Vela X-1: Flares & Off States

West Orange High SchoolManthan Kothari, Lucy Zipf, Neil Savalia, Brian Meise, Krish Pillai

IN TODAY’S PRESENTATION, WE WILL:

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 “off-states for the 20-40 keV range.

5. Present our findings regarding flares and off-states for the 1-10 keV range.

Why Flares & Off States?

• Literature Review– Kreykenbohm et al., 2008Discusses 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 findings.

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 changing.

– 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 switch)

– 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 pressure.

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.

Rise time

Fall Time

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 Softening

Off-States

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.