J/ApJ/747/77 Thermonuclear X-ray bursts. II. Eddington limit (Guver+, 2012) ================================================================================ Systematic uncertainties in the spectroscopic measurements of neutron-star masses and radii from thermonuclear X-ray bursts. II. Eddington limit. Guver T., Ozel F., Psaltis D. =2012ApJ...747...77G ================================================================================ ADC_Keywords: Binaries, X-ray ; Spectroscopy Keywords: stars: neutron - X-rays: binaries - X-rays: bursts Abstract: Time-resolved X-ray spectroscopy of thermonuclear bursts observed from low-mass X-ray binaries offer a unique tool to measure neutron-star masses and radii. In this paper, we continue our systematic analysis of all the X-ray bursts observed with Rossi X-ray Timing Explorer from X-ray binaries. We determine the events that show clear evidence for photospheric radius expansion and measure the Eddington limits for these accreting neutron stars using the bolometric fluxes attained at the touchdown moments of each X-ray burst. We employ a Bayesian technique to investigate the degree to which the Eddington limit for each source remains constant between bursts. We find that for sources with a large number of radius expansion bursts, systematic uncertainties are at a 5%-10% level. Moreover, in six sources with only pairs of Eddington-limited bursts, the distribution of fluxes is consistent with a ~10% fractional dispersion. This indicates that the spectroscopic measurements of neutron-star masses and radii using thermonuclear X-ray bursts can reach the level of accuracy required to distinguish between different neutron-star equations of state, provided that uncertainties related to the overall flux calibration of X-ray detectors are of comparable magnitude. Description: Galloway et al. (2008, Cat. J/ApJS/179/360) presented a catalog of Rossi X-ray Timing Explorer (RXTE) observations of X-ray bursts from 48 low-mass X-ray binaries. Following Paper I, we chose 12 X-ray binaries from this sample that show at least two PRE (photospheric radius expansion) events. We extracted time-resolved 2.5-25.0 keV X-ray spectra from all the RXTE/Proportional Counter Array (PCA) layers. We varied the exposure time between 0.25s and 1s to keep the signal-to-noise ratio constant based on the count rate during the burst. We also used a 16s spectrum, obtained prior to each burst, as background. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table1.dat 34 12 The number of photospheric radius expansion (PRE) events for each source table2.dat 49 80 Measured touchdown flux values from PRE events table3.dat 33 9 The measured touchdown fluxes -------------------------------------------------------------------------------- See also: J/ApJ/738/128 : RXTE observations of 4U 1728-34 (Seifina+, 2011) J/PAZh/37/651 : Hard X-ray bursts by INTEGRAL in 2003-2009 (Chelovekov+, 2011) J/ApJ/716/L109 : Radio and X-ray observations of Aql X-1 (Miller-Jones+, 2010) J/ApJS/179/360 : Thermonuclear X-ray bursts observed by RXTE (Galloway+, 2008) J/ApJ/685/436 : X-ray variability in 4U 1636-53 (Altamirano+, 2008) J/A+A/458/21 : INTEGRAL burst observations of 4U 1728-34 (Falanga+, 2006) Byte-by-byte Description of file: table1.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 16 A16 --- Name Source name 19- 21 I3 --- Nb Number of bursts (1) 24- 25 I2 --- Npre Number of PRE bursts (2) 28- 29 I2 --- Ngam Number of bursts exceeding {gamma} limit (3) 32- 33 I2 --- N.PRE ? Final number of PRE events 34 A1 --- f_N.PRE [d] Excluded burst (4) -------------------------------------------------------------------------------- Note (1): Values are adopted from Galloway et al. (2008, Cat. J/ApJS/179/360) and show the total number of X-ray bursts detected by RXTE. Note (2): The total number of X-ray bursts tagged as PRE or potentially PRE events in the Galloway et al. (2008, Cat. J/ApJS/179/360) catalog. Note (3): The number of remaining bursts with peak flux that exceeds the pre-burst emission by a factor of 10 (the {gamma} limit is defined by {gamma}=F_per_/F_Edd_) Note (4): d = As discussed in detail in Paper I (Guver et al. 2012ApJ...747...76G), we excluded the first burst observed from 4U 1724-307 from our analysis, since model fits of the X-ray spectra extracted from this burst cannot be fitted with a Planckian function and addition of absorption edges at several energies is needed (in't Zand & Weinberg, 2010A&A...520A..81I). -------------------------------------------------------------------------------- Byte-by-byte Description of file: table2.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 16 A16 --- Name Source name 18- 20 I3 --- BID [1/168] Burst identification number (1) 22- 32 F11.5 --- MJD [50128.8/54043.7] Modified Julian date 34- 38 F5.2 10-11W/m2 Flux [1.06/12.16] Touchdown flux at 2.5-25.0 keV (2) 40- 43 F4.2 10-11W/m2 e_Flux [0.06/0.4] Flux uncertainty (2) 45- 49 F5.2 --- Ratio [1.6/34] Normalization ratio A_peak_/A_cool_ (in cooling tail) -------------------------------------------------------------------------------- Note (1): Burst IDs and burst start times are adopted from Galloway et al. (2008, Cat. J/ApJS/179/360). Note (2): Values are given in units of 10^-8^erg/s/cm^2^ and are calculated using Equation (3) of Galloway et al. (2008, Cat. J/ApJS/179/360). -------------------------------------------------------------------------------- Byte-by-byte Description of file: table3.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 16 A16 --- Name Source name 18- 22 F5.2 10-11W/m2 Flux [1.19/10.44] Touchdown flux at 2.5-25.0 keV in units of 10^-8^erg/s/cm^2^ 24- 27 F4.2 10-11W/m2 eSys [0/2.3]? Systematic uncertainty (5) 28 A1 --- f_eSys [n] n = n/a 30- 33 F4.2 10-11W/m2 eForm [0/0.3] Formal uncertainty (6) -------------------------------------------------------------------------------- Note (5): Uncertainties in units of 10^-8^/erg/s/cm^2^. These reflect the most probable widths of the underlying distributions. Note (6): Uncertainties in units of 10^-8^/erg/s/cm^2^. These reflect the uncertainties in measuring the most probable values of the underlying distributions. See section 5 for further explanations. -------------------------------------------------------------------------------- History: From electronic version of the journal References: Guver et al. Paper I. 2012ApJ...747...76G ================================================================================ (End) Emmanuelle Perret [CDS] 27-Sep-2013