1. “Studying NH variations in Vela X-1”
TRAINEE WINTER PRESENTATION
“Studying NH variations in Vela X-1”
Trainee: Pedro Quintana
Supervisor: Peter Kretschmar
ESAC Pregraduate Project 2009

2. OUTLINE Aim of the Project Resources and Tools Vela X-1 - HMXB - Data
- Spectra
- Data Analysis
- Short Term Variability (GTI files)
- Discussion
Summary

3. AIM OF THE PROJECT
Analyze a large set of existing data and to determine the variations in NH
- Model fitting
- Error contours
Study the short variability of Vela X-1 by splitting the longer and brighter observations into shorter ones and analyzing them
Model fitting> determination of good model!!
- Creation of the GTI files
- Analysis and fitting of the spectra

4. RESOURCES AND TOOLS FTOOLS/XTETOOLS: Data reduction
XSPEC: Data analysis, model fitting, error calculation and plotting
IDL: Plotting and phase calculation
CShell: Creation of GTI files

5. VELA X-1 Eclipsing High Mass X-ray Binary (HMXB)
- 23 Mo B0.5Ib Super Giant
- 1.9 Mo Neutron Star
X-ray production via
wind accretion
X-ray luminosity changes
Stable pulse profile
X-ray luminosity changes unknown, probable caused by changes in the accretion rate due to variations in the stellar wind or the formation of the accretion disk.
Vela X-1 is a well known wind-accreting X-ray pulsar orbiting its supergiant companion in days. The source is
persistent but quite variable in its emission, demonstrating sometimes very active phases and sometimes rather calm
periods. From past observations it is known that the amount of absorbing gas between the source and the Earth is oftenvarying rapidly and strongly. The absorbing material is commonly described as "NH", number of hydrogen atoms per
square cm.
* Mauche et al. (2007a)

6. DATA. Instrument and Software
ROSSI X-RAY TIMING EXPLORER (1995)
Astronomical X-ray Sources
Instruments
- ASM (1.5 – 12 keV)
- PCA (2 – 60 keV)
- HEXTE (15 – 250 keV)
Software
- FTOOLS
PCA
- response matrix
- background subtraction
questions
- pca & hexte or just pca??

7. DATA. Spectra July 96 – October 08 9 Observations / 60+ datasets
Orbital Phase:
FEATURES
Powerlaw with exponential cutoff
Iron Kα line (~ 6.4 keV)
Soft excess (~ 3 keV)
Photoelectric absorption
Cyclotron absorption (~27 keV and ~54 keV)
Negro – normal
red – highly absorbed
green – highly absorbed / partial covering

8. DATA. Spectra
- Phabs due to a gas stream trailing the neutron star and due to circunstellar matter -> increased absorption at orbital phases >0.5
- erratic increases of NH by a factor of 10 at all orbital phases

9. Cutoff: better chi-squared but higher errors
Powerlaw with Exponential Cutoff Powerlaw
Cutoff: better chi-squared but higher errors

10. not even the cutoffpl works!! partial covering models
Powerlaw with Exponential Cutoff Powerlaw
not even the cutoffpl works!! partial covering models
(phabs1 x cutoffpl + phabs2 x cutoffpl) + gauss

11. NH vs. Orbital Phase

12. SHORT-TERM VARIABILITY
GTI Files
Determination of longer and brighter datasets
Splitting of these observations into shorter pieces (1ks) (GTI files)
Plotting
Individual fitting of every spectra
Error contours:
- α vs NH
- Iron line parameters
RESULTS:
A strong short term variability is observed when plotted.

13. Bad fitting for the highly absorbed ones
Bad fitting for the highly absorbed ones!! Could be corrected with a good model!

15. ERROR CONTOURS

16. SUMMARY
The RXTE observations of Vela X-1 demonstrate the variability of this source on many time scales.
The cyclotron absorption line features complicates the analysis and fitting of the spectra of Vela X-1. The fact that the classical high energy cutoff can introduce line-like residuals has led us to constrain our energy range and use simple models that give us a pretty decent fit for most of the datasets.
Although we are able to obtain acceptable fits for most of the data presented here, there is an obvious need for better models to fully exploit the high quality data current instruments deliver.

17. Thank you for your attention
ANY QUESTIONS??