| 1. |
- Axelsson, Magnus, et al.
(author)
-
Evolution of the 0.01-25 Hz power spectral components in Cygnus X-1
- 2005
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 438:3, s. 999-1012
-
Journal article (peer-reviewed)abstract
- Analyzing the archival data from the Rossi X-ray Timing Explorer (RXTE), we study the power density spectra (PDS) of Cygnus X-1 from 1996 to 2003 in the frequency range of 0.01-25 Hz. Using a model consisting of one or two Lorentzians and/or an exponentially cut-off power-law, we are able to achieve a good fit to the PDS during the observations. With our model we are also able to track the evolution of the Lorentzian components through all spectral states of the source. We confirm the relation between characteristic frequencies seen both in black hole candidate and neutron star sources, and show the changes in this relation during the transitional and soft states of the source. The connection between the Lorentzian components is investigated by analyzing similarities and differences in their behavior. We find that the spectral state of the source can be uniquely determined from the parameters of these components. The parameter correlations can all be described by continuous functions, which differ between components. We discuss our results in the context of relativistic precession model for the accretion disk, and show a remarkable agreement between the model prediction and the data in the hard state. We estimate a value for the specific angular momentum of a* = 0.49 (-0.57) in the case of prograde (retrograde) rotation and an estimate for the inner radius of 22 to 50 (25 to 55) gravitational radii. Additional assumptions are required to explain the soft state data, and attempting to invoke rotational reversal for state transitions shows that it is insufficient to explain the differences between the hard and soft state data. © ESO 2005.
|
|
| 2. |
|
|
| 3. |
- Axelsson, Magnus, et al.
(author)
-
Probing the temporal variability of Cygnus X-1 into the soft state
- 2006
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 452:3, s. 975-984
-
Journal article (peer-reviewed)abstract
- Building on results from previous studies of Cygnus X-1, we analyze Rossi X-ray Timing Explorer (RXTE) data taken when the source was in the soft and transitional spectral states. We look at the power spectrum in the 0.01-50 Hz range, using a model consisting of a cut-off power-law and two Lorentzian components. We are able to constrain the relation between the characteristic frequencies of the Lorentzian components, and show that it is consistent with a power-law relation having the same index (1.2) as previously reported for the hard state, but shifted by a factor ∌2. Furthermore, it is shown that the change in the frequency relation seen during the transitions can be explained by invoking a shift of one Lorentzian component to a higher harmonic, and we explore the possible support for this interpretation in the other component parameters. With the improved soft state results we study the evolution of the fractional variance for each temporal component. This approach indicates that the two Lorentzian components are connected to each other, and unrelated to the power-law component in the power spectrum, pointing to at least two separate emission components.
|
|
| 4. |
|
|
| 5. |
- Axelsson, Magnus, et al.
(author)
-
The width of gamma-ray burst spectra
- 2015
-
In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 447:4, s. 3150-3154
-
Journal article (peer-reviewed)abstract
- The emission processes active in the highly relativistic jets of gamma-ray bursts (GRBs) remain unknown. In this paper, we propose a new measure to describe spectra: the width of the EFE spectrum, a quantity dependent only on finding a good fit to the data. We apply this to the full sample of GRBs observed by Fermi/Gamma-ray Burst Monitor (GBM) and Compton Gamma-ray Observatory/Burst and Transient Source Experiment (BATSE). The results from the two instruments are fully consistent. We find that the median widths of spectra from long and short GRBs are significantly different (chance probability < 10(-6)). The width does not correlate with either duration or hardness, and this is thus a new, independent distinction between the two classes. Comparing the measured spectra with widths of spectra from fundamental emission processes - synchrotron and blackbody radiation - the results indicate that a large fraction of GRB spectra are too narrow to be explained by synchrotron radiation from a distribution of electron energies: for example, 78 per cent of long GRBs and 85 per cent of short GRBs are incompatible with the minimum width of standard slow cooling synchrotron emission from a Maxwellian distribution of electrons, with fast cooling spectra predicting even wider spectra. Photospheric emission can explain the spectra if mechanisms are invoked to give a spectrum much broader than a blackbody.
|
|
| 6. |
- Bagoly, Zsolt, et al.
(author)
-
Principal Component Analysis of Gamma-Ray Bursts' Spectra
- 2005
-
In: Nuovo Cimento C. ; 28:3, s. 295-298
-
Journal article (peer-reviewed)abstract
- The origin of dark bursts--i.e. that have no observed afterglows in X-ray, optical/NIR and radio ranges--is unclear yet. Different possibilities--instrumental biases, very high redshifts, extinction in the host galaxies--are discussed and shown to be important. On the other hand, the dark bursts should not form a new subgroup of long gamma-ray bursts themselves.
|
|
| 7. |
|
|
| 8. |
|
|
| 9. |
|
|
| 10. |
|
|
| 11. |
- Borgonovo, Luis, et al.
(author)
-
On the temporal variability classes found in long gamma-ray bursts with known redshift
- 2007
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 465:3, s. 765-775
-
Journal article (peer-reviewed)abstract
- Context: Based on the analysis of a small sample of BATSE and Konus gamma-ray bursts (GRBs) with know redshift it has been reported that the width of the autocorrelation function (ACF) shows a remarkable bimodal distribution in the rest-frame of the source. However, the origin of these two well-separated ACF classes remains unexplained. Aims: We study the properties of the bursts belonging to each ACF class and look for significant differences between them.Methods: We complement previous ACF analysis studying the corresponding power density spectra (PDS). With the addition of Beppo-SAX data and taken advantage of its broad-band capability, we not only increase the burst sample but we extend the analysis to X-ray energies.Results: The rest-frame PDS analysis at γ-ray energies shows that the two ACF classes are not simply characterised by a different low frequency cut-off, but they have a distinct variability as a whole in the studied frequency range. Both classes exhibit average PDS with power-law behaviour at high frequencies (f' ≥ 0.1 Hz) but significantly different slopes, with index values close to those of Brownian (-2) and Kolmogorov (-5/3) spectra for the narrow and broad classes respectively. The latter spectrum presents an additional PDS component, a low-frequency noise excess with a sharp cut-off at a characteristic frequency f'c ≃ 0.025 Hz, in conjunction with the small relative dispersion (~6%) of the ACF width in this class. At X-ray energies we find the power-law index unchanged for the broad class, but a significantly steeper slope in the narrow case (~-3). We interpret this as an indication that the broad class bursts have weaker spectral evolution than the narrow ones, as suggested also by our analysis of the ACF energy dependence. The low and high frequency PDS components may then arise from two radiating regions involving different emission mechanisms. We compare our GRB sample conditioned by afterglow detections with a complete, flux limited BATSE sample, finding a significant bias against narrow ACF bursts.
|
|
| 12. |
- Borgonovo, Luis, 1956-
(author)
-
Spectral and Temporal Studies of Gamma-Ray Bursts
- 2007
-
Doctoral thesis (other academic/artistic)abstract
- Gamma-ray bursts (GRBs) are sporadic flashes of light observed primarily in the gamma-ray band. Being the brightest explosions in the Universe since its birth, they are at present also the furthest astronomical sources detected. Since their serendipitous discovery in the late 1960s the study of GRBs has grown into one of the most active fields in astrophysics with ramifications in many other scientific areas.Despite intense studies many of the basic questions about the nature of GRBs remain unanswered. Long duration bursts are believed to be the result of ultra-relativistic outflows associated with the collapse of very massive stars. The mechanisms responsible for the emission, the geometry of the emitter, and the radiative processes involved are still a matter of research. Common multi-pulse bursts display a spectral evolution as complex as their light curves. However, it is unclear what produces the observed variability. The works presented in this thesis aim to build the necessary base to answer these open questions.A characterization of the spectral evolution is presented (based on time-resolved spectral analysis) that provides insight into the underlying emission processes and imposes severe constraints on current physical models (Paper I).We report the results of a multi-variate analysis on a broad range of GRB physical parameters covering temporal and spectral properties. Empirical relations were found that indicate a self-similar property in burst light curves and a luminosity correlation with potential use as a distance indicator (Paper II).Determining the relevant timescales of any astronomical phenomenon is essential to understand its associated physical processes. Linear methods in time-series analysis are powerful tools for the researcher that can provide insight into the underlying dynamics of the studied systems. For the first time these methods were used on GRB light curves correcting for cosmic time dilation effects which revealed two classes of variability. The possible origin of these classes is discussed (Papers III & IV).
|
|
| 13. |
|
|
| 14. |
- Borgonovo, Luis, et al.
(author)
-
Statistical Analysis of BATSE Gamma-Ray Bursts: Self-Similarity and the Amati Relation
- 2006
-
In: The Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 652:2, s. 1423-1435
-
Journal article (peer-reviewed)abstract
- The statistical properties of a complete, flux-limited sample of 197 long gamma-ray bursts (GRBs) detected by BATSE are studied. In order to bring forth their main characteristics, care was taken to define a representative set of 10 parameters. A multivariate analysis gives that ~70% of the total variation in parameter values is driven by only three principal components. The variation of the temporal parameters is clearly distinct from that of the spectral ones. A close correlation is found between the half-width of the autocorrelation function (τ) and the emission time (T50) most importantly, this correlation is self-similar in the sense that the mean values and dispersions of both τ and T50 scale with the duration of the burst (T90). It is shown that the Amati relation can be derived from the sample and that the scatter around this relation is correlated with the value of τ. Hence, τ has a role similar to that of the break in the afterglow light curve (tb) in the Ghirlanda-relation. In the standard GRB-scenario, the close relation between a global parameter (tb) and a local one (τ) indicates that some of the jet-properties do not vary much for different lines of sight. Finally, it is argued that the basic temporal and spectral properties are associated with individual pulses, while the overall properties of a burst is determined mainly by the number of pulses.
|
|
| 15. |
- Meszaros, A., et al.
(author)
-
On the origin of the dark gamma-ray bursts
- 2005
-
In: Nuovo cimento della societa italiana de fisica. C, Geophysics and space physics. - 1124-1896 .- 1826-9885. ; 28:3, s. 311-314
-
Journal article (peer-reviewed)abstract
- The origin of dark bursts-i.e. that have no observed afterglows in X-ray, optical/NIR and radio ranges-is unclear yet. Different possibilities-instrumental biases, very high redshifts, extinction in the host galaxies-are discussed and shown to be important. On the other hand, the dark bursts should not form a new subgroup of long gamma-ray bursts themselves.
|
|
