OBSERVATIONS OF MICROQUASAR CANDIDATES WITH THE MAGIC TELESCOPE

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OBSERVATIONS OF MICROQUASAR CANDIDATES WITH THE MAGIC TELESCOPE

J. RICO for the MAGIC Collaboration Institut de Fisica d’Altes Energies (IFAE) Edifici Cn. Universitat Autonoma de Barcelona

08193 Bellaterra (Barcelona) Spain E-mail: jrico@ifae.es

www.ifae.es

We report on the results from the observations in very high energy band (VHE, E

≥ 100 GeV) of the γ-ray binary LS I +61 303 and the black hole X-ray binary (BHXB) Cygnus X-1. LS I +61 303 was recently discovered at VHE by MAGIC [1] and here we present the preliminary results from an extensive observation campaign, comprising 112 observation hours covering 4 orbital cycles, aiming at determining the time-dependent features of the VHE emission.

Cygnus X-1 was observed for a total of 40 hours during 26 nights, spanning the period between June and November 2006. We report on the results of the searches for steady and variable γ-ray signals from Cyngus X-1, including the first experimental evidence for an intense flare, of duration between 1.5 and 24 hours.

Keywords: Very High Energy gamma-rays; binaries; LS I +61 303; Cygnus X-1

1. Introduction

Binary systems containing a compact object and a main-sequence star have been recently established as a new population of VHE emitters, by the de- tection of PSR B1259−63 [2], LS I+61 303 [1] and LS 5039 [3]. In PSR B1259 − 63 the VHE emission is thought to be produced by the interaction of the relativistic wind from a young pulsar with the out- flow of the companion star. Recent results may sug- gest that LS I +61 303 also contains a non-accreting neutron star [4], although this question is currently under debate (e.g. [5]). The situation is also unclear for of LS 5039 [6, 7], whose compact object nature is not known. To date, however, there has been no experimental evidence of VHE emission from any galactic black hole X-ray binary (BHXB) system. In the present paper we summarize recent results ob- tained with the MAGIC telescope on the study of binary systems, including a deep study of the time- dependent features of LS I+61 303 VHE emission, and the first solid experimental evidence of emission from a BHXB, namely Cygnus X-1.

2. LS I+61 303

LS I+61 303 is an X-ray binary system located at a distance of ∼ 2 kpc, composed of a B0 main se- quence star with a circumstellar disc and a compact object (probably a neutron star) orbiting around it in a highly eccentric (e = 0.72± 0.15) orbit. The orbital period is 26.496 days with the periastron passage at

phase φ = 0.23 ± 0.02 [8]. The orbital motion seems to modulate the emissions at radio [9, 10] and X- ray [11, 12] bands. LS I+61 303 is also positionally coincident with an EGRET γ-ray source [13], and variable emission at VHE has been recently detected with the MAGIC telescope [1]. Although previously considered a microquasar [14], recent VLBA imaging [4] over a full orbit showed the radio emission to come from angular scales smaller than about 7 mas (pro- jected size 14 AU) and to be cometary-like, roughly pointing away from the high-mass star. Based on these findings, it was suggested that LS I+61 303 is a pulsar wind nebula shaped by an anisotropic en- vironment.

LS I +61 303 was re-observed with MAGIC for 112 hours, covering 4 different orbital cycles from September to December 2006. Our measurements show that the VHE γ-ray emission from LS I+61 303 is variable. The integral γ-ray flux coming from the direction of LS I+61 303 in a day-by-day basis is presented in Fig. 1 and phase-folded (together with measurements from the 2005 campaign [1]) in Fig. 2.

The maximum flux is detected at around phase Φ =

0.6 − 0.7 for every orbital cycle. This points to the

possible periodicity of the VHE signal. Deeper stud-

ies on the matter will be presented elsewhere in the

near future. On the other hand, this level of flux is

not detected at periastron passage. Interestingly, a

secondary peak at phase ∼ 0.85 is seen in the last

observed cycle, with a flux of 17.0 ± 4.4 × 10

cm

s

. This corresponds to a similar level of flux de-

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tection seen in phase ∼ 0.6 (e.g. day 54035.11). How- ever, this high flux was not seen in the previous cy- cle, where an upper limit is set at phase 0.82 with 5.0 × 10

cm

s

or even the previous one, with φ = 0.81 and an upper limit of 1.9 × 10

cm

.

= 53991.2 MJD0

= 54017.7 MJD0

= 54044.2 MJD0

= 54070.7 MJD0

Orbital phase

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

-2 0 2

-2 0 2

]-1 s-2 cm-11F(E>300 GeV) [10

-2 0 2

-2 0 2

-0.5 0 0.5 1

Fig. 1. VHE (E > 300GeV) gamma-ray flux of LS I+61 303 as a function of the orbital phase for the four observed orbital cycles (4 upper panels) and averaged for the entire observation time (lowermost panel).

We have searched the data set for intra-night variability, covering 87 different observation nights, on time scales from 15 to 75 minutes. Most of the observation slots last 2-3 hours except for two of them, lasting more than 4 hours. All obtained nightly lightcurves are well fitted by a constant flux level.

The chi-square distribution for all different time bin- nings indicates that no flux level variations on time scales between 30-75 min occur in the source within the sensitivity of the MAGIC telescope.

The spectrum of LS I+61 303 does not ex- hibit changes in the spectral shape during differ- ent observed phases on time scales of one year. The VHE spectra derived from both campaigns between

∼ 300 GeV and ∼ 4 TeV are shown in Fig. 3. The

reference red dotted black line corresponds to the 2005 campaign data (averaged for phases 0.4-0.7), fitted by a power law with spectral index 2.6 ± 0.2(stat)±0.2(syst). For the 2006 campaign, the spec- tra for phases [0.5-0.6] and [0.6-0.7] are presented.

We found that one universal photon index is enough to describe the spectral behavior in all phase bins investigated.

Orbital Phase 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ]

s

cm

F(E>400GeV) [10

-20 -10 0 10

20

Period 2 Period 3 Period 4 Period 5 Period 6 Period 7 Period 8 Period 9 Period 10

preliminary

Fig. 2. Integral γ-ray flux (E > 400 GeV) lightcurve on a day-by-day basis folded with the orbital period of 26.496 days phaseogram. Each color represent a different orbital cycle.

E (GeV)

102 10³ 10⁴

)-1s-2cm-1dN/dE (TeV

10-15

10-14

10-13

10-12

10-11

10-10

10-9

Spectrum dN/dE

Cycle I

<0.7 φ Cycle II 0.6<

<0.6 φ Cycle II 0.5<

Preliminary

Fig. 3. Differential energy spectrum of LS I+61 303 for en- ergies between 300 GeV and 4 TeV and averaged for maximal flux orbital phases. The dashed line corresponds to the power law fit to fluxes in the 2005 campaign [1]. The blue and red points correspond to measured fluxes in the 2006 campaign for orbital phases 0.5-0.6 and 0.6-0.7 respectively.

3. Cygnus X-1

Cygnus X-1, located at a distance of 2.2 ± 0.2 kpc, is

composed of a 21 ± 8 M

BH orbiting an O9.7 Iab

companion of 40 ± 10 M

[15] in a circular orbit of 5.6

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days and inclination between 25

and 65

[16]. The X-ray source is likely powered by accretion. Obser- vations in the soft γ-ray range with COMPTEL [17]

and INTEGRAL [18] strongly suggest the presence of a higher energy, non-thermal component. Images with the VLBA have shown the presence of a highly collimated relativistic jet [19]. The interaction of the jet with the interstellar medium appears to be re- sponsible for a large-scale ( ∼ 5 pc diameter), ring- like, radio emitting structure [20].

Energy [GeV]

102 10³

]-1 TeV-1 s-2Flux (dN/dE dA dt) [cm

10-15 10-14 10-13 10-12 10-11 10-10 10-9 10-8

-1] -1 TeV -2 s 0.6 [cm

± -3.2 1 TeVE 10-12

×

±0.6) = (2.3 dA dt dE

dN

Crab Nebula Cyg X-1 2006-09-24 Cyg X-1 steady

MAGIC

Fig. 4. Differential energy spectrum from Cygnus X-1 corre- sponding to 78.9 minutes effective observation time (EOT) be- tween MJD 54002.928 and 54002.987 (2006-09-24). Also shown are the Crab nebula spectrum and the best fit of a power-law to the data and the 95% CL upper limits to the steady γ-ray flux.

Cygnus X-1 was observed with MAGIC between June and November 2006 (MJD=53942 to 54058) [21]. The data set comprises 40.0 good observation hours from 26 different nights. A search for steady γ-ray signals was performed for the entire recorded data sample, yielding no significant excess. This al- lows us to establish the first upper limits to the VHE γ-ray steady flux of Cygnus X-1 of the order of ≤ 1 − 5% of the Crab nebula flux (see Fig. 4).

Gamma-ray signals were also searched for on a daily basis. We obtain results compatible with background fluctuations at 99% CL for all searched samples except for MJD=54002.875 (2006-09-24). The data from 2006-09-24 were further subdivided into two halves to search for rapidly varying signals, obtain- ing 0.5σ and 4.9σ results for the first (75.5 minutes EOT starting at MJD 54002.875) and second (78.9 minutes EOT starting at MJD 54002.928) samples, respectively. The post-trial probability is conserva- tively estimated by assuming 52 trials (2 per observa- tion night) and corresponds to a significance of 4.1σ.

Crab Units

-0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 MAGIC

(150-2000 GeV)

Crab Units

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

SWIFT BAT (15-50 keV)

MJD-54000 [days]

-60 -40 -20 0 20 40 60

Crab Units

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

RXTE ASM (1.5-12 keV)

MJD-54000 [days]

2 4

Fig. 5. From top to bottom: MAGIC, Swift/BAT and RXTE/ASM measured fluxes from Cygnus X-1 as a func- tion of time. The left panels show the whole time spanned by MAGIC observations. The vertical, dotted blue lines de- limit the range zoomed in the right panels. The vertical red line marks the time of the MAGIC signal.

The distribution of excess events corresponding

to the 78.9 minutes EOT sample starting at MJD

54002.928 is consistent with a point source located

at the position of Cygnus X-1. A Gaussian fit yields

the location: α = 19

58

17

, δ = 35

12

8

with sta-

tistical and systematic uncertainties of 1.5

and 2

,

respectively, compatible within errors with the po-

sition of Cygnus X-1 and excluding the radio neb-

ula [20] at a distance of ∼ 8

. The energy spectrum

is shown in Fig. 4. It is well fitted (χ

/n.d.f. = 0.5)

by the following power law: dN/(dA dt dE) = (2.3 ±

0.6) × 10

(E/1 TeV)

cm

s

TeV

where

the quoted errors are statistical only. We estimate

the systematic uncertainty to be 35% on the overall

flux normalization and 0.2 in the determination of

the spectral index.

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The excess from the direction of Cygnus X- 1 occurred simultaneously with a hard X-ray flare detected by INTEGRAL [22], Swift/BAT and RXTE/ASM. Fig. 5 shows the correlation between MAGIC, Swift/BAT and RXTE/ASM light-curves.

The VHE excess was observed on the rising side of the first hard X-ray peak, 1–2 hours before its max- imum, while there is no clear change in soft X-rays.

Additionally, the MAGIC non-detection during the following night (yielding a 95% CL upper limit cor- responding to a flux ∼ 5 times lower than the one observed in the second half of 2006-09-24) occurred during the decay of the second hard X-ray peak. A possible explanation is that, during the night of 2006- 09-24, soft and hard X-rays were produced in differ- ent regions. Furthermore, hard X-rays and VHE γ- rays could be produced in regions linked by the colli- mated jet, e.g. the X-rays at the jet base and γ-rays at an interaction region between the jet and the stel- lar wind. These processes would have different physi- cal timescales, thus producing a shift in time between the VHE and X-ray peaks. Note that the distance from the compact object to the VHE production re- gion is constrained below 2

by MAGIC observations and therefore it is unrelated with the nearby radio emitting ring-like structure [20]. The observed VHE excess took place at phase 0.91 (phase 0.0 is when the BH is behind the O star). Currently, MAGIC obser- vations at this phase are available only for the night 2006-09-24, which precludes any possible analysis of periodicity for the VHE emission. The jet scenario, however, has some constraints. If the VHE emission were produced in the jet, well within the binary sys- tem, the photon conversion in the stellar radiation field would be very substantial, rendering unlikely a VHE detection [23]. Admittedly, the inclination of the orbit and the angle of propagation to the ob- server can affect this. Even without an explanation for a VHE flare, it is possible that the emission could have originated far from the compact object. Inter- actions of the jet with the stellar wind may lead to such a situation.

4. Summary

The time-dependent properties of the γ-ray binary LS I +61 303 have been deeply studied. We find a highly variable flux with clear hints of periodicity.

The variability is seen in timescales of 1 day but not in shorter times. The peak of the emission is always found at phase 0.6-0.7. For one observed orbital cy-

cle, also a secondary, intense peak was detected at later phases (0.85). The spectral shape is always well described by a power law with index 2.6, indepen- dently of the orbital phase or day investigated.

Regarding Cygnus X-1, for the first time we have found experimental evidence of VHE emission pro- duced by a Galactic stellar-mass BH. It is also the first evidence of VHE gamma-rays produced at an ac- creting binary system. Our results show that a possi- ble steady VHE flux is below the present IACT’s sen- sitivity and tight upper limits have been derived. On the other hand, we find evidence for an intense flaring episode during the inferior conjunction of the optical star, on a timescale shorter than 1 day with a rise time of about 1 hour, correlated with a hard X-ray flare observed by Swift and INTEGRAL. These re- sults imply the existence of a whole new phenomenol- ogy in the young field of VHE astrophysics of binary systems to be explored by present and future IACT’s.

We thank the IAC for the excellent working con- ditions at the Observatorio del Roque de los Mucha- chos in La Palma.

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