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Register a new userMonitoring the radio galaxy M87 with HAWC
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Tom\\'as Capistr\\'an
Publication date
2021
fields
Physics
and research's language is
English
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Studies of radio galaxies at TeV energies are fascinating because their jets are misaligned concerning our sightline. Thus, it provides us with a unique opportunity to study the structure of their jets, the radiative processes, and the acceleration mechanisms involved in them. In addition, some radio galaxies have presented variability in their emission, like the giant radio galaxy M87, which has reported several activity periods. Due to its duty cycle >95% and instantaneous field of view of 2 sr, HAWC provides daily monitoring of variable sources visible from the Northern Hemisphere. In this work, we show the results of monitoring M87 between January 2015 and December 2018. HAWCs observations are consistent with the low activity state reported by other instruments (like H.E.S.S and MAGIC). However, after September 2017 (~MJD 58000), the HAWC measurements of M87 show hints of higher activity.
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M87 is one of the closest (z=0.00436) extragalactic sources emitting at very-high-energies (VHE, E > 100 GeV). The aim of this work is to locate the region of the VHE gamma-ray emission and to describe the observed broadband spectral energy distribution (SED) during the low VHE gamma-ray state. The data from M87 collected between 2012 and 2015 as part of a MAGIC monitoring programme are analysed and combined with multi-wavelength data from Fermi-LAT, Chandra, HST, EVN, VLBA and the Liverpool Telescope. The averaged VHE gamma-ray spectrum can be fitted from 100GeV to 10TeV with a simple power law with a photon index of (-2.41 $pm$ 0.07), while the integral flux above 300GeV is $(1.44 pm 0.13) times 10^{-12} cm^{-2} s^{-1}$. During the campaign between 2012 and 2015, M87 is generally found in a low emission state at all observed wavelengths. The VHE gamma-ray flux from the present 2012-2015 M87 campaign is consistent with a constant flux with some hint of variability ($sim3sigma$) on a daily timescale in 2013. The low-state gamma-ray emission likely originates from the same region as the flare-state emission. Given the broadband SED, both a leptonic synchrotron self Compton and a hybrid photo-hadronic model reproduce the available data well, even if the latter is preferred. We note, however, that the energy stored in the magnetic field in the leptonic scenario is very low suggesting a matter dominated emission region.
The recently completed High Altitude Water Cherenkov (HAWC) gamma-ray observatory has been taking data with a partial array for more than one year and is now operating with >95% duty cycle in its full configuration. With an instantaneous field of view of 2 sr, two-thirds of the sky is surveyed every day at gamma-ray energies between approximately 100 GeV and 100 TeV. Any source location in the field of view can be monitored each day, with an exposure of up to $sim$ 6 hours. These unprecedented observational capabilities allow us to continuously scan the highly variable extra-galactic gamma-ray sky. By monitoring the flaring behavior of Active Galactic Nuclei we aim to significantly increase the observational data base for characterizing particle acceleration mechanisms in these sources and for studying cosmological properties like the extra-galactic background light. In this work we present first studies of data taken between June 2013 and July 2014 with a partial array configuration. Flux light curves, binned in week-long intervals, for the TeV-emitting blazars Markarian 421 and 501 are discussed with respect to indications of flaring states and we highlight coincident multi-wavelength observations. Results for both sources show indications of gamma-ray flare observations and demonstrate that a water Cherenkov detector can monitor TeV-scale variability of extra-galactic sources on weekly time scales. The analysis methods presented here can provide daily flux measurements with a minimum time interval of one transit and will be applied to new data from the completed HAWC array for monitoring of blazars and other transients.
We report on our search for very-long-term variability (weeks to years) in X-ray binaries (XRBs) in the giant elliptical galaxy M87. We have used archival Chandra imaging observations to characterise the long-term variability of 8 of the brightest members of the XRB population in M87. The peak brightness of some of the sources exceeded the ultra luminous X-ray source (ULX) threshold luminosity of ~ 10^{39} erg/s, and one source could exhibit dips or eclipses. We show that for one source, if it has similar modulation amplitude as in SS433, then period recoverability analysis on the current data would detect periodic modulations, but only for a narrow range of periods less than 120 days. We conclude that a dedicated monitoring campaign, with appropriately defined sampling, is essential if we are to investigate properly the nature of the long-term modulations such as those seen in Galactic sources.
The Event Horizon Telescope (EHT) has recently delivered the first resolved images of M87*, the supermassive black hole in the center of the M87 galaxy. These images were produced using 230 GHz observations performed in 2017 April. Additional observations are required to investigate the persistence of the primary image feature - a ring with azimuthal brightness asymmetry - and to quantify the image variability on event horizon scales. To address this need, we analyze M87* data collected with prototype EHT arrays in 2009, 2011, 2012, and 2013. While these observations do not contain enough information to produce images, they are sufficient to constrain simple geometric models. We develop a modeling approach based on the framework utilized for the 2017 EHT data analysis and validate our procedures using synthetic data. Applying the same approach to the observational data sets, we find the M87* morphology in 2009-2017 to be consistent with a persistent asymmetric ring of ~40 uas diameter. The position angle of the peak intensity varies in time. In particular, we find a significant difference between the position angle measured in 2013 and 2017. These variations are in broad agreement with predictions of a subset of general relativistic magnetohydrodynamic simulations. We show that quantifying the variability across multiple observational epochs has the potential to constrain the physical properties of the source, such as the accretion state or the black hole spin.
The relativistic jet in M87 offers a unique opportunity for understanding the detailed jet structure and emission processes due to its proximity. In particular, the peculiar jet region HST-1 at ~1 arcsecond (or 80 pc, projected) from the nucleus has attracted a great deal of interest in the last decade because of its superluminal motion and broadband radio-to-X-ray outbursts, which may be further connected to the gamma-ray productions up to TeV energies. Over the last five years, we have been doing an intensive monitoring of HST-1 with EVN at 5GHz in order to examine the detailed structural evolution and its possible connection to high-energy activities. While this program already yielded interesting results in terms of the detailed mas-scale structure, proper motion measurements and structural variations, the recent HST-1 brightness is continuously decreasing at this frequency. To counter this, we have shifted our monitoring frequency to 1.7GHz from October 2013. This strategy successfully recovered the fainter emission that was missed in the last 5GHz session. Moreover, we again discovered the sudden emergence of a new component at the upstream edge of HST-1, demonstrating that the use of EVN 1.7GHz is indeed powerful to probe the current weak nature of HST-1. Here we report early results from the 1.7GHz monitoring as well as further progress on the long-term kinematic study.
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