suggestive of edges in ‘t Zand & Weinberg 2010, A&A 520, 81

• None have (yet) been observed with Chandra or XMM-Newton

• Only upper limits (τ<0.2-0.3) from weaker, more typical radius-expansion bursts (Galloway et al. 2010, ApJ 724, 417)

Thermonuclear burst spectroscopy with LOFT

The view from the edge with LOFT

• Expected 3-sigma limit on edge optical depth for 0.25-s integrations during typical burst spectra is < 0.1

• This is plenty to fully resolve the behaviour in a superexpansion burst (τ up to 3)

• Can also probe much deeper in “typical” radius-expansion bursts to further test for features in (e.g.) 4U 1728-34

Thermonuclear burst spectroscopy with LOFT

2. Hard excess• Non-Planckian spectra

are observed intermittently (?); overrepresented in short (& radius expansion) bursts

• Characteristically exhibit flattening, resembling mild comptonisation

• Possible causes include variations in the persistent flux during the burst e.g. Walker 1992, ApJ 385, 642

Thermonuclear burst spectroscopy with LOFT

The picture with LOFT• Much higher signal

permits much more stringent tests of the blackbody paradigm

• Perhaps possible to distinguish between accretion (background) variability and intrinsic deviations

Thermonuclear burst spectroscopy with LOFT

• Also permits much deeper searches for (Doppler broadened) spectral features

Summary and prospects

• LOFT will permit exquisite time-resolved spectroscopy of thermonuclear bursts

• This capability will provide detailed information on spectral phenomena which are presently weakly-detected, or at insufficient strength/resolution to provide good diagnostic information

• These data will have important implications to the determination of neutron star mass and radii, and hence the equation of state

• Will also improve our capability to use bursts as probes and diagnostics of nuclear burning processes

Thermonuclear burst spectroscopy with LOFT