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Vela X-1:
Flares & Off States
West Orange High School
Manthan Kothari, Lucy Zipf, Neil Savalia,
Brian Meise, Krish Pillai
1. Discuss how we chose our project.
2. Describe the Vela X-1 system.
3. Present the Characteristics & Models for flaring
behavior for the 20-40 keV range.
4. Present the Characteristics & Models for “offstates for the 20-40 keV range.
5. Present our findings regarding flares and offstates for the 1-10 keV range.
Why Flares & Off States?
• Literature Review
– Kreykenbohm et al., 2008
Discusses flaring behavior of Vela X-1 for 20-40 keV X-Ray
energy range based on data from the INTEGRAL
(International Gamma-Ray Astrophysics Laboratory,
Launched in 2002) satellite.
• RAI:
We look at X-Rays in the 1-10 keV range from the Exosat
(European Space Agency X-Ray Observatory, finished its
mission in 1986). So, we decided to compare Vela X-1’s
flaring behavior in this energy range versus Kreykenbohm’s
Where is Vela X-1?
Parameters of Vela X-1
1. Compact Object
– Period (from the Power Spectrum) of the
compact object is ~283 seconds and it is not
– The compact object is a neutron star based
on its luminosity (from the energy spectrum
flux) of ~1036 ergs sec-1
– It is a pulsar because it is in a MXRB and has
a Power law Model fit.
Literature support: Kretschmar, 2004, Charles and Seward, 1995, Kreykenbohm, 2008.
2. Companion OB Star
– The luminosity (from the distance modulus)
of the OB star companion HD77581 is
63,000 times that of the sun.
– The radius is (using Stephan-Boltzmann)
21 times that of the sun.
Literature support: Kaper, 1997, Kretschmar, 2004.
3. System
-- Using values for the orbital speed and the orbital
period (from literature), we found the orbital radius of
HD77581 to be 2.6x109 m and the orbital radius of
the neutron star to be 3.45x1010 m .
– Therefore the radius of the neutron star’s orbit is
~50 solar radii (~1.7RHD77581).
– MHD77581~24Msun (using Kepler’s 3rd Law and Center of
Mass independently)
– This tells us this is a close MXRB meaning solar
winds account for accretion of matter onto the NS.
Literature support: Quaintrall et al., 2003, Van Paradijs et al., 1976.
Kretschmar et al.,2004, Willems et al., 2005.
Kreykenbohm’s Flares (20-40 keV)
• Characteristics
– Long Flares
• TRise/TTotal > 0.5
• Hardness Plot does not change.
– Short Flares
• TRise/TTotal < 0.3
• Hardness Plot does change:
the soft x-rays increase
Kreykenbohm’s Flare Models
Flip-Flop Instability
The Lucy Situation
Kreykenbohm’s Off-States
• Characteristics
– Occurs suddenly without a transition phase (almost like a
– Not an eclipse but count rate drops to below detection limits,
almost 0.
Kreykenbohm’s Off-State Models
• The “Biggest Loser” Model
– Dense blobs of stellar winds (thanks to the
close binary)
• Propeller Effect
– Inhibition of Accretion via balancing of
infalling ram pressure and the magnetic
Our Research Questions
• Do light curves in the 1-10 keV range
exhibit flaring and off-state behaviors?
• If these behaviors are present, do they
have the same characteristics as in the
20-40 keV range?
• If they’re present but with different
characteristics, what model(s) might
account for the difference?
Flaring Behaviors
Average cts/sec ~ 40 cts/sec +/- 0.03 cts/sec
Rise Time = ~2500 secs.
Flare Time = ~5000 secs.
Therefore, this is a LONG FLARE.
Pre-Rise Time: 40 cts/sec +/- .447cts/sec
This indicates
spectral softening.
Average cts/sec ~ 38 cts/sec +/- 0.031 cts/sec
Rise Time = ~1000 secs.
Flare Time = ~3000 secs.
Therefore, this is a SHORT FLARE.
Pre-Rise Time:
25 cts/sec +/- .35 cts/sec
This does not
indicate Spectral
Hardness plot fluctuates for an
off-state which is consistent with
Kreykenbohm’s results.
Summary of Our Findings
• Flares
– Flare behavior for 1-10 keV range is consistent
with Kreykenbohm’s 20-40 keV data.
– Hardness plots for flaring behavior were just the
opposite of the Kreykenbohm results!!!!
– Result: we have evidence that
(1) supports the flare models but
(2) is inconsistent with hardness plot
results found by the Kreykenbohm study
• Off-States
– One of the off-states was consistent with
Kreykenbohm time wise while the other one
was not (for a longer period of time).
– The hardness plots for both off-states are
consistent with Kreykenbohm’s 20-40 keV
range results.
Future Research
• Write a paper presenting our findings
• Look at other MXRB to compare Vela X-1
flaring behaviors with those sources.