arXiv subjects

Philip J. Diamond

Publications and source records attributed to Philip J. Diamond.

11 recordsLinked to original sources

arXiv2013

The spatiokinematical structure of H_2O and OH masers in the "water fountain" source IRAS 18460-0151

Using the Very Long Baseline Array and the European VLBI Network, we have observed 22.2 GHz H_2O and 1612 MHz OH masers in the "water fountain" source IRAS 18460-0151. The H_2O maser spectrum has a very wide line-of-sight velocity range (~310 km/s) and consists of three groups of emission features at the blue-shifted (-68 km/s <~ V_LSR <~ -17 km/s) and red-shifted (V_LSR ~= 240 km/s) edges as well as around the systemic velocity (112 km/s <~ V_LSR <~ 133 km/s). The first two H_2O spectral components exhibit a highly-collimated high-velocity bipolar jet on the sky, with an angular separation of ~120 milliarcseconds (mas) (240 AU in linear length) and a three-dimensional flow velocity of ~160 km/s. The flow dynamical age is estimated to be only ~6 yr (at the time of the observation epochs of 2006--2007). Interestingly, the systemic velocity component clearly exhibits a spherically-expanding outflow with a radius of ~36 AU and a flow velocity of ~9 km/s. On the other hand, the OH maser spectrum shows double peaks with a velocity separation of ~25 km/s (V_LSR=$111--116 and 138--141 km/s), as typically seen in circumstellar envelopes of OH/IR stars. The angular offset between the velocity-integrated brightness peaks of the two high-velocity H_2O components is ~25 mas (50 AU). The offset direction and the alignment of the red-shifted maser spots are roughly perpendicular to the axis of the H_2O maser flow. High-accuracy astrometry for the H_2O and OH masers demonstrates that the collimated fast jet and the slowly expanding outflow originate from a single or multiple sources which are located within 15 mas (30 AU). On the other hand, the estimated systemic velocity of the collimated jet (V_sys ~87--113 km/s) has a large uncertainty. This makes it difficult to provide strong constraints on models of the central stellar system of IRAS 18460-0151.

astro-ph.SR↗

arXiv2011

Resolution of the Compact Radio Continuum Sources in Arp220

We present 2 cm and 3.6 cm wavelength very long baseline interferometry images of the compact radio continuum sources in the nearby ultra-luminous infrared galaxy Arp220. Based on their radio spectra and variability properties, we confirm these sources to be a mixture of supernovae (SNe) and supernova remnants (SNRs). Of the 17 detected sources we resolve 7 at both wavelengths. The SNe generally only have upper size limits. In contrast all the SNRs are resolved with diameters {\geq} 0.27 pc. This size limit is consistent with them having just entered their Sedov phase while embedded in an interstellar medium (ISM) of density 10^4 cm^{-3} . These objects lie on the diameter-luminosity correlation for SNRs (and so also on the diameter-surface brightness relation) and extend these correlations to very small sources. The data are consistent with the relation L {\propto} D^{-9/4}. Revised equipartition arguments adjusted to a magnetic field to relativistic particle energy density ratio of 1% combined with a reasonable synchrotron-emitting volume filling factor of 10% give estimated magnetic field strengths in the SNR shells of ~ 15-50 mG. The SNR shell magnetic fields are unlikely to come from compression of ambient ISM fields and must instead be internally generated. We set an upper limit of 7 mG for the ISM magnetic field. The estimated energy in relativistic particles, 2%-20% of the explosion kinetic energy, is consistent with estimates from models that fit the IR-radio correlation in compact starburst galaxies.

astro-ph.CO↗

arXiv2010

Compact source resolution and rapid variability in Arp220

We present multi-epoch global VLBI observations at 2 cm, 3.6 cm and 6 cm of the compact radio sources in Arp220. We resolve many sources and estimate sizes, expansion velocities and source classes. We find most source properties are consistent with them being either radio supernovae (SNe) or supernova remnants (SNRs). We extend the luminosity-diameter relation for SNRs to very small sources and argue this supports models where shell magnetic fields are internally amplified. We also detect one probable SN/SNR transition object candidate and three highly variable sources with possible superluminal motion (approximately 4c) of jet-like features near rapidly varying almost stationary components. These enigmatic sources, which show similarities to the recently discovered superluminal source in M82, might be associated with an AGN or a new radio source class (e.g. intermediate-mass black holes or micro-blazars).

astro-ph.CO↗

arXiv2006

The radio spectra of the compact sources in Arp 220: A mixed population of supernovae and supernova remnants

We report the first detection at multiple radio wavelengths (13, 6 and 3.6 cm) of the compact sources within both nuclei of the Ultra Luminous Infra-Red Galaxy Arp 220. We present the radio spectra of the 18 detected sources. In just over half of the sources we find that these spectra and other properties are consistent with the standard model of powerful Type IIn supernovae interacting with their pre-explosion stellar wind. The rate of appearance of new radio sources identified with these supernova events suggests that an unusually large fraction of core collapse supernovae in Arp 220 are highly luminous; possibly implying a radically different stellar initial mass function or stellar evolution compared to galactic disks. Another possible explanation invokes very short (~3 x 10^5 year) intense (~10^3 M_Sol year^-1) star formation episodes with a duty cycle of ~10%. A second group of our detected sources, consisting of the brightest and longest monitored sources at 18 cm do not easily fit the radio supernova model. These sources show a range of spectral indexes from -0.2 to -1.9. We propose that these are young supernova remnants which have just begun interacting with a surrounding ISM with a density between 10^4 and 10^5 cm^-3. One of these sources is probably resolved at 3.6 cm wavelength with a diameter 0.9 pc. In the western nucleus we estimate that the ionized component of the ISM gives rise to foreground free-free absorption with opacity at 18 cm of <0.6 along the majority of lines of sight. Other sources may be affected by absorption with opacity in the range 1 to 2. These values are consistent with previous models as fitted to the radio recombination lines and the continuum spectrum.

astro-ph↗

arXiv2006

VLBI Images of 49 Radio Supernovae in Arp 220

We have used a Very Long Baseline Interferometry (VLBI) array at 18cm wavelength to image the nucleus of the luminous IR galaxy Arp 220 at ~1 pc linear resolution, and with very high sensitivity. The resulting map has an rms of 5.5 microJy/beam, and careful image analysis results in 49 confirmed point sources ranging in flux density from 1.2 mJy down to ~60 microJy. Comparison with high sensitivity data from 12 months earlier reveals at least four new sources. The favored interpretation of these sources is that they are radio supernovae, and if all new supernovae are detectable at this sensitivity, a resulting estimate of the supernova rate in the Arp 220 system is 4 +/- 2 per year. The implied star formation rate is sufficient to power the entire observed far-infrared luminosity of the galaxy. The two nuclei of Arp 220 exhibit striking similarities in their radio properties, though the western nucleus is more compact, and appears to be ~3 times more luminous than the eastern nucleus. There are also some puzzling differences, and differential free-free absorption, synchrotron aging and expansion losses may all be playing a role. Comparison with the nearby starburst galaxy M82 supports the hypothesis that the activity in Arp 220 is essentially a scaled-up version of that in M82.

astro-ph↗

arXiv2003

A birth and growth of a collimated molecular jet from an AGB star

With the VLBA, we have observed water masers associated with the OH/IR star, W43A, which trace a birth and growth of a molecular jet. The water masers exhibit the collimated distribution (1700 AU: 20 AU) and fast motions (+/-150 km/s). The maser distribution was well fit by a precessing jet model. The jet length has extended by 800 AU within 8 years, indicating that the extension rate is roughly equivalent to the jet speed (150 km/s). Very likely the jet was born around the year 1960. An elongated planetary nebula will be formed by such a jet during an extremely short period (<<1000 yrs) of the transition to form a ptoto-planetary nebula. Then fading of the water maser jet will be observed during our human life.

astro-ph↗

arXiv2003

VLBI Imaging of Luminous Infrared Galaxies: AGN Cores in Mrk231, UGC 5101 & NGC 7469

We report 18cm VLBI continuum imaging observations at 5 mas resolution for UGC 5101, NGC 7469, and Mrk 231, all part of a sample of Luminous Infrared Galaxies which have been shown to have strong VLBI radio cores. The radio morphology of these three systems on VLBI scales is AGN-like, with well-defined ridgelines and high-brightness yet spatially resolved components. The structure and flux densities of these VLBI components are not consistent with starburst generated radio supernovae of the type found in Arp 220. On scales of 100pc the radio continuum in all three objects appears to be dominated by an AGN, not a starburst. Radio emission on larger scales may well originate in a less compact circumnuclear star-forming region. Confirming and extending VLBI imaging of Mrk 231 by Ulvestad et al. (1999), our continuum image shows a triple structure, with a core and two lobes, classifying it as a Compact Symmetric Object (CS0). If the southern (primary) lobe/hot-spot in Mrk 231 is confined by ram pressure, we estimate a lobe advance speed, $v_a \sim 10^{-4}c$, and an age for the jet/compact source, $< 10^6 yr$. We have also imaged the 1667 MHz OH maser emission in Mrk 231, which is extended on scales of 50--100 milliarcsec (40--80 pc) and probably coincides with the inner region of the disk which is seen in CO emission and HI absorption. Among OH megamasers studied at high sensitivity with mas resolution, Mrk 231 is unique in the stringent upper limits placed upon the flux density of compact OH structures of the type found in Arp 220 and other LIGs. It is possible that the circumnuclear environment of Mrk 231 has been sufficiently disrupted by the emergent QSO that the cool, dense clouds necessary for such compact masers no longer exist.

astro-ph↗

arXiv2003

Location of H2O maser in the double-nuclei system of NGC6240

We performed VLA observations of the 22 GHz H2O maser emission in the merging galaxy NGC6240, which hosts the well-known double active nuclei. In a previous paper, we reported on the first solid detection of the H2O maser emission in 2001. After two abortive attempts due to the weakness and probable variability of the emission, the maser was detected with the VLA in June 2002. The emission is unresolved at ~ 0.3 arcsecond and coincides with the southern 22 GHz continuum peak to ~ 0.007 arcsecond (~ 3 pc: D = 97 Mpc). The detection of the maser in the southern nucleus indicates that nuclear activity of the galaxy,which is significant in X-ray and far-infrared (FIR) bands, lies mainly in the southern nucleus, and the nucleus without a high brightness peak could be explained by thick dust emitting FIR radiation. We favour the idea that the maser in NGC6240 is associated with the AGN-activity.

astro-ph↗

arXiv2003

Continuum and spectral line observations of the OH Megamaser galaxy Arp 220

We present MERLIN observations of the continuum (both 1.6 and 5 GHz) and OH maser emission towards Arp220. the correct spatial configuration of the various componnents of the galaxy is revealed. In the eastern component the masers are shown to be generally coincident with the larger scale continuum emission; in the west, the masers and continuum do not generally arise from the same location. A velocity gradient (0.32+/-0.03km/s/pc) is found in the eastern nuclear region in MERLIN scales; this gradient is three times smaller than seen in OH and implies that the OH gas lies inside the HI. A re-analysis of previously presented global VLBI data (Lonsdale et al. 1998) reveals a very high velocity gradient (18.67+/-0.12km/s/pc) in one component, possibly the site of a heavily obscured AGN.

astro-ph↗

arXiv2003

The 3-D kinematics of water masers around the semiregular variable RT Virginis

We report observations of water masers around the semiregular variable RT Virginis (RT Vir), which have been made with the Very Long Baseline Array (VLBA) of the National Radio Astronomy Observatory (NRAO) at five epochs, each separated by three weeks of time. We detected about 60 maser features at each epoch. Overall, 61 features, detected at least twice, were tracked by their radial velocities and proper motions. The 3-D maser kinematics exhibited a circumstellar envelope that is expanding roughly spherically with a velocity of about 8 km/s. Asymmetries in both the spatial and velocity distributions of the maser features were found in the envelope, but less significant than that found in other semiregular variables. Systematic radial-velocity drifts of individual maser features were found with amplitudes of <= 2 km/s/yr. For one maser feature, we found a quadratic position shift with time along a straight line on the sky. This apparent motion indicates an acceleration with an amplitude of 33 km/s/yr, implying the passage of a shock wave driven by the stellar pulsation of RT Vir. The acceleration motion is likely seen only on the sky plane because of a large velocity gradient formed in the accelerating maser region. We estimated the distance to RT Vir to be about 220 pc on the basis of both the statistical parallax and model-fitting methods for the maser kinematics.

astro-ph↗

arXiv2001

VLBI study of water maser emission in the Seyfert 2 galaxy NGC5793. I: Imaging blueshifted emission and the parsec-scale jet

We present the first result of VLBI observations of the blueshifted water maser emission from the type 2 Seyfert galaxy NGC5793, which we combine with new and previous VLBI observations of continuum emission at 1.7, 5.0, 8.4, 15, and 22 GHz. Maser emission was detected earlier in single-dish observations and found to have both red- and blueshifted features relative to the systemic velocity. We could image only the blueshifted emission, which is located 3.6 pc southwest of the 22 GHz continuum peak. The blueshifted emission was found to originate in two clusters that are separated by 0.7 milliarcsecond (0.16 pc). No compact continuum emission was found within 3.6 pc of the maser spot. A compact continuum source showing a marginally inverted spectrum between 1.7 and 5.0 GHz was found 4.2 pc southwest of the maser position. The spectral turnover might be due to synchrotron self-absorption caused by a shock in the jet owing to collision with dense gas, or it might be due to free-free absorption in an ionized screen possibly the inner part of a disk, foreground to the jet. The water maser may be part of a maser disk. If so, it would be rotating in the opposite sense to the highly inclined galactic disk observed in CO emission. We estimate a binding mass within 1 pc of the presumed nucleus to be on the order of 10^7 Msun. Alternatively, the maser emission could result from the amplification of a radio jet by foreground circumnuclear molecular gas. In this case, the high blueshift of the maser emission might mean that the masing region is moving outward away from the molecular gas surrounding an active nucleus.

astro-ph↗