Radio Galaxy M87 Research Articles

A new bound on axion-like particles

Axion-like particles (ALPs) and photons can quantum mechanically interconvert when propagating through magnetic fields, and ALP-photon conversion may induce oscillatory features in the spectra of astrophysical sources. We use deep (370 ks), short frame time Chandra observations of the bright nucleus at the centre of the radio galaxy M87 in the Virgo cluster to search for signatures of light ALPs. The absence of substantial irregularities in the X-ray power-law spectrum leads to a new upper limit on the photon-ALP coupling, gaγ: using a very conservative model of the cluster magnetic field consistent with Faraday rotation measurements from M87 and M84, we find g_{a\\gamma} < 2.6 \ imes 10^{-12} {\\rm GeV}^{-1} at 95% confidence level for ALP masses ma ⩽ 10−13 eV. Other consistent magnetic field models lead to stronger limits of gaγ ≲ 1.1–1.5 × 10−12 GeV−1. These bounds are all stronger than the limit inferred from the absence of a gamma-ray burst from SN1987A, and rule out a substantial fraction of the parameter space accessible to future experiments such as ALPS-II and IAXO.

HIGH-SENSITIVITY 86 GHz (3.5 mm) VLBI OBSERVATIONS OF M87: DEEP IMAGING OF THE JET BASE AT A RESOLUTION OF 10 SCHWARZSCHILD RADII

ABSTRACT We report on results from new high-sensitivity, high-resolution 86 GHz (3.5 mm) observations of the jet base in the nearby radio galaxy M87, obtained by the Very Long Baseline Array in conjunction with the Green Bank Telescope. The resulting image has a dynamic range exceeding 1500 to 1, the highest ever achieved for this jet at this frequency, resolving and imaging a detailed jet formation/collimation structure down to ∼10 Schwarzschild radii ( R s ?> ). The obtained 86 GHz image clearly confirms some important jet features known at lower frequencies, i.e., a jet base with a wide opening angle, a limb-brightened intensity profile, a parabola-shape collimation profile and a counter jet. The limb-brightened structure is already well developed at &lt; 0.2 ?> mas ( &lt; 28 ?> R s ?> , projected) from the core, where the corresponding apparent opening angle becomes as wide as ∼100°. The subsequent jet collimation near the black hole evolves in a complicated manner; there is a “constricted” structure at tens of R s ?> from the core, where the jet cross section is locally shrinking. We suggest that external pressure support from the inner part of the radiatively inefficient accretion flow may be dynamically important in shaping/confining the footprint of the magnetized jet. We also present the first 86 GHz polarimetric experiment using very long baseline interferometry for this source, where a highly polarized (∼20%) feature is detected near the jet base, indicating the presence of a well-ordered magnetic field. As a by-product, we additionally report a 43/86 GHz polarimetric result for our calibrator 3C 273, suggesting an extreme rotation measure near the core.

A strong radio brightening at the jet base of M87 during the elevated very-high-energy γ-ray state in 2012

AbstractThe nearby radio galaxy M87 offers a unique opportunity for exploring the connection between γ-ray production and jet formation at an unprecedented linear resolution. However, the origin and location of the γ-rays in this source is still elusive. Based on previous radio/TeV correlation events, the unresolved jet base (radio core) and the peculiar knot HST-1 at &gt;120 pc from the nucleus are proposed as candidate site(s) of γ-ray production. Here we report our intensive, high-resolution radio monitoring observations of the M87 jet with the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network (EVN) from February 2011 to October 2012. During this period, an elevated level of the M87 flux is reported at TeV with VERITAS. We detected a remarkable flux increase in the radio core with VERA at 22/43 GHz coincident with the VHE activity. Meanwhile, HST-1 remained quiescent in terms of its flux density and structure at radio. These results strongly suggest that the TeV γ-ray activity in 2012 originates in the jet base within 0.03 pc (projected) from the central supermassive black hole.

Very Long Baseline Interferometry observations: the closest look at the cores of AGN

AbstractVery Long Baseline Interferometry (VLBI) provides the highest angular resolution achievable in astronomy, reaching sub-milliarcsecond scales. For radio loud AGNs, this offers the unique opportunity to directly image and monitor the fine details of the jet structure, approaching the event horizon in the nearest and most supermassive black holes, like in M87. After a quick review of various VLBI facilities, we present results from recent VLBI monitoring projects on two remarkable radio and high energy sources: the radio galaxy M87 and the BL Lac object Mrk421. For the latter, we present a detailed analysis of the jet structure in total intensity and polarization through the whole 2011, during which a multi-wavelength campaign took place. We reveal flux density variability but no significant changes in the jet structure. In M87, we detect and follow the evolution of the core and of the jet feature HST-1; we reveal superluminal motion of components within HST-1, with a possible connection between ejection of new such components and the occurrence of very high energy flares. Estimates on the physical parameters for both sources are given (Doppler factor, viewing angle, magnetic field, etc.).

OPTICAL PROPER MOTION MEASUREMENTS OF THE M87 JET: NEW RESULTS FROM THE HUBBLE SPACE TELESCOPE

We report new results from an HST archival program to study proper motions in the optical jet of the nearby radio galaxy M87. Using over 13 years of archival imaging, we reach accuracies below 0.1c in measuring the apparent velocities of individual knots in the jet. We confirm previous findings of speeds up to 4.5c in the inner 6" of the jet, and report new speeds for optical components in the outer part of the jet. We find evidence of significant motion transverse to the jet axis on the order of 0.6c in the inner jet features, and superluminal velocities parallel and transverse to the jet in the outer knot components, with an apparent ordering of velocity vectors possibly consistent with a helical jet pattern. Previous results suggested a global deceleration over the length of the jet in the form of decreasing maximum speeds of knot components from HST-1 outward, but our results suggest that superluminal speeds persist out to knot C, with large differentials in very nearby features all along the jet. We find significant apparent accelerations in directions parallel and transverse to the jet axis, along with evidence for stationary features in knots D, E, and I. These results are expected to place important constraints on detailed models of kpc-scale relativistic jets.

Probing the inner jet of M87; from the jet base to HST-1

The giant radio galaxy M87 accompanies one of the nearest Active-Galactic-Nuclei jets showing the intense radiation through radio to TeV gamma-ray. Its proximity and the large mass of the central black hole provide an excellent advantage to probe the sites of jet formation and gamma-ray production. Here we review some of our recent studies for the inner jet of M87 based on the multi-frequency and multi-epoch VLBI observations, especially focusing on the two remarkable regions i.e., the jet base near the black hole and the peculiar feature HST-1. Our multi-frequency observations with the phase-referencing technique revealed the detailed structure of the jet base region regarding the location of the central engine and the collimation profile. In terms of HST-1, the intense multi-epoch VLBI monitoring constrained the accurate kinematic properties and the structural variations in this complicated feature, together with a possible connection to the gamma-ray activities. At the end of this contribution, we briefly describe our new monitoring project for M87 with VERA, which permits a detailed study on the structural evolution in the jet base region.

A cosmic ray cocoon along the X-ray jet of M87?

Relativistic jets propagating through an ambient medium must produce some observational effects along their side boundaries because of interactions across the large velocity gradient. One possible effect of such an interaction would be a sheared magnetic field structure at the jet boundaries, leading to a characteristic radio polarization pattern. As proposed by Ostrowski, another effect can come from the generation of a high energy cosmic ray component at the boundary, producing dynamic effects on the medium surrounding the jet and forming a cocoon dominated by cosmic rays with a decreased thermal gas emissivity. We selected this process for our first attempt to look for the effects of this type of interaction. We analyzed the Chandra X-ray data for the radio galaxy M87 in order to verify if the expected regions of diminished emissivity may be present near the spectacular X-ray jet in this source. The detailed analysis of the data, merged from 42 separate observations, shows signatures of lower emissivity surrounding the jet. In particular we detect an intensity dip along the part of the jet, which would be approximately 150 pc x 2 kpc in size, if situated along the jet which is inclined toward us. Due to a highly non-uniform X-ray background in the central region we are not able to claim the discovery of a cosmic ray cocoon around the M87 jet: we only have demonstrated that the data show morphological structures which could be accounted for if a cosmic ray cocoon exists.

PROBING THE CENTRAL BLACK HOLE IN M87 WITH GAMMA-RAYS

Recent high-sensitivity observation of the nearby radio galaxy M87 has provided important insights into the central engine that drives the large-scale outflows seen in radio, optical and X-rays. This review summarizes the observational status achieved in the high energy (HE &lt; 100 GeV) and very high energy (VHE &gt; 100 GeV) gamma-ray domains, and discusses the theoretical progress in understanding the physical origin of this emission and its relation to the activity of the central black hole.

VERITAS Very High Energy Gamma-ray Observations of Galaxies

Remarkable progress has been made in very high energy (VHE; E ≥ 100 GeV) gamma-ray astrophysics in the last decade. The VHE source catalog has increased tenfold, with a wide variety of source classes, and includes more than 45 extragalactic objects. Currently, the Very Energetic Radiation Imaging Telescope Array System (VERITAS) has made observations of numerous source candidates, both galactic and extragalactic, including more than 100 galaxies. The last 18 months of observations are especially noteworthy as they have been completed in a new array configuration, following the relocation of one of the four 12–meter telescopes. This new configuration has yielded a significant increase in the VERITAS sensitivity to gamma-ray sources above 100 GeV, resulting in several new source detections in the first few months of operations alone. To date, the VERITAS extragalactic source catalog includes various blazars, the radio galaxy M87 and the starburst galaxy M82. Highlights from the VERITAS extragalactic observation program are summarized here.

VERITAS OBSERVATIONS OF DAY-SCALE FLARING OF M 87 IN 2010 APRIL

VERITAS has been monitoring the very-high-energy (VHE; >100GeV) gamma-ray activity of the radio galaxy M87 since 2007. During 2008, flaring activity on a timescale of a few days was observed with a peak flux of (0.70 +- 0.16) X 10^{-11} cm^{-2} s^{-1} at energies above 350GeV. In 2010 April, VERITAS detected a flare from M87 with peak flux of (2.71 +- 0.68) X 10^{-11} cm^{-2} s^{-1} for E>350GeV. The source was observed for six consecutive nights during the flare, resulting in a total of 21 hr of good quality data. The most rapid flux variation occurred on the trailing edge of the flare with an exponential flux decay time of 0.90^{+0.22}_{-0.15} days. The shortest detected exponential rise time is three times as long, at 2.87^{+1.65}_{-0.99} days. The quality of the data sample is such that spectral analysis can be performed for three periods: rising flux, peak flux, and falling flux. The spectra obtained are consistent with power-law forms. The spectral index at the peak of the flare is equal to 2.19 +- 0.07. There is some indication that the spectrum is softer in the falling phase of the flare than the peak phase, with a confidence level corresponding to 3.6 standard deviations. We discuss the implications of these results for the acceleration and cooling rates of VHE electrons in M87 and the constraints they provide on the physical size of the emitting region.

Dissipation efficiency of reconfinement shocks in relativistic jets

Abstract We calculate the dissipation efficiency of relativistic reconfinement shocks. Building on previous work, we consider different distributions of the external pressure. The average dissipation efficiency εdiss is a function of the product of two parameters — the jet Lorentz factor Γj and the reconfinement angle Θr, which is related to the opening angle Θj and the external pressure index η. The spatial distribution of the dissipation rate strongly depends on η. We discuss the significance of these results for the properties of relativistic jets in gamma-ray bursts and active galactic nuclei and propose that reconfinement shocks may explain a very high dissipation efficiency of the former and a moderate dissipation efficiency of the latter. Finally, we estimate the dissipation efficiency of the reconfinement shock associated with the quasi-stationary knot HST-1 in the jet of radio galaxy M87 and show that it is roughly consistent with the observational constraints.

The blazar-like radio structure of the TeV source IC 310

Context. The radio galaxy IC310 in the Perseus cluster has recently been detected in the gamma-ray regime at GeV and TeV energies. The TeV emission shows time variability and an extraordinarily hard spectrum, even harder than the spectrum of the similar nearby gamma-ray emitting radio galaxy M87. Aims. High-resolution studies of the radio morphology help to constrain the geometry of the jet on sub-pc scales and to find out where the high-energy emission might come from. Methods. We analyzed May 2011 VLBA data of IC310 at a wavelength of 3.6 cm, revealing the parsec-scale radio structure of this source. We compared our findings with more information available from contemporary single-dish flux density measurements with the 100-m Effelsberg radio telescope. Results. We have detected a one-sided core-jet structure with blazar-like, beamed radio emission oriented along the same position angle as the kiloparsec scale radio structure observed in the past by connected interferometers. Doppler-boosting favoritism is consistent with an angle of theta < 38 degrees between the jet axis and the line-of-sight, i.e., very likely within the boundary dividing low-luminosity radio galaxies and BL Lac objects in unified schemes. Conclusions. The stability of the jet orientation from parsec to kiloparsec scales in IC310 argues against its classification as a headtail radio galaxy; i.e., there is no indication of an interaction with the intracluster medium that would determine the direction of the tail. IC310 seems to represent a low-luminosity FRI radio galaxy at a borderline angle to reveal its BL Lac-type central engine.

AN EXPERIMENT TO LOCATE THE SITE IN M87 OF TeV FLARING

We describe a Chandra ToO project designed to isolate the site of TeV flaring in the radio galaxy M87. To date, we have triggered the Chandra observations only once (2010 April) and by the time of our first observation, the TeV flare had ended. However, we found that the X-ray intensity of the unresolved nucleus was at an elevated level at the time of our first of 9 observations. Of the ~70 observations we have made of the M87 jet covering 9 years, the nucleus was measured at &gt; 1 keV/s only 3 times. Two of these occasions can be associated with TeV flaring, and at the time of the third event, there were no TeV monitoring activities. We discuss the implications of these data.

AN EXPERIMENT TO LOCATE THE SITE OF TeV FLARING IN M87

We describe a Chandra X-ray ToO project designed to isolate the site of TeV flaring in the radio galaxy M87. To date, we have triggered the Chandra observations only once (2010 April) and by the time of the first of our 9 observations, the TeV flare had ended. However, we found that the X-ray intensity of the unresolved nucleus was at an elevated level for our first observation. Of the more than 60 Chandra observations we have made of the M87 jet covering 9 years, the nucleus was measured at a comparably high level only 3 times. Two of these occasions can be associated with TeV flaring, and at the time of the third event, there were no TeV monitoring activities. From the rapidity of the intensity drop of the nucleus, we infer that the size of the emitting region is of order a few light days times the unknown beaming factor; comparable to the same sort of estimate for the TeV emitting region. We also find evidence of spectral evolution in the X-ray band which seems consistent with radiative losses affecting the non-thermal population of the emitting electrons within the unresolved nucleus.

Broad escape from the abyss

New ultra-high-resolution images of the radio galaxy M87 show the origin of a jet very close to the galaxy's black hole, revealing that jets start as broad flows before straightening into nearly cylindrical beams. See Letter p.185 Radio jets from active galactic nuclei, such as the nearby galaxy M87, are thought to be powered by the accretion of material into a supermassive black hole. The relative position of this 'central engine' and the bright radio core that marks the base of the jet remain the subject of much speculation. New observations of M87 at six frequencies have been used to determine the position of the radio core to an accuracy of ∼ 20 microarcseconds. The data reveal that the central engine is located close to the radio core, within a distance of 14–23 Schwarzschild radii at 43 GHz.

Signatures of large-scale magnetic fields in active galactic nuclei jets: transverse asymmetries

We investigate the emission properties that a large-scale helical magnetic field imprints on AGN jet synchrotron radiation. A cylindrically symmetric relativistic jet and large-scale helical magnetic field produce significant asymmetrical features in transverse profiles of fractional linear polarization, intensity, Faraday rotation, and spectral index. The asymmetrical features of these transverse profiles correlate with one another in ways specified by the handedness of the helical field, the jet viewing angle (theta_ob), and the bulk Lorentz factor of the flow (Gamma). Thus, these correlations may be used to determine the structure of the magnetic field in the jet. In the case of radio galaxies (theta_ob~1 radian) and a subclass of blazars with particularly small viewing angles (theta_ob << 1/Gamma), we find an edge-brightened intensity profile that is similar to that observed in the radio galaxy M87. We present observations of the AGNs 3C 78 and NRAO 140 that display the type of transverse asymmetries that may be produced by large-scale helical magnetic fields.

NIKA: A millimeter-wave kinetic inductance camera

Current generation millimeter wavelength detectors suffer from scaling limits imposed by complex cryogenic readout electronics. To circumvent this it is imperative to investigate technologies that intrinsically incorporate strong multiplexing. One possible solution is the kinetic inductance detector (KID). In order to assess the potential of this nascent technology, a prototype instrument optimized for the 2 mm atmospheric window was constructed. Known as the N\'eel IRAM KIDs Array (NIKA), it was recently tested at the Institute for Millimetric Radio Astronomy (IRAM) 30-meter telescope at Pico Veleta, Spain. The measurement resulted in the imaging of a number of sources, including planets, quasars, and galaxies. The images for Mars, radio star MWC349, quasar 3C345, and galaxy M87 are presented. From these results, the optical NEP was calculated to be around $1 \times 10^{-15}$ W$ / $Hz$^{1/2}$. A factor of 10 improvement is expected to be readily feasible by improvements in the detector materials and reduction of performance-degrading spurious radiation.

The far-infrared view of M87 as seen by the Herschel Space Observatory

AbstractThe origin of the far-infrared emission from the nearby radio galaxy M87 remains a matter of debate. Some studies find evidence of a far-infrared excess due to thermal dust emission, whereas others propose that the far-infrared emission can be explained by synchrotron emission without the need for an additional dust emission component. We observed M87 with PACS and SPIRE as part of the Herschel Virgo Cluster Survey (HeViCS). We compare the new Herschel data with a synchrotron model based on infrared, submm and radio data to investigate the origin of the far-infrared emission. We find that both the integrated SED and the Herschel surface brightness maps are adequately explained by synchrotron emission. At odds with previous claims, we find no evidence of a diffuse dust component in M87.

TheHerschelVirgo Cluster Survey

The origin of the far-infrared emission from the nearby radio galaxy M87 remains a matter of debate. Some studies find evidence of a far-infrared excess due to thermal dust emission, whereas others propose that the far-infrared emission can be explained by synchrotron emission without the need for an additional dust emission component. We present Herschel PACS and SPIRE observations of M87, taken as part of the science demonstration phase observations of the Herschel Virgo Cluster Survey. We compare these data with a synchrotron model based on mid-infrared, far-infrared, submm and radio data from the literature to investigate the origin of the far-infrared emission. Both the integrated SED and the Herschel surface brightness maps are adequately explained by synchrotron emission. At odds with previous claims, we find no evidence of a diffuse dust component in M87, which is not unexpected in the harsh X-ray environment of this radio galaxy sitting at the core of the Virgo Cluster.

HIGH-ENERGY PARTICLE INTERACTIONS IN THE INNER JET OF THE RADIO GALAXY M87

Recent observations with the High Energy Stereoscopic System (HESS) have revealed strong and variable high energy gamma-ray emission from the radio galaxy M87. The origin of such emission is uncertain, but the rapid variability indicates that it should be produced close to the central engine of the source. In this work, a lepto-hadronic one-zone model is applied to the available multiwavelength data of M87. The different energy losses for both primary and secondary particles are calculated. Then, the different contributions to the spectral energy distribution through interactions with matter, radiation and magnetic fields are obtained, in good accordance with the observations.