Physical properties of the gamma-ray binary LS 5039 ... · Gamma-Ray Binaries Gamma-ray binaries:...

Physicalpropertiesofthegamma-raybinaryLS 5039through

lowandhighfrequencyradioobservations

Benito Marcote

M. Ribó, J. M. Paredes, C. H. Ishwara-Chandra

HEPRO VOctober 7, 2015

Gamma-Ray Binaries

Gamma-ray binaries: binary systems which host a compact object orbitinga high mass star that have the non-thermal maximum of the Spectral EnergyDistribution in γ-rays (Paredes et al. 2013, Dubus 2013).

System Main star P/ daysLS 5039 O6.5 V 3.91FGL J1018.6–5856 O6 V 16.6LS I +61 303 B0 Ve 26.5HESS J0632+057 B0 Vpe 315.0PSR B1259–63 O9.5 Ve 1236.7Cygnus X-3 WR 0.2Cygnus X-1 ?? O9.7 Ve 5.6MWC 656 ?? Be 60.4

Red: known gamma-ray binariesGreen: X-ray binaries with gamma-rayemission

−0.15 −0.10 −0.05 0.00 0.05 0.10AU

−0.15

−0.10

−0.05

0.00

0.05

0.10

AU

Benito Marcote [email protected] HEPRO V - La Plata 2/232/23

The gamma-ray binary LS 5039

αJ2000 = 18h 26m 15.06s

δJ2000 = −14 50′ 54.3′′

O6.5 V main-sequence star (23 ± 3 M⊙)Compact object, NS or BH (1–5 M⊙)

P ≈ 3.9 de = 0.35 ± 0.04d = 2.5 ± 0.5 kpc

X-rays: periodicGeV light-curve: periodic (anticorrelated)TeV light-curve: periodicRadio: persistent, small variabilitywithout orbital modulation

Aharonian et al. (2005), Casares et al.(2005), Kishishita et al. (2009), Abdo etal. (2009), Casares et al. (2012), Zabalzaet al. (2013), Collmar & Zhang (2014)

−5 −3 −1 1 3 5 7 9 11 13log (photon energy [eV])

28

30

32

34

36

38

log(εL

ε [er

g/s]

)

star

sync.

disk

corona

SSC

star IC

corona IC

int. Bremsstr.

ext. Bremsstr.

disk IC

EGRET

COMPTEL

BATSERXTE

VLA

FIGURE 1. Computed SED for LS 5039 compared with observational data. The arrows represent upperlimits.

This means that with reasonable parameter values, the assumptions of our model areconsistent with the general features of the spectrum of LS 5039 from radio to TeVgamma-ray energies.

ACKNOWLEDGMENTS

V.B-R. and J.M.P. acknowledge partial support by DGI of the spanish Ministerio deEducación y Ciencia (MEC) under grant AYA2004-07171-C02-01, as well as additionalsupport from the European Regional Development Fund (ERDF/FEDER). V.B-R. is sup-ported by the DGI of the MEC under the fellowship BES-2002-2699. G.E.R is supportedby the Argentine Agencies CONICET and ANPCyT (PICT 03-13291). This research hasbenefited from a cooperation agreement supported by Fundación Antorchas.

REFERENCES

1. Paredes, J.M., Martí, J., Ribó, M., & Massi, M., 2000, Science, 288, 23402. Strong, A. W., et al. 2001, AIP Conference Proceedings, 587, 213. Collmar, W., 2003, Proc. 4th Agile Science Workshop, p.177, Frascati (Rome) on 11-13 June 20034. Casares, J., Ribó, M., Ribas, I., Paredes, J.M. et al., 2005, MNRAS, submitted.5. Bosch-Ramon, V., Paredes, J.M., Ribó, M. et al., 2005a, ApJ, 628, 3886. Bosch-Ramon, V., Romero, G.E., & Paredes, J.M. 2005b, A&A, submitted

SED of LS 5039 (Paredes et al. 2005)


The gamma-ray binary LS 5039

αJ2000 = 18h 26m 15.06s

δJ2000 = −14 50′ 54.3′′

O6.5 V main-sequence star (23 ± 3 M⊙)Compact object, NS or BH (1–5 M⊙)

P ≈ 3.9 de = 0.35 ± 0.04d = 2.5 ± 0.5 kpc

X-rays: periodicGeV light-curve: periodic (anticorrelated)TeV light-curve: periodicRadio: persistent, small variabilitywithout orbital modulation

Aharonian et al. (2005), Casares et al.(2005), Kishishita et al. (2009), Abdo etal. (2009), Casares et al. (2012), Zabalzaet al. (2013), Collmar & Zhang (2014)

0

5

10

15

F1−

10ke

V/

10−1

2er

gcm−2

s−1

0

1

2

3

F10−3

0M

eV/

10−5

cm−2

s−1

0.0

0.5

1.0

1.5

F0.

1−1

GeV/

10−6

cm−2

s−1

0.0 0.5 1.0 1.5 2.0φ (P = 3.90603 d,MJD0 = 51942.59)

0

2

4

F>

1Te

V/

10−1

2cm−2

s−1


Previous radio observations of LS 5039• Variability from Martí et al. (1998)

J. Martı et al.: The system LS 5039: a new massive radio emitting X-ray binary L73

−0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2log Frequency (GHz)

1.1

1.2

1.3

1.4

1.5

1.6

1.7

log

Flux

Den

sity

(mJy

)

1998 Feb 11

1998 Apr 09

1998 Mar 10

1998 May 12

Fig. 2. The radio spectrum of LS 5039 on different epochs during the1998 observations with the VLA. Error bars not shown are smaller thanthe symbol size.

Table 2. Optical photometry of LS 5039.

Date Julian Day V R I

(JD−2400000)

1998 Jun 2 50966.5 11.35 − −

1998 Jun 3 50967.6 11.33 − −

1998 Jun 7 50971.6 11.35 10.64 9.901998 Jun 8 50972.5 11.39 10.69 9.95

and they were reduced using standard procedures based on theIRAF image processing system. In Fig. 3 we show a 30 s expo-sure of the LS 5039 field in the V -band. Differential photometrywas performed against two nearby comparison stars (C1 and C2in Fig. 3).

Based on several observations of Landolt et al. (1992) stan-dards, the adopted magnitudes of the comparison stars areV = 12.53, R = 12.01, I = 11.55 and V = 10.10, R = 9.36,I = 8.68 for C1 and C2, respectively. This absolute photom-etry is accurate to ±0.03 mag. Relative variations in the finalphotometric results of Table 2 can be nevertheless traced at the±0.01 mag level.

4. Discussion

4.1. Astrometric identification

The radio position derived from the VLA-A configuration mapsin Fig. 1 is found to be: αJ2000 = 18h26m15.s056±0.s001and δJ2000 = −1450′54.′′24±0.′′01. These coordinates agreewithin 0.′′1 with those of LS 5039 in the optical USNO-A1.0catalogue, whose typical astrometric error is about 0.′′25. There-fore, we conclude that the identification of the LS 5039 radiocounterpart on the basis of astrometrical coincidence at the sub-arcsec level is virtually certain. From source count analysis (e.g.Ledden et al. 1980), the a priori probability of having a back-

LS 5039 V-Band

DEC

LIN

ATI

ON

(J20

00)

RIGHT ASCENSION (J2000)18 26 20 15 10 05

-14 49 00

30

50 00

30

51 00

30

52 00

30

53 00

LS5039

ROSAT

C1

C2

Fig. 3. CCD image of LS 5039 obtained on 1998 June 7 with the 1.52mOAN telescope in the V -band. The circle with 35

′′ radius representsthe 90% confidence position from the ROSAT PSPC pointed observa-tions quoted in M97. The two comparison stars used for differentialphotometry are labeled as C1 and C2.

ground extragalactic source with S20cm ≃ 30 mJy, or brighter,within 0.′′2 of the LS 5039 optical position is as low as <

∼ 10−7.To our knowledge, this discovery increases to four the numberof confirmedmassiveX-ray binarieswith associated radio emis-sion, the other three beingSS433,CygnusX-1 andLSI+61303.

4.2. Radio behavior

The radio spectra ofLS5039 inFig. 2 are very suggestive of non-thermal radio emission.A typical one can bewell represented bySν = (52±1)mJy (ν/1GHz)−0.46±0.01. Given the unresolvedangular size (θ ≤0.′′1), the brightness temperature

Tb = 1.76 × 103K! ν

GHz

"−2#

θ

arcsec

$−2 #

Sν

mJy

$

(1)

can be estimated, for example at the 20 cm wavelength, asTb ≥ 4 × 106 K. Such a high value, together with the signifi-cantly negative spectral index, safely rules out a thermal emis-sion mechanism. Non-thermal synchrotron radiation remainstherefore as the most plausible interpretation for the LS 5039radio emission. We can also obtain a lower limit to the sourceangular size by preventing catastrophic inverse Compton lossesto occur (Tb ≤ 1012 K). This condition yields θ ≥ 0.2 mas,implying that further progress in studying the LS 5039 struc-ture may be feasible by the use of VLBI techniques. We ignoreat present if the radio emission is originated in collimated mil-liarcsec radio jets or, alternatively, some other mechanism is atwork.

Assuming equipartition arguments for synchrotron radiosources (Pacholczyk 1970), the total energy content and mag-netic field in LS 5039 can be expressed as:

Etotal = c13(1 + k)4/7φ3/7R9/7L4/7

rad(2)

LETT

ER

Spectral index α = −0.46 ± 0.01, variability < ±25 %


Understanding the emission of Gamma-Ray BinariesYoung non-accreting pulsar scenario

Strong shock between both winds:• Relativistic pair plasma wind from

the pulsar• Stellar wind from the massive

companion starOriginally proposed by Maraschi & Treves(1981), re-proposed by Dubus (2006)

• High energy emission produced bysynchrotron and inverse Compton.

• Radio flux dominated by thesynchrotron emission (Bosch-Ramon2009)

V. Zabalza et al.: GeV-TeV emission model for LS 5039

Star

Shocked pulsar wind

Shocked stellar wind

Windstandoff

Coriolisturnover

Orbitalmotion

Pulsar

Fig. 1. Sketch of the proposed scenario at the periastron of aclose-orbit system similar to LS 5039. The region of the CDwhere significant turbulence, and therefore wind mixing, areexpected to take place is indicated with a wavy line. The redregions indicate the two proposed emitter locations.

simulations of Bosch-Ramon et al. (2012), and will therefore beused throughout this work.

The presence and properties of this quasi-perpendicularstrong shock at the Coriolis turnover location sets apart thecharacteristics of the interaction of stellar and pulsar wind withrespect to that of binary stellar systems. The non-relativisticstellar wind in binary stellar systems can be smoothly deflectedby sound waves, as found in the simulations of Lamberts et al.(2012). However, sound wave deflection would not affect analready trans-sonic relativistic pulsar wind, leading to the for-mation of a quasi-perpendicular strong shock because of struc-ture bending. We note that this approach may not be valid forw Æ 1.2102, as numerical simulations indicate that a recon-finement shock, with different characteristics to those exploredabove, will develop in the pulsar wind for those cases wherethe stellar wind vastly overpowers the pulsar wind (Bogovalovet al., 2008).

3. Location and properties of the non-thermal emitters

A sketch of the system, as described in the previous section, canbe found in Fig. 1. The influence of the orbital motion on theshape of the pulsar wind zone gives rise to two distinct regionswhere most of the energy of the pulsar wind is deposited inquasi-parallel shocks: the wind standoff and Coriolis turnoverlocations. The intermediate regions of the pulsar wind shock,i.e., those not marked in red in Fig. 1, are not expected to con-tribute significantly to the non-thermal emission of the systemgiven the high obliquity of the impact of the pulsar wind andcorrespondingly low energy transfer into non-thermal parti-cles.

The pulsar wind termination region close to the wind stand-off location has been generally considered as the preferred loca-tion for acceleration of electrons up to ultrarelativistic energies,which would give rise to the observed non-thermal broadbandemission through synchrotron and IC processes (e.g., Dubus,2006). However, the intense stellar photon field at this location,and corresponding high opacities owing to photon-photon pairproduction, precludes VHE gamma-rays above 40 GeV fromreaching the observer for some orbital phases with clear VHEdetections (Khangulyan et al., 2008a). On the other hand, anemitter with constant particle injection located at the windstandoff is compatible with the phenomenology exhibited bythe source at GeV energies. The modulation is mainly driven bythe change in the IC interaction angle, with enhanced (reduced)emission at superior (inferior) conjunction. For the allowedorbital inclination angles (20–60, Casares et al., 2005), themodulation is too strong to reproduce the Fermi lightcurve, butthis effect may be mitigated by Doppler boosting. As a proxy forthe complex hydrodynamical properties of the flow (Bogovalovet al., 2008), we have considered an effective bulk flow velocityof 0.15c in the direction of the axis of symmetry of the CD apex,which changes along the orbit (see CD aberration in Sect. 2)but is always close to the radial direction. The bulk flow veloc-ity of 0.15c used here applies exclusively to the Doppler effectaveraged over the CD cone and with respect to the observerline-of-sight. Therefore, it is still possible, and, given the sim-ulations of Bogovalov et al. (2008), probable, that the intrinsicbulk flow velocity of the shocked pulsar wind is higher.

The pulsar wind termination shock at the Coriolis turnoverlocation is a good candidate for the VHE emitter. The reducedstellar photon field density implies pair production absorptioncharacteristics consistent with the observed spectra at VHE. Forscenarios in which the stellar wind ram pressure is dominant,the pulsar wind solid angle subtended by the Coriolis turnovershock is much lower than that of the apex region. This is con-sistent with the reduced emission power of the TeV componentwith respect to the GeV component.

The magnetic field of the shocked pulsar wind is gener-ally well known in plerions from observations (e.g., Kennel &Coroniti, 1984), but the so-called sigma problem precludes onefrom deriving it theoretically. The situation in gamma-ray bi-naries is even more complex, as there might be a reduction ofthe post shock magnetic field strength owing to the reaccel-eration of the flow (Bogovalov et al., 2012). Since knowledgeof the post-shock magnetic field could only be obtained via amagnetohydrodynamical simulation of the system, we havechosen to parametrize the magnetic field energy density as afraction of the pre-shock equipartition magnetic field. Wedefine the latter at the balance of the magnetic field energydensity and the kinetic energy density of the pulsar wind atthe shock. Therefore, for a constant value of , the post-shockmagnetic field strength will behave along the orbit as B / 1/rp,where rp is the distance of the shock to the pulsar.

3.1. Maximum electron energy

A distinct feature of the GeV and TeV spectra of LS 5039 are thecutoffs present at a few GeV and above 10 TeV, respectively. Theformer is apparently constant along the orbit, whereas owingto the softer TeV spectrum during superior conjunction the lat-ter has only been detected clearly during inferior conjunction.These spectral cutoffs can be directly related to the maximumenergy of the electrons in the respective particle populations.The maximum energy that can be reached in any acceleration

3

Model for LS 5039 (Zabalza et al. 2012)


Previous radio observations of LS 5039

• VLBI observations: Moldón et al. (2012)

Observations along one orbital period.Dominant core emission (≲ 1 mas, or 3 AU).Extended emission orbitally modulated at mas scales (<10% of thetotal flux density emission).


Previous radio observations of LS 5039

• At low frequencies, only a few observations have been publishedwith contradictory results.

100 1000 10000Frequency (MHz)

5

10

50

100

Flux

dens

ity(m

Jy)

Pandey et al. (2007) APandey et al. (2007) BMartı et al. (1998)

Adapted from Pandey et al. (2007)

L2 S. Bhattacharyya et al.

synchrotron self-absorption (SSA) was responsible for the opticallythick radio spectrum, then the magnetic field was found to be inthe range 10−2−1 G. The size of the emitting region would be∼(4.5–10) × 1013 cm and the location of the source would be at!(4.5–10)×1013 cm. Hence it is important to understand the actualprocess responsible for the absorption of radio emission.

Here we present the results of radio observations of LS 5039 takenwith the Giant Metrewave Radio Telescope (GMRT) in the 200–1280 MHz frequency range. We study the low-frequency spectrumduring the periastron and apastron passage of the compact star andattempt to understand its possible consequences. The observationand the data analysis are discussed in the next section. The resultsare discussed in Section 3, and finally we conclude the Letter inSection 4.

2 O B S E RVAT I O N S A N D DATA R E D U C T I O N

The GMRT consists of 30 steerable antennas of 45-m diameter inan approximate ‘Y’ shape similar to the VLA but with each antennain a fixed position. 14 antennas are randomly placed within a central1 × 1 km2 (the ‘central square’) and the remaining antennas formthe irregular Y shape (six on each arm) over a total extent of about25 km. We refer the reader for details about the GMRT array tohttp://gmrt.ncra.tifr.res.in and Swarup et al. (1991). We observedLS 5039 at 1280 MHz on 2006 February 23 and simultaneouslyobserved at 234 and 614 MHz on 2006 March 13.

The observations were made in standard fashion, with each sourceobservation (30 min) interspersed with observations of the phasecalibrator (4 min). The primary flux density calibrator was either3C 48 or 3C 286, with all flux densities being on the scale of Baarset al. (1977). Either of these flux calibrators was observed for 20 minat the beginning and end of each observing session.

The data recorded with the GMRT were converted to FITS for-mat and analysed with the Astronomical Image Processing System(AIPS) using standard procedures. During the data reduction, wehad to discard a fraction of the data affected by radio frequencyinterference and system malfunctions. After editing, the data werecalibrated and collapsed into fewer channels. A self-calibration onthe data was used to correct for the phase-related errors and improvethe image quality. The observation dates, the observed frequenciesand the corresponding spectral phases are given in Table 1.

3 R ESULTS AND DISCUSSION

The spectra obtained for the two orbital phase ranges correspondingto the periastron and apastron passage are shown in Fig. 1 alongwith the data points from our previous observations (Paper I) andarchival data points from Martı, Paredes & Ribo (1998) and Riboet al. (2008). The flux points obtained from Ribo et al. (2008) donot match with the points obtained by Martı et al. (1998). Thisis because the observation by Ribo et al. (2008) with very long

Table 1. Log of GMRT observa-tions.

MJD Frequency Phase(MHz) range

54573 234 0.63–0.70605.2 0.63–0.70

54670 1280 0.65–0.7254684 1280 0.98–0.0554688 605.2 0.99–0.06

Figure 1. Radio spectrum of LS 5039 as observed with the GMRT. Thearchival data are also plotted.

baseline interferometry (VLBI) resolved the source and we haveused the flux values reported for the core. The overall flux fromthe core and the wings is of the same order as reported by Martıet al. (1998). It is evident that the spectra remain optically thickand the flux values at frequencies 605 and 1280 MHz are almostequal during the periastron (phase range: 0.65–0.72) and apastron(phase range: 0.99–0.06) passages. Because of the poor quality ofthe data at 234 MHz, we could not produce a source image for boththe observing spells. Compared to the flux values of our previousobservations, the present flux values fall within the error bars of theprevious observations. It is to be noted that the exposure time for ourprevious observations was less than that of the present observations.

The optically thick spectrum at radio frequencies can be produceddue to the absorption of radiation either by free–free absorption or bySSA in the emitting region of the source. The free–free absorptionof radio waves may occur in LS 5039 due to the presence of stellarwind from the primary star. The relativistic jet where the radioemission takes place by the synchrotron process moves through anenvironment of such a stellar wind. For the time being, let us assumethat the wind from the companion star is spherically symmetric. Ifso, the free–free absorption opacity is given by (Rybicki & Lightman1986)

αffν = 0.018T −(3/2)Z2n2ν−2gff, (1)

where T is the temperature of the stellar wind, n is the numberdensity of the stellar wind and ν is the frequency of radiation.

For a binary orbit inclined at an angle i with respect to the skyplane, the jet will make an angle i with the direction of observation.It is assumed that the jet is perpendicular to the orbital plane. If thesemimajor axis of the orbit is a and the eccentricity is e, then theequation of the orbit is given by

ρ2 = a2(1 − e2)(1 − e2 cos2 θ )

, (2)

where θ is the phase angle and θ = 2πφ, 0 ≤ φ ≤ 1 is the orbitalphase. From Fig. 2, using simple geometry, one can write

O′C2 = ρ2 + a2e2 − 2ρae cos θ . (3)

If the radio-emitting region is located at a height z from the base ofthe jet, then

s2 = O′C2 + z2 = ρ2 + a2e2 − 2ρae cos θ + z2. (4)

C⃝ 2011 The Authors, MNRAS 421, L1–L5Monthly Notices of the Royal Astronomical Society C⃝ 2011 RAS

Bhattacharyya et al. (2012)Godambe et al. (2008)


Radio observationsWork published in Marcote et al. (2015)

• 1.4 – 15 GHz (VLA)Monitoring in 1998, 2002

• 154, 235 & 610 MHz (GMRT)Archival observations 2004–08

• 154 MHz – 5 GHz (GMRT & WSRT)Two quasi-simultaneous observationsin 2013.


Summary of the observations analyzed in this work


High-frequency variability along the orbitVLA monitoring in 2002

• Persistent flux density emission.• Variability on timescales as

short as one day.

• Variability < ±25 %

• No visible orbital modulation

Flux density values at all frequencies compatible with the ones reportedat other epochs (e.g. Martí et al. 1998).


High-frequency variability along the orbitVLA monitoring in 2002

• Dashed lines represent thepower-laws determined from5.0 and 8.5 GHz

• We observe a ≈ power-lawat these GHz frequenciesmost of the times

• Slight curvature below2 GHz

• Average spectral index:α = −0.57 ± 0.12


Low-frequency variability along the orbitArchival GMRT observations 2004–2008


Low-frequency variability along the orbitArchival GMRT observations 2004–2008

• Observations spread over 4 yr

• Persistent emission

• Variability at > 6σ

• Average spectral indexα = +0.5 ± 0.8

• Hints of orbital modulation?


Non-simultaneous spectrum of LS 5039Combining data from 1998 to 2013

• Small variability along the years(< 25% ∀ ν)

• “Similar” profile in average• Turnover at ∼ 0.5 GHz• Source undetected at 150 MHz

• Only two data at 2.3 GHz(no statistics)

• The mean square errors havebeen used in the average data

0.2 0.5 1 2 5 10 20None

5

10

20

50

S/

mJy

VLA 1998VLA 2002GMRT 2004–2008GMRT-WSRT 2013

0.2 0.5 1 2 5 10 / GHz

5

10

20

50

S/

mJy


Modeling the LS 5039 spectrumA first approximation (toy model)

From VLBI observations (Moldón et al. 2012) we known that most ofthe radio emission comes from a compact core ≲ 1 mas (∼ 3 AU) tobe compared with the 0.19 AU of the semi major axis.

We have built a very simple model to understand the spectrum:• Compact core ; one-zone model• No orbital modulation ; symmetric emitting region (spheric)• For simplicity ; isotropic and homogeneous• We consider the presence of a synchrotron emitting plasma• Turnover produced either by SSA, FFA or Razin effect (and

combinations of them)


Modeling the LS 5039 spectrumA first approximation (toy model)

We have built a very simple model to understand the spectrum:• Synchrotron emission, with a particle injection:

n(E)dE = KE−pdE

• Synchrotron self-absorption (SSA):

κSSA ∝ KB(p+2)/2ν−(p+4)/2

• Free-free absorption (FFA):

κFFA ∝ n2eT−3/2

w ν−2

• Razin effect:

Sν ; Sνe−νR/ν , νR ≡ 20neB−1


Non-simultaneous spectrum of LS 5039Combining data from 1998 to 2013

• The average spectrum can befitted by typical models: SSA Synchrotron + FFA SSA + Razin FFA + Razin SSA + FFA

• SSA+Razin effect is the best fitto the data

• But the other fits are notstatistically rejected

• Small differences between all ofthem


Quasi-simultaneous spectrum of LS 5039GMRT & WSRT campaign in 2013 July 19 and 21


Quasi-simultaneous spectrum of LS 5039GMRT & WSRT campaign in 2013 July 19 and 21

Two 0.15–5 GHz spectra at orbital phases ϕ ≈ 0.9 and 0.4.

• Although similar to the averagespectrum, we observe subtledifferences between the twoepochs.

• The turnover at ∼ 0.5 GHz ispersistent.

• Stronger emission on 2013 July19.

• 2013 July 19: pure SSA spec.• 2013 July 21: SSA+Razin spec.• FFA provides poor fits

GMRT data: 235 and 610 MHzWSRT data: 2.3 and 5.0 GHz154 MHz GMRT data taken every other day(on 2013 July 18, 20 and 22)


Modeling the LS 5039 spectrum

The three spectra show a similar shape but with subtle differences:• Avg. spectrum: SSA+Razin• July 19 spectrum: SSA• July 21 spectrum: SSA+Razin

Fit p ΩB−1/2 KℓB(p+2)/2 νR[10−16 G−1/2

] [10 3 cm G(p+2)/2

] [10 8 Hz

]Avg. spectrum 2.16 ± 0.04 500 ± 800 3 ± 5 4.1 ± 0.2July 19 1.867 ± 0.014 3.9 ± 0.3 (2.1 ± 0.9)× 106 –July 21 2.24 ± 0.08 200 ± 600 0.4 ± 1.7 4.1 ± 0.7

• We can also compare these results with the free-free opacityinferred from the stellar wind (the region must be optically thin toFFA)


Modeling the LS 5039 spectrumBuilding a coherent picture from the fits and the free-free opacity:• Avg. spectrum: SSA+Razin• July 19 spectrum: SSA• July 21 spectrum: SSA+Razin• Coherent picture with:

ℓ ∼ 0.85 mas (∼ 2.5 AU)

B ∼ 20 mGne ∼ 4 × 105 cm−3

M ∼ 5 × 10−8 M⊙ yr−1

where:ℓ: linear size of the emitting region,B: module of the magnetic field,ne: electron density of the non-

relativistic plasma,M: mass-loss rate of the companion

star.


Modeling the LS 5039 spectrum

• A significant mixing of the non-relativistic wind inside thesynchrotron radio emitting relativistic plasma, even close to∼ 100%, is observed.

• The derived mass-loss rate (model dependent, in agreement withthe last results, Casares, in prep.) implies that the wind is clumpy.

• The presence of Razin effect, widely observed in Colliding WindBinaries, could give further support to the scenario of the youngnon-accreting pulsar.


ConclusionsMarcote et al. (2015), MNRAS, 451, 59Marcote (2015), PhD Thesis, Universitat de Barcelona

• We report day to day variability, trends on week timescales, andthe absence of orbital variability. Variability < ±25 % is observedat all frequencies (0.23–15 GHz) even on year timescales.

• Persistent turnover at around ∼ 0.5 GHz.• No detection up to now at 150 MHz• The considered simple model can explains the observed spectra,

indicating that the turnover is dominated by SSA• A contribution of Razin effect is also observed at some epochs.• Even with this simple model we have obtained a coherent picture

that explains the observed spectra.• Future multifrequency campaigns with more accurate results

(specially at low frequencies) are required to compare with moredetailed models.


Thank You…!…and see also the poster aboutsimilar studies conducted for thegamma-ray binary LS I +61 303(no. 20)!

LATEX TikZposter

Orbital and superorbital variability of LS I +61 303 at lowradio frequencies with GMRT and LOFAR

B. Marcote1, M. Ribó, J. M. Paredes, C.H. Ishwara-Chandra, J. D. Swinbank, J. W. Broderick, S. Markoff, R. Fender,R. A. M. J. Wijers, G. G. Pooley, A. J. Stewart, M. E. Bell, R. P. Breton, D. Carbone, S. Corbel, J. Eislöffel, H. Falcke,

J.-M. Grießmeier, M. Kuniyoshi, M. Pietka, A. Rowlinson, M. Serylak, A. J. van der Horst, J. van Leeuwen, M. W. Wise, P. Zarka1Departament d’Astronomia i Meteorologia, Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB)

Orbital and superorbital variability of LS I +61 303 at lowradio frequencies with GMRT and LOFAR

B. Marcote1, M. Ribó, J. M. Paredes, C.H. Ishwara-Chandra, J. D. Swinbank, J. W. Broderick, S. Markoff, R. Fender,R. A. M. J. Wijers, G. G. Pooley, A. J. Stewart, M. E. Bell, R. P. Breton, D. Carbone, S. Corbel, J. Eislöffel, H. Falcke,

J.-M. Grießmeier, M. Kuniyoshi, M. Pietka, A. Rowlinson, M. Serylak, A. J. van der Horst, J. van Leeuwen, M. W. Wise, P. Zarka1Departament d’Astronomia i Meteorologia, Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB)

LS I +61 303 is a gamma-ray binary that exhibits an outburst at GHz frequen-cies each orbital cycle of 26.5 d and a superorbital modulation with a period of4.6 yr. We have performed a detailed study of the low-frequency radio emission ofLS I +61 303 by analysing all the archival GMRT data at 150, 235 and 610 MHz,and conducting regular LOFAR observations within the Radio Sky Monitor (RSM)at 150 MHz. We have detected the source for the first time at 150 MHz, whichis also the first detection of a gamma-ray binary at such a low frequency. Wehave obtained the light-curves of the source at 150, 235 and 610 MHz, all of themshowing orbital modulation. The light-curves at 235 and 610 MHz also showthe existence of superorbital variability. A comparison with contemporaneous15-GHz data shows remarkable differences with these light-curves. At 15 GHzwe see clear outbursts, whereas at low frequencies we see variability with widemaxima. The light-curve at 235 MHz seems to be anticorrelated with the oneat 610 MHz, implying a shift of ∼0.5 orbital phases in the maxima. We modelthe shifts between the maxima at different frequencies as due to changes in thephysical parameters of the emitting region assuming either free-free absorptionor synchrotron self-absorption, obtaining expansion velocities for this region closeto the stellar wind velocity with both mechanisms. The inferred values would givefurther support to the young non-accreting pulsar wind scenario. All these resultsare published in Marcote et al. (2015, submitted).

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Detection of LS I +61 303 in the 2008 Feb 18 154-MHz GMRT observation (left)and in the 2013 July 14 149-MHz LOFAR observation after removing baselinesbelow 0.2 kλ (right).

Flux density values of LS I +61 303 as a function of the MJD (left) and folded withthe orbital period (right) from the analyzed GMRT and RT data. The black data(left bottom) shows the spectral index in the range of 235–610 MHz. We observean orbital modulated light-curve. The 610-MHz data shows a quasi-sinusoidalmodulation with a slower decay with respect to the 15-GHz data. The 235-MHzlight-curve is almost anticorrelated with the one observed at 610 MHz.

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Folded light-curve of LS I +61 303with the orbital period from the 150-MHz data. We observe an or-bitally modulated emission, with theoutburst slightly delayed with re-spect to the contemporaneous 15-GHz data. We note that these 150-MHz observations were taken at dif-ferent superorbital phase (φso ∼ 0.8)compared to the ones at 235 and610 MHz (0.2). Therefore we can notdirectly compare the behavior of thesource from these two data sets.

Field of LS I +61 303 seen by POSS II (Reid et al. 1991) at optical red F band(top), and by LOFAR at 149 MHz on 2013 July 14 (bottom).

Expected shifts in orbital phase (∆φ)for the maxima in the flux densityemission between a frequency νand 15 GHz for different velocitiesof expansion of the radio emittingregion considering synchrotronself-absorption with different de-pendences of the magnetic field withthe size of the emitting region (νSSA,top figures) or free-free absorption(νFFA, top right column).

Inferred expansion velocity, assumingFFA, as a function of the mass-loss rate.The vertical dashed line denotes the ex-pected mass-loss rate. We compare theobtained velocities with the mean stellarwind velocity (solid line) and its lower ex-pected value (dotted line).


Back-up slides


Accurate GMRT data reduction (Marcote et al. 2015, Appendix)

Analyzing the GMRT data we found the reasons of the differencesbetween Pandey et al. (2007) and Godambe et al. (2008):• At low frequencies, the contribution of the Galactic diffuse

emission is quite high within the Galactic Plane.• It must be removed to recover the right flux densities in the final

image. Otherwise the flux densities will be underestimate.• Usually done automatically in most telescopes. Not in the GMRT.• Godambe et al. (2008) did not take into account this correction.• Pandey et al. (2007) used the Haslam approximation (see Marcote et

al. 2015), extrapolating the emission seen at 408 MHz.• We have conducted dedicated non correlated observations with the

GMRT to directly measure the Galactic contribution in the field ofLS 5039 to properly substract it.

• We have seen significant differences in these two methods for thefield of LS 5039 (compatible with the comparison in Sirothia 2009).


Razin effectThe synchrotron emission propagates through a plasma, which presentsa refractive index n. Always that n < 1 the beaming effect is partiallysuppressed. Although at high frequencies the effect is negligible, atlow frequencies (with ν ≪ νp, where νp is the plasma frequency) itsuppresses the beaming effect since

n2 = 1 −(νpν

)2

and the beaming effect goes asθb ≈ γ−1 =

√1 − n2β2

• Presence of a thermal plasma surrounding the emitting region.• Attenuation of the synchrotron radiation at low frequencies.• Widely reported in Colliding Wind Binaries, solar wind,…• A good approximation is an exponential attenuation at low

frequencies (Dougherty et al. 2003):Sν ∝ Sνe−νR/ν , νR ≡ 20neB−1


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