Constraining Pulsar Emission Physics through Radio/Gamma-Ray Correlation of Crab Giant Pulses

0.79 MB·English

by  A. V. Bilous

2009 Fermi Symposium, Washington, D.C., Nov. 2-5 1
Constraining Pulsar Emission Physics through Radio/Gamma-Ray
Correlation of Crab Giant Pulses
A.V.Bilous
DepartmentofAstronomy,UniversityofVirginia,POBox400325,Charlottesville,VA22904
V.I.Kondratiev
0 NetherlandsInstituteforRadioAstronomy(ASTRON),Postbus2,7990AADwingeloo,TheNetherlands
1 M.A.McLaughlin,M.Mickaliger,D.R.Lorimer
0
DepartmentofPhysics,WestVirginiaUniversity,Morgantown,WV26506
2
S.M.Ransom

n NRAO,Charlottesville,VA22903
a M.Lyutikov
J
DepartmentofPhysics,PurdueUniversity,525NorthwesternAvenue,WestLafayette,IN47907-2036

8 B.Stappers
Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester,
] ManchesterM139PL,UK
E
G.I.Langston
H
NRAO,GreenBank,WV24944
.
h
p
-
o To constrain the giant pulse (GP) emission mechanism and test the model of Lyutikov [1] of GP emission,
we are carrying out a campaign of simultaneous observations of the Crab pulsar between γ-rays (Fermi) and
r
t radio wavelengths. The correlation between times of arrival of radio GPs and high-energy photons, whether
s
it exists or not, will allow us to choose between different origins of GP emission and further constrain the
a
emission physics. Our foremost goal was testing whether radio GPs are due to changes in the coherence of
[
the radio emission mechanism, variations in the pair creation rate in the pulsar magnetosphere, or changes

2 in the beaming direction. Accomplishing this goal requires an enormous number of simultaneous radio GPs
v and γ-photons. Thus, we organized a radio observations campaign using the 42-ft telescope at the Jodrell
4 Bank Observatory (UK), the 140-ft telescope, and the 100-m Robert C. Byrd Green Bank Telescope (GBT)
at the Green Bank Observatory (WV). While the observations with the two first ones are ongoing, here we
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presentthepreliminaryresultsof20hrsofobservationswiththeGBTatthehighfrequencyof8.9GHz. These
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particularobservationswereaimedtoprobethemodelofGPemissionbyLyutikov[1]whichpredictsthatGPs
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atfrequencies>4GHzshouldbeaccompaniedbyγ-rayphotons ofenergiesof1-100GeV.
.
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1
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1. Introduction of the radio emission, variations in the pair creation
0
: rate in the magnetosphere, or changes in the beam-
v ing direction. If GP phenomenonis due to changesin
i The Crab pulsar was discovered by Staelin &
X the coherence of the radio emission mechanism, then
Reifenstein in 1968 [2] by its remarkably bright gi-
one would expect little correlation of the radio GPs
r ant pulses (GPs). These relatively rare bursts of in-
a with the high-energy emission. However, if the GPs
tense radio emission last a few nanoseconds to a few
are due to changes in the actual rate of pair creation
microseconds, and are clearly a special form of pul-
inthe pulsar magnetosphere,one wouldexpect anin-
sar radio emission [e.g. 3, 4]. GPs generally occur
creased flux at high energies at the time of the GPs.
only in certain narrow ranges of pulse phase that are
Similarly,becausetheradioGPandγ-raycomponents
often coincident with pulses seen at X-ray and γ-ray
arealigned,oneexpectsthattheycomefromthesame
energies [5]. Popov et al. [6] propose that all radio
placeinthepulsarmagnetosphere. Therefore,ifaGP
emission from the Crab (except for that in the pre-
occurs from a beam direction alteration, one would
cursor) is composed entirely of GPs, consistent with
expect to see an increase also in the high-energy flux.
the alignmentofthe GPandhigh-energycomponents
seen in other GP pulsars [3, 7]. The attempt to carry out simultaneous radio/γ-
Crab pulsar shows pulsed emission across the en- ray observations (50-220 keV energy range of
tire electromagnetic spectrum (see Fig. 1, left), re- CGRO/OSSE) and correlate time of arrivals (TOAs)
flecting different radiationprocesses in pulsar magne- of GPs was undertaken before by Lundgren et al. [5].
tosphere — from coherent curvature or synchrotron They did not have enough sensitivity to correlate
(radio)to incoherentsynchrotron(optical andX-ray) TOAs for single events and averaged their data in
and incoherent curvature (γ-ray). Similar to other 2-min intervals. They were only able to put an up-
sporadic, variability phenomena in pulsar radio emis- per limit on γ-ray flux increase of a factor of 2.5
sion, representedby nulling pulsars [e.g. 9], intermit- concurrent with radio GPs. This suggested that GP
tent pulsars [10], and rotation radio transients [11], mechanism is largely based on changes in coherence
GP emission can be due to changes in the coherence and not changes in pair production rates or beaming.
2 2009 Fermi Symposium, Washington, D.C., Nov. 2-5
Yet, Shearer et al. [12] performed simultaneous ra-
dio/optical observations of the Crab pulsar, and they
found a weak correlation, that optical pulses coinci-
dentwithradioGPswereonaverage3%brighterthan
others. In contrast to the Lundgren et al. work, this
observation clearly points to variations in magneto-
sphericparticledensityasthecauseoftheradiogiant
pulses.
Also, Lyutikov [1] proposed a more specific, quan-
titative model ofGP emissionin which CrabGPs are
generatedonclosedmagneticfieldlines nearthe light
cylinderviaanomalouscyclotronresonanceontheor-
dinary mode. One clear prediction of this model is
that radio GPs (at least those at radio frequencies
> 4 GHz) should be accompanied by γ-ray photons,
as the high energy beam is expected to produce cur-
vature radiation at energies ∼ ¯hγ3Ω ∼ 1–100 GeV,
depending on the exact value of the Lorentz factor
γ. These energies fall perfectly into the energy range
of the Fermi mission, and so this hypothesis can also
be tested through high-frequency radio observations
concurrent with Fermi.
InSections2and3belowwedescribeperformedra-
Figure 1: Left. The average profile of theCrab pulsar
dio observationsand Fermi data used in further anal-
from radio to γ-rays (from thepaper of Moffett &
ysis. Section 4 presents the obtained preliminary re- Hankins[8]). Right. Average Crab pulsar radio profile
sults. We describe the correlation analysis between for theGBT session on Sep 25, 2009.
radio GPs and Fermi photons in Section 4.1. Conclu-
sions are made in Section 5.
above 100 MeV and with the angular separation θ <
Max(1.6−3lg(E/1000 MeV),1.3)◦ from the nominal
2. Radio observations pulsarpositionwereselected(e.g.[13]). Totalnumber
of photons per 8 hrs of simultaneous observations is
The radio observations were carried out during the 70(11above1GeV).Photonswerebarycenteredwith
September2009withthe100-mRobertC.ByrdGreen TEMPO2usingthe sameephemerisasforradioGPs.
BankTelescope(GBT)usingthenewGreenBankUl-
timate Pulsar Processor (GUPPI) at a frequency of
8.9 GHz. The total bandwidth of 800 MHz was split
4. Results
into 256 frequency channels, and the total intensity
wasrecordedwith the sampling interval2.56–3.84µs.
Therewere10observingsessionsforatotalof∼20hrs. The number of detected radio GPs stronger than
The raw data from every session were dedispersed 7σ and γ-photons with the energies above 100 MeV
with the current DM of the Crab Pulsar using the during contemporaneous time for each observing ses-
PRESTO package, and searched for all the single- sion is given in the Table I. Taking into account the
pulse events. The lists of events were presented in contributionfrom the Crab nebula, the 1σ sensitivity
TEMPO format and converted to barycentric refer- in our observations was about 480 mJy. In total, we
ence frame for further analysis with Fermi data. Fig- found more than 85,000 GPs stronger than 7σ, with
ure1,rightshowstheaverageprofile(top)oftheCrab about simultaneous 39,450GPs with Fermi. The cor-
Pulsar together with the subintegrations during the responding total number of photons detected is 70.
course of observations for the most fruitful sessionon Figures2and3presentthefluxdensityofradioGPs
Sep 25. The interpulse (IP) and high-frequency com- andenergyofFermiphotonsandtheirhistogramsver-
ponents (HFCs) are clearly seen, with the weak peak sus the phase of their occurrence within the Crab pe-
after HFC2 being the main pulse (MP). riod. It is evident that most of GPs occurred at the
phases of the MP and IP. In agreementwith Hankins
& Eilek [14] radio GPs occur more frequently at the
3. Fermi data phases of IP, while GPs in MP are stronger. Due to
smallphotons sample, no definitive conclusioncanbe
For each observing radio session we extracted LAT put forward, whether there is an average increase in
data fordiffuse classofevents. Photonswithenergies countsratein Fermiprofileduringthe eventsofradio
2009 Fermi Symposium, Washington, D.C., Nov. 2-5 3
Figure2: FluxdensityofGPs andenergyFermiphotonsvs. thephaseoftheiroccurrencewithin theCrab period. The
radio profile from one of theGBT sessions is shown in thebackground (gray).
inherent to the pulsar. Hence, if so and there is a
Table I Observations summary. The columns are as
correlationbetween time of arrivals of radio GPs and
follows: theday in Septemberwhen observations were
high-energyphotons,onewouldexpectFermiphotons
happened;Tradio,γ, theduration of radio session
also come within clumps of GPs. Figure 4 shows the
simultaneously with Fermi;NGPs,thenumberofdetected
GPs stronger than 7σ duringcontemporaneous time with time series of radio GPs and Fermi photons for every
Fermi; Nγ, the numberof Fermi photonsoccurred with observingsession. Apparently photons do also have a
energy >100 MeV. tendencytooccurinclumps,andinsomecasesduring
the increase of the flux in radio. The latter, however,
Day Tradio,γ NGPs Nγ
does require an additional analysis.
(Sep 2009) (s) (>7σ) (>100 MeV)
12 1551 4 7
14 3304 4166 11 4.1. Correlation Analysis
19 3077 4096 7
For every photon we searched for a GP within the
20 1779 1069 2
set of different time windows between 1 ms and 10 s.
21 1801 154 2
Since the rate of GPs varies significantly from session
22 3871 1567 6
to session (see Fig. 4), the search was performed for
23 4710 9764 10
each day separately. The results are presented in the
24 1260 264 1 Table II. It is obvious, that the higher the average
25 7446 18299 19 GPrateand/orlengthofcorrelationtimewindow,the
28 2795 65 5 morematchesbetweenradioGPsandFermiphotons.
Total: 31594 39448 70 To estimate the probability P of getting the same
number of matches by pure chance (i.e. when there
is no true correlation), we performed a Monte-Carlo
simulationassigningarandomtimeofarrivalforeach
GPs. Severalpulses,eithergiantpulsesorregularsin- γ-photonforeveryobservingsession. Then,wecalcu-
gle pulses, were also detected at the phases of HFCs lated the number of matches between real radio GPs
and even MP precursor. Though such pulses were re- and simulated photons and repeated this procedure
ported by Jessner et al. [15] as well, their GP nature Nsim = 10,000 times. Knowing the real number of
istobe checked,andtheycouldonlyrepresentstrong matches k for every observing session and every time
single pulses of regular emission. window, we calculated P as Nk/Nsim, where Nk is
At high radio frequency of 8.9 GHz GPs occur in the numberofperformedsimulationswith k matches.
burstsorclumpsduetoscintillationswithacharacter- The results ofthis simulationarepresentedonFig-
istic scintillation time of about 10-20 min. However, ure 5. It is clear that the probability of getting the
scintillations can mask the intrinsic flux variability recorded number of matches between GPs and γ-
4 2009 Fermi Symposium, Washington, D.C., Nov. 2-5
Figure 3: Histograms of GPs and Fermi photonsduring thesimultaneous time. For illustrative purposes radio and
gamma scaled profiles are shown (gray). Scaled radio profile is from GBT session on Sep 25, 2009, and γ-profileis
Fermi profile accumulated duringSep,2009.
Figure 4: Time series of radio GPs and Fermi photonsduring10 observing sessions. The values on Y-axesshows either
themaximum fluxdensity in radio (left) or the maximum photon energy (right) in thecorresponding subplot.
photons by pure chance is very high, and, thus, the Due to a small number of contemporaneous pho-
presenceofintrinsiccorrelationisstillundertheques- tons, no definitive inference can be made about an
tion. averageincreaseincountsrateinFermiprofileduring
the events of radio GPs. Current preliminary results
indicate that fraction of photons closely accompany-
ing high-frequency GPs is certainly less than 10%. In
5. Conclusions
order to estimate this value more precisely, one needs
to continue simultaneous observations, accumulating
morephotonsandregisteringmoreGPs. Suchacam-
No obvious correlation was found between Fermi
paign of Fermi/radio observations using 140-ft tele-
photons ofenergies > 100MeV and radio giant pulses
scope at Green Bank Observatory (WV) and 42-ft
at the frequency of 8.9 GHz.
2009 Fermi Symposium, Washington, D.C., Nov. 2-5 5
Figure 5: The probability that the numberof matches between GP and γ- photon within given time window is dueto
purechance (nocorrelation). Each line corresponds to different observation date. The simulation was performed for
each observing session separately dueto different rates of radio GPs.
for very high energy photons >∼100 GeV. At this en-
Table II Correlation results between radio GPs and
ergy range Fermi, though sensitive, can not provide
Fermi photonsfor each observing session. Column 1
extensive sample of photons in reasonable time for
shows theday in September2009 when observation was
correlation, so Cherenkov detectors are much more
happened,column 2givestheaverage GPrateduringthe
promising.
session, and columns 3–7 givethe numberof matches
between radio GPs and Fermi photonsfor different time Simple identification of time of arrivals of photons
windows from 1 ms to 10 s. showsthattheyalsocomeingroupssimilarlytoradio
Day GP rate Time window GPs. Though in case of GPs this is caused by scin-
(Sep 2009) (s−1) 1 ms 10 ms 100 ms 1 s 10 s tillations, they could potentially mask the flux vari-
ability intrinsic to the Crab pulsar as well. Then, we
12 0.0026 – – – – –
would expect groups of photons to concentrate with
14 1.26 1 1 4 9 10
fraction of stronger GPs. And in fact, we do see the
19 1.33 – – 2 5 7
tendency ofsomegroupsofphotonstocluster around
20 0.6 – – – – 2 strongGPs(Fig.4). Usingthecurrentdataset,weare
21 0.0855 – – – – 1 planning to do a more thoroughanalysis of TOAcor-
22 0.4 – – – 1 4 relation between individual photons and radio GPs.
23 2.073 – 1 4 7 9 There could be a time delay between time of arrival
of Fermi photon and corresponding GP due to, for
24 0.21 – – 1 1 1
instance, different travel paths in the pulsar magne-
25 2.458 – – 6 13 15
tosphere, or non-simultaneity in emitting radio and
28 0.023 – – – 2 2
gamma-ray. Wewillintroducethedelaybetweenpho-
tonandGPtimeseriesandrunthecorrelationanaly-
sis for many such trials. Also, γ-photons may accom-
pany only the brightest GPs or the clump of giant
telescopeatJodrellBankObservatoryatlowfrequen- pulses as a whole.
cies is ongoing now. Non-correlation, if indeed true,
may favor for coherence change as a reason for GP
emission rather than variations in pair creation rate
in the pulsar magnetosphere, or changes in beaming Acknowledgments
direction. In particular, the model of Lyutikov [1]
predictscorrelationbetweenhigh-energyphotonsand The National Radio Astronomy Observatory is a
radio GPs at frequencies 4-10 GHz. Again, if radio facility of the National Science Foundation operated
and γ-rays are indeed not correlated, then model ei- under cooperative agreement by Associated Universi-
therhastobetweaked,orsuchcorrelationexistsonly ties, Inc.
6 2009 Fermi Symposium, Washington, D.C., Nov. 2-5
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