Gamma-ray flaring activity of NGC 1275 in 2016-2017 measured by MAGIC
Key Words.:galaxies: active – galaxies: jets – galaxies: individual: NGC 1275 – gamma rays: galaxies
We report on the detection of flaring activity from the Fanaroff-Riley I radio galaxy NGC 1275 in very-high-energy (VHE, E 100 GeV) gamma rays with the MAGIC telescopes.
The observations were performed between 2016 September and 2017 February, as part of a monitoring program. The brightest outburst with times the Crab Nebula flux above 100 GeV (C.U.) was observed during the night between 2016 December 31 and 2017 January 1. The flux is fifty times higher than the mean flux previously measured in two observational campaigns between 2009 October and 2010 February and between 2010 August and 2011 February. Significant variability of the day-by-day light curve was measured. The shortest flux-doubling time-scales was found to be of min. The spectra calculated for this period are harder and show a significant curvature with respect to the ones obtained in the previous campaigns. The combined spectrum of the MAGIC data during the strongest flare state and simultaneous data from the Fermi-LAT around 2017 January 1 follows a power-law with an exponential cutoff at the energy GeV. We further present simultaneous optical flux density measurements in the R-band obtained with the KVA telescope and investigate the correlation between the optical and gamma-ray emission.
Due to possible internal pair-production, the fast flux variability constrains the Doppler factor to values which are inconsistent with a large viewing angle as observed in the radio band. We investigate different scenarios for the explanation of fast gamma-ray variability, namely emission from: magnetospheric gaps, relativistic blobs propagating in the jet (mini-jets) or external cloud (or star) entering the jet. We find that the only plausible model to account for the luminosities here observed would be the production of gamma rays in a magnetospheric gap around the central black hole only in the eventuality of an enhancement of the magnetic field threading the hole from its equipartition value with the gas pressure in the accretion flow. The observed gamma-ray flare therefore challenges all the discussed models for fast variability of VHE gamma-ray emission in active galactic nuclei.
The majority of gamma-ray detected Active Galactic Nuclei (AGN), namely blazars (Ajello et al., 2017), are characterized by a small angle between the jet axis and the line of sight of the observer (viewing angle ). Doppler boosting of their non-thermal emission, conventionally explained as due to an emitting region moving relativistically along the jet axis, accommodates the enormous luminosities observed ( erg s), along with features like fast flux variability. In this broadly accepted scenario (Mastichiadis & Kirk, 1997) the gamma-ray emission is produced via Comptonization of internal or external radiation fields.
Variabilities shorter than the light crossing time at the black hole event horizon could challenge the aforementioned model: even though they can still be described by adapting extreme parameters, such as large Doppler factors (Begelman et al., 2008), theoretical alternatives were formulated over the years (Giannios et al., 2010; Levinson & Rieger, 2011; Tavecchio & Ghisellini, 2014; Barkov et al., 2012b; Hirotani & Pu, 2016).
Observing huge gamma-ray luminosities and fast variability in non-blazar AGN, like radio galaxies (Acciari et al., 2009; Aleksić et al., 2014b), in which a larger viewing angle () can eventually cause a de-boosting of the radiation, it is therefore an intriguing phenomenon. It provides room for discussion of the aforementioned alternative models, especially in case of the most extreme phenomena observed in (Aleksić et al., 2014b).
Gamma-ray observations and exploration of the variability of non-blazar AGN is crucial to provide insights into the location and physical processes behind extragalactic non-thermal emission.
NGC 1275, also known as 3C 84, is the central galaxy of the Perseus cluster with a redshift of (Falco et al., 1999). While the optical spectrum shows strong nuclear emission lines typical of a Seyfert galaxy (Humason, 1932; Khachikian & Weedman, 1974), the morphology in the radio band reveals a Fanaroff-Riley I type (Vermeulen et al., 1994; Buttiglione et al., 2010). The viewing angle of NCG 1275 was inferred from radio interferometers to be by Walker et al. (1994) and Fujita & Nagai (2017) on (sub)-parsec scales. The sub-parsec radio jet shows a new component (C3) which appeared about ten years ago and keeps growing in brightness as it moves downstream the jet (Nagai et al., 2010, 2012). Flux variability has been detected in various frequency bands: in radio (Dutson et al., 2014), optical (Aleksić et al., 2014a), and X-rays (Fukazawa et al., 2016), although the emission is often affected by other contributions such as the host galaxy (Aleksić et al., 2014a) or filaments (Fabian et al., 2011) so that the AGN emission is difficult to extract.
After the early detection with the COS B satellite (Strong et al., 1982), NGC 1275 was observed in high-energy (HE, MeV) gamma-ray regime with Fermi-LAT (Abdo et al., 2009) and later measured with MAGIC and VERITAS in VHE (Aleksić et al., 2012a, 2014a; Benbow, 2015). While the measurements with Fermi-LAT yielded flux variability on time scales of d (Brown & Adams, 2011), MAGIC measurements showed marginal flux changes on monthly scales. A detailed analysis of Fermi-LAT data during 2008â2017 can be found in Baghmanyan et al. (2017). Recently flaring activity in VHE band was reported MAGIC and VERITAS (Mirzoyan, 2016; Mukherjee & VERITAS Collaboration, 2016; Mirzoyan, 2017; Mukherjee & VERITAS Collaboration, 2017)
This paper is structured as follows: in section 2 we report on the results of the MAGIC observations during the period between 2016 September and 2017 February and on the analysis of simultaneous Fermi-LAT data. Gamma-ray light curves are produced along with VHE spectra for different flux states. A joint MAGIC and Fermi-LAT spectrum is shown for the night with the strongest flux. Measurements with the KVA telescope in the optical band allow us to present also an optical - gamma-ray emission correlation study. In section 3 the physics discussion, supported by the considerations in Levinson & Rieger (2011) and Hirotani & Pu (2016); Hirotani et al. (2016) along with some of the analytical parametrization provided in Aharonian et al. (2017), tries to identify the emission model more suitable to explain the observed gamma-ray fluxes.
2 Observational results
The observations here reported were conducted with the MAGIC (Major Atmospheric Gamma Imaging Cherenkov) telescopes (Aleksić et al., 2012b, 2016), two 17 m-diameter Imaging Atmospheric Cherenkov Telescopes located at the Canary island of La Palma, Spain, designed to perform gamma-ray astronomy in the energy range from 50 GeV to 50 TeV (Aleksić et al., 2012b, 2016).
NGC 1275 was observed between 2016 September and 2017 February (MJD 57637.1–57811.9) for 63 hours mostly under dark conditions as part of a monitoring program. Seven hours of data affected by non-optimal weather conditions were discarded. The analysis of the data was performed using the standard analysis chain described in Aleksić et al. (2016). The data cover the zenith distance range of 111low zenith angles and low night sky background levels reflect in lower energy threshold and sensitivity (Aleksić et al., 2016).
Following Aleksić et al. (2016), we consider for the spectra the following systematic errors: 11% for the flux normalization, 15% for the energy scale and 0.15 for the photon index. The absorption due to the extragalactic background light (EBL) is only marginal for the redshift and the calculated energy range of the spectrum of NGC 1275. The cutoff in the VHE spectrum due to the EBL is expected to be at energies TeV as discussed in Ahnen et al. (2016).
2.1.1 VHE Flux Variability
The mean flux between 2016 September to 2017 February equals cm s above 100 GeV whereas previous measurements yielded cm s and cm s during 2009-2010 and 2010-2011, published in Aleksić et al. (2014a). Thus, the mean flux reported here is seven to nine times higher.
The VHE daily light curve is shown in the upper panel of Fig. 1 and is calculated assuming a power-law index of . Fitting the daily light curve with a constant function yields a /d.o.f. of . Several nights around 2016 November until 2017 January were identified with a clear high flux. We investigated the intra-night light curves for these nights but did not find strong hints for variability during the observation of typically 1-3 h. For the night with the highest flux, 2017 January 01, 8 data runs for a total observation time of 2.4 h were collected. A fit of the run-wise LC with a constant flux returned a probability of 0.09. No individual data point deviates by more than 2 sigma from a constant fit value.
In order to estimate the time-scale at which the flux has doubled, we fit the daily light curve during the brightest outburst around MJD 57751.00–57754.02, as shown in Fig. 2, with the function:
where is the baseline flux, the normalization flux at the time , and is the flux-doubling time-scale. Fixing to cm s and to MJD yields a flux-doubling time-scale of min (/d.o.f. of , probability of 0.49) for the rising part of the flare in 2016 December and 2017 January. Note that we only fit the measured data points, not taking into account the upper limits, the result is however consistent with them.
2.1.2 VHE Spectral analysis
power-law with exponential cutoff: with the cutoff energy given in units of TeV
We divided the dataset according to the following flux states:
flux C.U.: night of 2017-01-01 (MJD 57753.90–57754.08);
0.4 C.U. ¡ flux ¡ 1 C.U.: nights of 2017-01-02 and 2017-01-03 (MJD 57754.89-57756.03);
flux C.U.: remaining nights of the monitored period, noting that this low state sample also includes some weaker flares.
For each of this dataset we calculate the spectrum.
The spectral energy distributions (SEDs) are shown in Fig. 3 together with the long-term averaged result from observations in 2009 to 2014. Continuous line represent the result of a fit using the whole energy band while individual spectral points are calculated using the unfolding method in Albert et al. (2007). Due to a clear curvature, we fitted the SED with a power-law with exponential cutoff and with a log-parabola function. Parameters of the spectral fit to the VHE SED obtained with MAGIC data are given in Table 1.
The power-law with exponential cutoff provides the highest probability for the fit, although the log-parabola fit can not be excluded. The cutoff energies at around 250–560 GeV can not be a result of the EBL since its effect would start to dominate at energies above 10 TeV.
Above 1 TeV, NGC 1275 could still be detected with a significance of 8 using all the data and the equation derived by Li & Ma (1983). Long-term observations presented in Ahnen et al. (2016) for a total of 253 h between 2009–2014 did not yield in a detection above 1 TeV.
In order to further investigate the presence of a cutoff in the brightest state, to provide a more constrained spectral information, and to search for a correlation between the optical and gamma-ray emission, we analyse data from the Fermi Large Area Telescope (LAT) (Atwood et al., 2009; Ackermann et al., 2012) for 2017 January 1 as well as over a longer time period covering the MAGIC observation window.
The LAT is an imaging high-energy gamma-ray telescope on board the Fermi satellite, covering the energy range from about 20 MeV to more than 300 GeV. Its field of view covers about 20% of the sky at any time and, when working in survey mode, covers the whole sky every three hours.
The data are reduced and analysed using fermipy222http://fermipy.readthedocs.io/en/latest/ (Wood et al., 2017) with the latest release of the Pass 8 Fermi Science Tools 333https://fermi.gsfc.nasa.gov/ssc/data/analysis/documentation/. We use the instrument response functions (IRFs) P8R2_SOURCE_V6, the isotropic diffuse background template iso_P8R2_SOURCE_V6_v06 and the galactic diffuse background emission model gll_iem_v06 (Acero et al., 2016).
We select all the photons in a region of interest (ROI) of radius around the coordinates of NGC 1275, and perform a binned likelihood analysis using three bins per energy decade in an energy range from 100 MeV to 10 GeV for the spectrum evaluation and the energy range 0.1–300 GeV for the light curve. All the 3FGL (third Fermi Gamma-ray LAT catalog, Acero et al. (2015)) sources within from the source position are included in our model, along with the galactic and isotropic diffuse emission.
2.2.1 Fermi-LAT light curve analysis
The light curve analysis is performed using Fermi-LAT data encompassing MAGIC and KVA observation windows. The data from MJD 57619.5 to MJD 57820.5 are divided in 24 hours bins (bin center at midnight), and in each time bin a likelihood analysis is performed. The normalizations of all the sources within a radius of 5 from the source position are let free to vary, while the spectral indexes are fixed to the catalog value. The normalizations of the diffuse components are kept fixed. NGC 1275 spectrum is modeled with a simple power-law. The resulting integrated flux in an energy range of 0.1 GeV to 300.0 GeV is shown in the middle panel of Fig. 1. All the time bins in which the Likelihood fit returned a TS25 are represented as upper limits. The mean flux was estimated to be cm s. We fit the light curve with a constant function in the time range from MJD 57620.0 to 57820.0. This yield a /d.o.f. of without considering the upper limits.
2.2.2 Fermi-LAT spectral analysis
Given the short observation time of MAGIC (few hours per night) simultaneity of data for a spectral analysis is limited by the minimal exposure time necessary to get a reliable spectral analysis of the LAT data; considering the short variability observed of 10 h, for the spectral analysis of 2017 January 1 we selected a time span of 12 hours, centered around midnight: MJD 57753.75–57754.25. All the normalizations of sources within a radius of 5 from NGC 1275 position are left free in the fit while the spectral indexes are fixed to the catalog value. The normalizations of the diffuse components is also fixed given the difficulty to fit their contribution in such a small integration time. NGC 1275 is modeled with a PowerLaw2:
, allowing the errors on the integrated flux () to be evaluated directly by likelihood. Converting to a simple PowerLaw form: , and evaluating as the point at which and show the minimum correlation (decorrelation energy), instead of arbitrarily fixing it, we obtain:
MeV cm s, , GeV.
In the 12 h dataset the source shows a test statistic (TS) of 55.83 (determined from the likelihood ratio of the source / no source hypothesis), showing a significance of 7.5.
2.3 Combined Fermi-LAT and MAGIC spectral analysis for 2017 January 1
A method for a joint spectral analysis of Fermi-LAT and MAGIC data is described in Moralejo et al. (2017). The spectral parameters are obtained maximizing a Poissonian likelihood describing the observed number of VHE events in the sky region around the source, and in three close-by background control regions, in each energy bin. The rate of excess events per (estimated) energy bin is then folded with the IRFs obtained from Monte Carlo simulations to obtain the expected flux. The Fermi-LAT information is used in the following way to constrain the fit: assuming that the Fermi data are fitted with a simple power-law, two additional terms are introduced in the Likelihood to anchor the high energy (HE) spectrum to the VHE one: , . Where and are the outcome of the Fermi power-law based spectral analysis and and are normalization and spectral index at the decorrelation energy. The allowed functions for the fit are always nested models of a simple power-law. The results of the joint fit follow in Table 2 and are shown in Fig. 4.
The joint analysis fit with Fermi-LAT data confirms that for the brightest flare a power-law with exponential cutoff is preferred for fitting the spectrum. The position of the cutoff agrees with what estimated only using MAGIC data.
In the optical, NGC 1275 is being monitored within the Tuorla blazar monitoring program 444http://users.utu.fi/kani/1m since 2009 October. In this work we discuss the observations in the R-band (640 nm) performed with the Kungliga Vetenskaps Akademien (KVA) 35 cm telescope in the time period covering the MAGIC observations.
For the data reduction, the standard analysis pipeline is applied. Magnitudes are measured with differential photometry with an aperture of 5.0 and comparison stars from Fiorucci et al. (1998).
The resulting flux densities corrected for the host galaxy and the galactic extinction (Schlafly & Finkbeiner, 2011) are shown in the bottom panel of Fig. 1. To study the flux variability, we fit the light curve from KVA in the R-band between 2016 September and 2017 February with a constant function and obtain a /d.o.f. of , thus implying significant flux variability. The mean flux during the period presented in this work is calculated to be mJy. This is 1.6 times higher than the mean flux found during observations between 2009 October and 2011 February (Aleksić et al., 2014a).
2.5 Optical-Gamma-ray correlation analysis
We further investigate the correlation between the optical and the gamma-ray emission during the observation period. In Fig. 5 we plot the daily gamma-ray fluxes obtained by MAGIC and LAT measurements versus the flux density measurements in the R-band from KVA for observations performed within MJD 0.5. The linear correlation coefficient (Bravais-Pearson coefficient) is calculated to be 0.50 and 0.52 for MAGIC-KVA and LAT-KVA, respectively (upper limits are not used for the coefficient calculation). Thus, the optical emission is much less correlated with the gamma-ray flux with respect to the value of 0.79 (LAT-KVA) reported in Aleksić et al. (2014a). Without the flaring nights (2017 January 1 to 2017 January 3), the coefficient increases to 0.82 and 0.72 for MAGIC-KVA and LAT-KVA. Similar to the study performed in Aleksić et al. (2014a), we also fit the correlation plots with a linear () and a quadratic function (). In a standard SSC flaring scenario, quadratic and linear correlation are expected between the optical and gamma rays, depending on the physical parameter that is causing the flare, see discussion in Aleksić et al. (2014a). For LAT-KVA we find a for the linear correlation and for the quadratic. For MAGIC-KVA we find a for the linear correlation and for the quadratic. Thus no correlation between gamma-ray and optical fluxes is found.
3.1 Size of the emitting region
The flux variability gives the possibility to estimate the size of the gamma-ray emitting region. Assuming a spherical region, we can estimate its radius using causality arguments.
The radius of the emission source at redshift is constrained by the variability time scale and can be estimated with , where is the Doppler factor to describe the relativistic beaming. The shortest flux-doubling time-scale found in this work is 611 min. This correspond to a size of a spherical emission region of cm. Wilman et al. (2005) and Scharwächter et al. (2013) measured a mass of the central black hole of NGC 1275 of and , respectively. These masses correspond to a gravitational radius of cm and cm, respectively, and a light crossing time of min and min, respectively. Thus, the observed variability time scale is much larger than the event horizon light crossing-time but indicate an emission region one or two orders of magnitude smaller than the one proposed in Aleksić et al. (2014a) or Tavecchio & Ghisellini (2014).
It is therefore necessary to investigate if this small emission region is transparent for TeV photons to escape. Taking into account that 1 TeV photons were observed (see Fig. 3), we calculate the Doppler factor which is necessary to avoid internal absorption via -pair production. Following Eq. 9 in Abdo et al. (2011) for the optical depth for pair-production, we can establish the following condition in order to observe TeV photons
where is the Thomson cross section, is the luminosity distance, the assumed peak of the target SED, the energy of the hitting photon (1 TeV in this case), the redshift of the source, is the size of the emitting region, the mass of the electron and the speed of light. A simultaneous flux measurement in the eV energy band (where the absorption with the TeV photons is supposed to happen) is available within the KVA dataset. For the night 2017 January 01 (MJD = 57753.94) a flux of erg cm s is observed, yielding a Doppler factor of . This measurement, although corrected for the host-galaxy contribution, could be related to a region different than the one causing this particular VHE flare, as the lack of gamma-optical correlation is suggesting. We try then to provide another lower limit for the Doppler factor assuming a leptonic radiative model. From theoretical considerations we know that even in the most extreme flares the Compton dominance (i.e. the ratio between inverse Compton and the synchrotron peak) can not be greater than two orders of magnitude (Zacharias & Schlickeiser, 2012; Ghisellini et al., 2010) and we use this constraint to derive as the peak of the synchrotron component. The SED Compton peak measured in section 2.3 is erg cm s (note that the log-parabola is used for this estimate, lacking the power-law with exponential cutoff SED of a local maximum, spectral index ¡ -2). We get , with the conservative assumption . Using the afore mentioned Doppler factor values, we find the viewing angle and , for any Lorentz factor . These are smaller than the results from radio observations, reporting . For a large , a large viewing angle may even lead to a de-boosting. Hovatta et al. (2009) found a very small Doppler factor of 0.3 for NGC 1275 based on the observation of the variability brightness temperature in the radio band while =2 and 4 was assumed in Aleksić et al. (2014c) to model the source with a SSC scenario, these values are difficult to accommodate within the constraints found above.
3.2 Physical models
Since the lack of gamma-optical radiation correlation and the transparency constraint given by the small emission region hamper the use of a classical SSC leptonic scenario, we consider the feasibility of the following alternatives.
3.2.1 Spine-layer model
First we examine the spine-layer model in Tavecchio & Ghisellini (2014). This model is based on a structured jet hypothesis and has been suggested to fit the broad-band emission of NGC 1275. The fast spine with Lorentz factor in the inner part of the cylindrical jet is surrounded by a slower layer with . The low energy emission (radio to X-ray) in the broad-band SED should be dominated by the spine, whereas the high-energy emission should be predominantly produced by the layer.
An evidence of the structural configuration is given by the detection of a limb-brightened structure of the inner parsec-scale jet in high-resolution radio data reported by Nagai et al. (2014). Tavecchio & Ghisellini (2014) claimed that assuming is incompatible with a scenario involving internal -pair production. Indeed, in this work we clearly detected photons TeV for which the optical depth becomes in this model since the requirement of a slow for a large leads to insufficient Doppler boosting. Thus, the spine-layer model from Tavecchio & Ghisellini (2014) it is not suitable to explain our data.
In what follows we study then the scenarios suggesting fast variability outlined in Aharonian et al. (2017), namely the magnetospheric model, the mini-jets model and the cloud-jet interaction.
3.2.2 Magnetospheric model
As first described in Blandford & Znajek (1977), a spinning black hole embedded in an external magnetic field can build up a force-free magnetosphere ( along the magnetic field lines), supported by an electron-positron plasma generated by pair cascading. The injection of charges in the magnetosphere is commonly explained with the pair cascading of self-annihilating MeV photons produced from a radiatively inefficient accretion flow (RIAF) via free-free emission. During phases of low accretion the minimal charge density (where is the angular frequency of the dragged magnetic field lines), required to keep the magnetosphere force-free, could not be sustained. Charge-starved region (gaps) with a significant component of parallel to would then arise as electrostatic accelerators for the leptons. Curvature and Inverse Compton (IC) photons generated by the leptons within the gap and synchrotron and IC photons from the leptons cascaded outside will made the electromagnetic radiation of the source. This model has already been applied to radio galaxies, in particular in Neronov & Aharonian (2007) and Levinson & Rieger (2011) to M87 and Sgr A* and in Hirotani & Pu (2016) to IC 310.
Since the gap can release only a part of the electromagnetic power extracted from the BH, the Blandford-Znajek (BZ) mechanism poses an upper limit to the gap luminosity
where , denotes the dimensionless BH spin parameter ( its angular momentum), the intensity of the magnetic field threading the BH. Assuming that in the RIAF the equipartition magnetic pressure is half the gas pressure (Mahadevan, 1997; Levinson & Rieger, 2011) allows to set , where is the accretion rate measured in units of the Eddington rate: and is the efficiency of conversion from mass to radiant energy, typically assumed . Plugging this magnetic field value in Eq. 3 produces
To create a gap, the electron-positron density of the pairs cascaded by the MeV RIAF photons has to become less than . As shown in (Levinson & Rieger, 2011) and remarked in Hirotani et al. (2016) (Eq. 8) the ratio of these two charge densities is only dependent on and . The condition for a gap to be open yields , that substituted in Eq. 4 returns, for our case
using and assuming .
Therefore the gamma-ray luminosity measured in the highest flux night overcomes by 3 orders of magnitudes the upper limit imposed by the BZ total power.
To obtain a more precise upper limit on the maximum gamma-ray luminosity we can examine Fig. 25 of Hirotani et al. (2016) that depicts the gamma luminosity for curvature and IC processes in a BH with mass (same order of magnitude of NGC 1275 BH mass estimated both in Wilman et al. (2005) and scharwachter2013). In the ballpark of this BH mass we see that the minimum accretion rate needed to sustain pair production in the gap is for which a maximum gamma-luminosity (IC dominated) of can be attained.
The constraint on the total luminosity (both the BZ upper limit and the power actually radiated via curvature and IC processes) is strongly dependent on the assumption that the magnetic field of the RIAF is at the equipartition (simplification from Eq. 3 to Eq. 4). To explain the huge gamma-ray luminosity in the IC310 flare detected in Aleksić et al. (2014b) () and its 3 orders of magnitude overcoming the allowed BZ power () Hirotani & Pu (2016) contemplated, for an extremely rotating BH (), an enhancement of magnetic field due to compilation of plasma near the BH horizon. To accommodate the measured erg s the BZ limit has to be increased, eventually overcoming the jet power , estimated to be for NGC 1275 (see next paragraph for more details). Such an increase of magnetic field should be sustainable only on timescales smaller than the jet propagation timescale, i.e. the gap could be opened with a small duty cycle. The gamma-ray luminosity we report in this paper can be framed in a magnetospheric scenario sustained by a RIAF only in the hypothesis of an enhancement of the disk magnetic field in the proximity of the BH horizon from its equipartition value. This would imply allowing for the BZ luminosity a value larger than the jet power, sustainable only within a small duty cycle, as in the event of a flare. As remarked in Hirotani & Pu (2016) a complete numerical simulation is needed to investigate the possibility of such an enhancement of B near the black hole horizon.
3.2.3 Mini-jets model
In the mini-jets model (Giannios et al., 2009, 2010) it is assumed that the main jet with Lorentz factor contains several mini-jets with which are produced, e.g., by dissipation of magnetic energy in strongly magnetized plasma regions. Their relative motion with respect to the main jet results in a higher emitted Lorentz factor which can solve the opacity problem occurring when fast VHE variability is observed. In case of NGC 1275 we may see the emission of the mini-jets pointing outside the jet cone. The lower limit of the jet luminosity required for the mini-jets scenario can be calculated with Eq. 37 in Aharonian et al. (2017):
where is the luminosity in gamma rays, accounts for the conversion efficiency from the jet material to the outflow, and from the outflow to radiation; , and are the jet bulk Lorentz factor and the normalized viewing angle, respectively. We assume a filling factor of the mini-jets inside the jet of which corresponds to the total number of mini-jets during a flaring event, the duty cycle of flares, their duration, and the variability time scale. With the parameters given above, we derive a minimum jet luminosity of erg s. In Abdo et al. (2009), a jet luminosity of erg s was found for NGC 1275 based on the modeling of the broad-band SED with a single-zone synchrotron self-Compton model assuming either one proton per radiating electron or a ten times higher energy density of the protons than the electrons. Using a different method, Dunn & Fabian (2004) found a total power of erg s required to inflate the radio lobes of NGC 1275 against the pressure of the hot cluster gas. The two numbers are likely compatible given the entrainment (work against the interstellar and intergalactic medium). Thus, the mini-jets model can account for the observed gamma-ray emission assuming the range of the jet luminosity inferred by Abdo et al. (2009) but has difficulties to explain the measurements in case of smaller jet power of NGC 1275. The situation further worsen for a higher filling factor.
3.2.4 Cloud in jet model
In the cloud-jet interaction model (Barkov et al., 2012b, a) a VHE flare is explained by an obstacle moving through the jet or vice versa. For example, such an obstactle can be a star with a high mass-loss rate causing the formation of a cloud out of the lost material due to pressure in the jet. Interactions of colliding protons at a bow shock located at the jet-cloud interface produce a single peak in a VHE light curve. For this scenario, the lower limit of the jet luminosity can be estimated with Eq. 43 in Aharonian et al. (2017):
This gives a minimal jet power of erg s required to explain the NGC 1275 observations with a cloud-jet interaction model. This result clearly exceeds the inferred values for the jet power.
In the estimations presented a normalized viewing angle of was assumed for NGC 1275. The required jet powers in the mini-jets as well as in the cloud-jet interaction model would increase if a larger value for is assumed, hence, making those scenarios more unlikely.
In this work we present VHE gamma-ray data of NGC 1275 measured in 2016 September to 2017 February with the MAGIC telescopes. We found several nights in which NGC 1275 was in a high state with respect to the flux previously measured in the 2009–2011 campaigns. For the brightest flare around 2017 January 1 a value fifty times higher was measured, characterized by a flux-doubling times-scale of min which equals to 22 times the light crossing time at the black hole event horizon. The spectra from different flux states are generally harder than the ones from previous campaigns and can not be described with a simple power-law function. The combined spectral analysis of Fermi-LAT and MAGIC data from 2017 January 1 yields good fit results when assuming a power-law function with exponential cutoff revealing a cutoff energy of GeV.
Furthermore, investigating the correlation of the optical and gamma-ray emission by comparing MAGIC, Fermi-LAT and KVA (R-band) light curves, we find no correlated variability of the optical flux density around the time of the VHE flare.
Considering the observations in the lights of different emission models, the fast flux variability is constraining the size of the gamma-ray emission region to a value one or two orders of magnitude smaller than the one used within the SSC scenario proposed in (Aleksić et al., 2014a) or the spine-layer in Tavecchio & Ghisellini (2014). A higher Doppler factor than the one assumed in (Aleksić et al., 2014a) would be needed to avoid absorption in the SSC scenario, implying a viewing angle in tension with the large value observed in radio. Absorption of the highest energy photons via pair production in such a small emitting region would be also dominant in a spine-sheath scenario, thus excluding this theoretical model standing the significant emission above 1 TeV measured. Among the alternative scenarios for fast variability presented in Aharonian et al. (2017), the mini-jets model and the cloud-jet interaction, probably fail because of a large jet power necessary to reach the observed gamma-ray luminosity. A hard limit on the maximum luminosity expected for a magnetospheric model can be estimated from the maximum extractable BZ power (under the assumption that the magnetic field in the disc is at the equipartition value) and from the condition on the accretion rate in Eddington units needed to open a gap (Eq. 8 and 4 in Levinson & Rieger (2011) and Hirotani et al. (2016) respectively). The only possibility to fit the enormous (10 erg s) luminosity measured in this paper for the higher flaring state within the strong upper limit posed by the BZ power (10 erg s), as suggested in Hirotani & Pu (2016), would be an enhancement of the magnetic field threading the BH horizon from its equipartition value, increasing the extractable BZ power even beyond the jet power. This increase has to happen on a time scale smaller with respect to the jet propagation timescale (e.g. during a flaring event) and yet has to be proved by numerical studies. The luminosities and the corresponding fast variability hereby reported pose a challenge to the actual models for fast variability of VHE gamma-ray emission in AGN.
Acknowledgements.We would like to thank the Instituto de Astrofísica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The financial support of the German BMBF and MPG, the Italian INFN and INAF, the Swiss National Fund SNF, the ERDF under the Spanish MINECO (FPA2015-69818-P, FPA2012-36668, FPA2015-68378-P, FPA2015-69210-C6-2-R, FPA2015-69210-C6-4-R, FPA2015-69210-C6-6-R, AYA2015-71042-P, AYA2016-76012-C3-1-P, ESP2015-71662-C2-2-P, CSD2009-00064), and the Japanese JSPS and MEXT is gratefully acknowledged. This work was also supported by the Spanish Centro de Excelencia “Severo Ochoa” SEV-2012-0234 and SEV-2015-0548, and Unidad de Excelencia “María de Maeztu” MDM-2014-0369, by the Croatian Science Foundation (HrZZ) Project IP-2016-06-9782 and the University of Rijeka Project 126.96.36.199.02, by the DFG Collaborative Research Centers SFB823/C4 and SFB876/C3, the Polish National Research Centre grant UMO-2016/22/M/ST9/00382 and by the Brazilian MCTIC, CNPq and FAPERJ.
We would like to thank F. Rieger for useful discussions.
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