VHE observations of microquasars with MAGIC

Gamma-ray emission from microquasars has been theoretically predicted for several years. How-ever, it was not until less than a decade that gamma rays were detected from this type of systems.

In 2009, two satellites AGILE and Fermi-LAT reported, for the first time, excess in energies above 100 MeV from the high-mass microquasar Cygnus X-3 (Tavani et al. 2009b, Fermi LAT Collaboration et al. 2009b). Another firmly established microquasar, composed as well with a high-mass companion, Cygnus X-1, presented a hint of steady emission reported by Malyshev et al. (2013a). Hints of transient radiation from this source was also reported byAGILE(Sabatini et al. 2010a, Rushton et al. 2012a and Sabatini et al. 2013a).

No VHE gamma-ray emission has been detected up to now from microquasars. Although pur-sued for many IACTs, this complicated task of disentangling the TeV regime in these systems is still not fulfilled, mostly due to the extremely good sensitivity required by the instruments.

MAGIC had performed deep observations on microquasars since it starts operation in 2004, looking to Cygnus X-1, Cygnus X-3, SS 433, GRS 1915+105 or Scorpius X-1. In 2006, MAGIC published the detection of LS I+61 303 (Albert et al. 2006), however although it was classified as microquasar at the beginning, it is currently accepted to be consistent with a pulsar wind scenario.

In this thesis, I present the latest results of two high-mass and one low-mass microquasars:

Cygnus X-1, Cygnus X-3 and V404 Cygni. The former was observed in a long-term campaign from 2007 to 2014 accumulating∼ 100 hours of good quality data. Such a deep campaign was motivated by a hint of signal detected by MAGIC in 2006 at the level of 4.1σin the direction of this source (Albert et al. 2007). Cygnus X-3 was, at the time of starting this thesis, the best candi-date for searching VHE gamma-ray emission given that was at the moment the only microquasar detected in the gamma-ray band. It was observed following a strict follow-up observations cam-paign: we performed daily analysis of public Fermi-LAT data and according to the results on the MeV-GeV regime, MAGIC observations were triggered. V404 Cygni was observed during an extreme outburst that the system underwent on June 2015 that lasted several days. MAGIC could observed the source at its maximum activity thanks to the automatic Gamma-ray Burst pro-cedure. With all these observations, MAGIC has been able to provide very useful information and shed light on microquasars in the VHE regime.

Cygnus X-1 4

4.1 History

Cygnus X-1 is one of the brightest and most studied X-ray sources in our Galaxy and a firmly established stellar-mass BH X-ray binary system. Discovered in the early stage of the X-ray as-tronomy (Bolton 1972), the system is located in the Cygnus region (l=71.32^◦and b=+3.09^◦) at a distance of 1.86⁺_−0.11^0.12kpc from the Earth (Reid et al. 2011). It is comprised of a (14.81±0.98)M

BH and a O9.7 Iab type supergiant companion star with a mass of (19.16±1.90) M(Orosz et al.

2011). Nevertheless, the most plausible mass range of the donor star has been recently increased to 25-35 Mby Zi´ołkowski (2014). This system is the only HMXB for which the compact object has been clearly identified as BH.

The assumption that Cygnus X-1 ranks among the microquasars was accepted after the detection, with the VLBA instrument, of a highly collimated one-sided relativistic radio-jet that extends

∼ 15 mas from the source (opening angle< 2^◦and velocity≥ 0.6c, Stirling et al. 2001). These jets are thought to create a 5 pc diameter ring-like structure observed in radio that extends up to 10¹⁹cm from the BH (Gallo et al. 2005). The total power carried by these relativistic outflows is 10³⁶⁻³⁷ erg s⁻¹ (Gallo et al. 2005; Russell & Fender 2010).

The binary system moves following a slightly elliptical orbit with eccentricity of 0.018 (Orosz et al. 2011), orbital period of 5.6 days (Brocksopp et al. 1999) and an inclination angle of the orbital plane to our line of sight of 27.1±0.8^◦ (Orosz et al. 2011). The superior conjunction phase of the compact object, when the companion star is interposed between the BH and the ob-server (see Figure 4.2), corresponds to phase 0, assuming the ephemeridesT₀=52872.788 HJD taken from Gies et al. (2008). As mentioned in Chapter 3.1, the X-ray binaries generally

suf-Figure 4.1:Alexander Jamieson’s Celestial Atlas representation of the Cygnus Constellation (1822). The location of Cygnus X-1 corresponds to theηsymbol in the neck of the swan figure.

fer flux periodicity at their own orbital period. Cygnus X-1 shows this kind of modulation both in X-ray and radio wavelengths (Wen et al. 1999, Brocksopp et al. 1999, Szostek & Zdziarski 2007), which may be caused by absorption or scattering by the wind of the donnor star over the radiation emitted from the compact object. Besides this modulation, several X-ray binary systems also present flux variations at periods much longer than their respective orbital period.

This effect is known as superorbital modulation and is thought to be caused by the precession of the accretion disk or jet (Poutanen et al. 2008). The X-ray superorbital period of Cygnus X-1 was under debate for several years. Initally, it was estimated to be∼ 290 d by Priedhorsky et al.

(1983). Later, a large number of authors claimed a superorbital periodicity of half this value,

∼ 150 d (e.g., Brocksopp et al. 1999 or more recent Lachowicz et al. 2006). The latest results confirm again a superorbital period of ∼ 300 d, as suggested by Rico (2008) and confirmed by Zdziarski et al. (2011).

Given that is composed of a BH, Cygnus X-1 displays the two canonical X-ray spectral states of a BH transient system (see Chapter 3.2.3), the HS and the SS (Esin et al. 1998), and the course that it follows through all the different states is well defined by the HID (Fender et al. 2004).

Therefore, its X-ray spectrum can be described as the sum of two components: a blackbody-like emission coming from the disk and dominant during the SS state, and a power-law tail, most likely originated due to IC of disk photons by hot thermal electrons in the corona, and dominant during the HS state. As we have seen in Chapter 3.2.3, there is a relation between

0.5 0.0 0.5 AU

0.5 0.0 0.5

AU

To observer Periastron

(

φ

.

Apastron

φ

=0 . 5 φ

=0 . 0

Figure 4.2: Schematic of the Cygnus X-1 orbit where the superior (φS C = 0) and inferior conjunction (φIC = 0.5) are marked. The almost circular orbit (eccentricity 0.018) that follows the BH is on scale with the companion star (in grey) of 16.4R(radius taken from Orosz et al. 2011, as well as the periastron and apastron phases). Neither the inclination of the orbit with the line of sight or the longitude of the ascending node were considered here. AU stands for astronomical units.

radio and X-ray: whilst in the HS, microquasars display steady relativistic synchrotron jets at GHz frequencies, except for some unusual flares in Cygnus X-1 (Fender et al. 2006), during SS that emission is strongly quenched. However, Cygnus X-1 is a persistent X-ray source never fully disk-dominated, i.e. even during its SS the system presents a strong power-law component and evidences of an unresolved compact jet during this state (Rushton et al. 2012b). Nevertheless, this jet is 3–5 times weaker than the one observed during the HS that reached 0.6c(Gallo et al.

2005). Thus, there is a constant level of radio emission around 10–15 mJy, that extends with no cutoff, up to the IR band, where the contribution from the O9.7 Iab type donor dominates, hindering the measurement of such cutoff.

Observations with COMPTEL when Cygnus X-1 remained in the SS suggested, for the first time, the existence of non-thermal component beyond MeV (McConnell et al. 2002). This result gave rise to an increase of the interest for this source in the gamma-ray regime. Nevertheless, observations with INTEGRAL excluded the existence of this MeV tail in the SS, but probed, in turn, the presence of such non-thermal hard emission during HS, when the jets were present (Rodriguez et al. 2015b). INTEGRAL-IBIS also reported a hard tail in HS which was shown to be polarized in the energy range of 0.4-2 MeV at a level of∼ 70% with a polarization angle of (40.0±14.3)^◦(Laurent et al. 2011, Jourdain et al. 2012). The origin site of this polarized MeV tail

was suggested to be the jets where ultra-relativistic electrons would produce it via synchrotron.

The corona was also considered as source of this radiation, where a population of secondary leptons would emit synchrotron soft gamma rays (Romero et al. 2014).

Steady high-energy gamma-ray emission during HS was hinted by Malyshev et al. (2013b) at the level of 4σin the energy range of 0.1–10 GeV by using 3.8 years ofFermi-LAT data. At the time of this thesis, the 7.5 years ofPass 8Fermi-LAT data was released (see Chapter 2.6.2.1).

Given the hint spotted in the past, with more available data and better sensitivity, the analysis of this Cygnus X-1 data could clearly provide new information on accreting X-ray binaries. The data used and the results of such analysis can be found in Section 4.2. Besides this persistent emission, the source underwent 3 preceding episodes of transient activity detected by AGILE.

The first two flaring events occurred during HS on the 16th of October 2009, with an integral flux of (2.32±0.66)×10⁻⁶ph cm⁻²s⁻¹between 0.1 and 3 GeV (Sabatini et al. 2010b), and on the 24th March 2010, with an integral flux of 2.50×10⁻⁶ ph cm⁻² s⁻¹ for energies above 100 MeV (Bulgarelli et al. 2010). The third one, on the 30th of June 2010 with a flux of (1.45± 0.78)× 10⁻⁶ ph cm⁻² s⁻¹ also for energies above 100 MeV (Sabatini et al. 2013b), took place during the intermediate state when the source was leaving the HS but just before an atypical radio flare (Rushton et al. 2012b). All these episodes lasted only 1–2 days. An independent analysis performed by Bodaghee et al. (2013) using 3.6 years ofFermi-LAT data confirmed, at the level of 3–4σ, transient emission from Cygnus X-1, although not coincident with the AGILE flares (between one and two days before the event reported by Sabatini et al. 2010b).

MAGIC observed the source in the past for a total of 40 hours, spanning in 26 nights be-tween June and November of 2006. At that period, the observations were carried out with the stand-alone MAGIC telescope, MAGIC I. Although no significant excess for steady gamma-ray emission using the all data sample was found, during the daily basis analysis a hint on the 24th of September 2006 (MJD=54002.96), corresponding to an orbital phase of 0.9 (i.e. close to the superior conjunction of the compact object) was spotted (Albert et al. 2007). This search yielded an evidence of gamma rays at 4.9σ(4.1σ after trials) in an effective time of 79 minutes. This excess took place at the maximum superorbital modulation of the source and simultaneously with the rising edge of a hard X-ray flare detected by INTEGRAL, Swift-BAT andRXTE-ASM (Malzac et al. 2008). The energy spectrum computed for this day is well defined by a simple power-law of dφ/dE=(2.3±0.6)×10⁻¹²(E/1TeV)^−3.2±0.6cm⁻²s⁻¹TeV⁻¹. VERITAS Collaboration also observed Cygnus X-1 on 2007 without any significant detection (Guenette et al. 2009) and therefore, former MAGIC results were the first experimental hint of VHE emission from a stellar BH binary. Consequently, both HE and VHE hints triggered a deep campaign on Cygnus X-1, whose results are shown in this chapter.