Variability analysis in low count rate sources

We have analyzed a 90 ksec long observation of the bright nucleus of M33 made with Chandra /ACIS. We detected low-amplitude (∼ 10%) highly significant variability on timescales of ∼ 5000 sec. We also find associated spectral variability. The two main spectral components (a power-law with Γ ∼ 2 and a multicolor disk -MCD-with kT∼ 0.9 − 1.2 keV) vary in relative flux. The MCD temperature also increases with increasing MCD flux. The pattern of variability is reminiscent of (but not identical to) galactic black hole binaries. A ∼ 15 M ⊙ accreting black hole may explain this source.

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“…d Variability timescale from the Collura test (Collura et al 1987) Fig. 4.-Spectrum of M33-X8 and residuals over the best fit model ( Table 2).…”

Section: Discussionmentioning

confidence: 99%

Spectral Variability of the Nucleus of M33 in aChandra/ACIS Observation

Parola

Damiani

Fabbiano

et al. 2003

ApJ

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“…The reduced χ 2 can serve as a measure of the variation amplitude (Collura et al 1987). For about 10,000 sources above 40 counts, the reduced χ 2 ranges from 0.04 (steady) to 10 (extremely variable).…”

Section: Source Variabilitymentioning

confidence: 99%

CHANDRA ACIS SURVEY OF X-RAY POINT SOURCES IN 383 NEARBY GALAXIES. I. THE SOURCE CATALOG

Liu

2010

ApJS

104

112

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The Chandra data archive is a treasure for various studies, and in this paper we exploit this valuable resource to study the X-ray point source populations in nearby galaxies. By 2007 December 14, 383 galaxies within 40 Mpc with isophotal major axis above 1 arcmin had been observed by 626 public ACIS observations, most of which were for the first time analyzed by this survey to study the X-ray point sources. Uniform data analysis procedures are applied to the 626 ACIS observations and lead to the detection of 28,099 point sources, which belong to 17,599 independent sources. These include 8700 sources observed twice or more and 1000 sources observed 10 times or more, providing us a wealth of data to study the long-term variability of these X-ray sources. Cross-correlation of these sources with galaxy isophotes led to 8519 sources within the D 25 isophotes of 351 galaxies, 3305 sources between the D 25 and 2D 25 isophotes of 309 galaxies, and additionally 5735 sources outside 2D 25 isophotes of galaxies. This survey has produced a uniform catalog, by far the largest, of 11,824 X-ray point sources within 2D 25 isophotes of 380 galaxies. Contamination analysis using the log N-log S relation shows that 74% of sources within 2D 25 isophotes above 10 39 erg s −1 , 71% of sources above 10 38 erg s −1 , 63% of sources above 10 37 erg s −1 , and 56% of all sources are truly associated with galaxies. Meticulous efforts have identified 234 X-ray sources with galactic nuclei of nearby galaxies. This archival survey leads to 300 ultraluminous X-ray sources (ULXs) with L X (0.3-8 keV) 2 × 10 39 erg s −1 within D 25 isophotes, 179 ULXs between D 25 and 2D 25 isophotes, and a total of 479 ULXs within 188 host galaxies, with about 324 ULXs truly associated with host galaxies based on the contamination analysis. About 4% of the sources exhibited at least one supersoft phase, and 70 sources are classified as ultraluminous supersoft sources with L X (0.3-8 keV)2 × 10 38 erg s −1 . With a uniform data set and good statistics, this survey enables future works on various topics, such as X-ray luminosity functions for the ordinary X-ray binary populations in different types of galaxies, and X-ray properties of galactic nuclei.

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“…We have performed a detailed search of short-term variability using the phase-averaged χ 2 method proposed by Collura et al (1987). Following this procedure we have binned the photon arrival times in equal duration intervals, ranging from 50 s to 8000 s; then we have compared each binned light curve with a constant one.…”

Section: Short-term Variabilitymentioning

confidence: 99%

“…With this procedure it is possible to test, in a statistically significant way, the presence of variability, and then, assuming a pulsed variability model, to evaluate the characteristic time scale τ and the fractional variability level V eff /I vs. the bin size b, where V eff is the amplitude of the variable component and I is the average number of counts per bin (see Collura et al 1987, for more details). We have first applied this procedure to the arrival times of all the photons in each spectrum, finding that the emission is compatible with the hypothesis of a constant source, at high statistical confidence level.…”

Section: Short-term Variabilitymentioning

confidence: 99%

On coronal structures and their variability in active stars: The case of Capella observed with Chandra/LETGS

Argiroffi

Maggio

Peres

2003

A&A

Self Cite

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Abstract. In this paper we present a detailed analysis of two X-ray spectra of Capella, taken eleven months apart with the Low Energy Transmission Grating Spectrometer (LETGS) of the Chandra Observatory. We have studied variability of the coronal emission over different time scales, both in the whole X-ray band and in narrow temperature ranges identified by lines. The comparison of the two observations shows that the whole coronal emission of Capella in March 2000 was 3% higher than in February 2001; there also appears to be a tendency, albeit a marginal one, for the hottest lines to show the largest changes between the two observations. A detailed search for short-term variability (on time scales ranging from 10 2 to 10 4 s) in the emission of individual lines shows that in all cases the emission is compatible with a constant source; the firm upper limits of 5%−10% to the source variability on short time scales suggests that the intense X-ray emission is due to stable coronal structures and not to flaring activity. We have also determined the coronal thermal structure, as described with the emission measure distribution vs. temperature and with the help of plasma density, derived from the analysis of the O , Ne , Mg  and Si  He-like ion triplets. The emission measure distribution, em(T ), and the element abundances, have been reconstructed with the Markov-Chain Monte Carlo method by Kashyap & Drake; the em(T ) presents a previously known sharp peak around log T = 6.8−6.9, but we have also found evidence of a small amount of plasma at T > 10 7 K. With the help of the em(T ) and the density values we have estimated the pressure and volume of the emitting plasma at different temperatures, and we have derived information about the structure of individual loops and about the population of loops having different maximum temperatures. Our results indicate that loops with higher maximum temperature have higher pressure and smaller volume than lower temperature loops.

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Variability analysis in low count rate sources

Cited by 22 publications

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Spectral Variability of the Nucleus of M33 in aChandra/ACIS Observation

Spectral Variability of the Nucleus of M33 in aChandra/ACIS Observation

CHANDRA ACIS SURVEY OF X-RAY POINT SOURCES IN 383 NEARBY GALAXIES. I. THE SOURCE CATALOG

On coronal structures and their variability in active stars: The case of Capella observed with Chandra/LETGS

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