Browsing by Author "Bagchi, Joydeep"

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BeppoSAX observation of the rich cluster of galaxies Abell 85
(2001-01-06) Lima Neto, G. B.; Pislar, V.; Bagchi, Joydeep
We report the observation of the Intra-Cluster Medium (ICM) of Abell 85 by the X-ray satellite Bep- poSAX. We have both analysed the spectrum obtained in the central 8 arcmin circular region centred on the Very Steep Spectrum Radio Source (VSSRS) and the spec- tra from a number of sub-regions. Analysis of the spec- tra allowed us to independently obtain new estimates of the temperature, metallicity and line-of-sight hydro- gen density column, both globally (T = 6.6 ± 0.3 keV, Z = 0.38 ± 0.06Z⊙ and NH = 5.5+0.9 −0.71020cm−2) and lo- cally. These measures are in good agreement with previ- ous measures based on ROSAT and ASCA data. In the region of the VSRSS, we have tried to disentangle the thermal from the non-thermal X-ray emission. Although we could not do this unambiguously, we have nonethe- less estimated the extended magnetic ﬁeld using the radio spectrum available for this region.We obtain a lower limit intensity of 0.9µG, consistent with our previous estimate. We also derive α-elements/iron abundance ratios that turn out to be higher than 1. Such a result tends to support the burst model for elliptical galaxies, where a strong galac- tic wind develops early in the galaxy history and type II supernovae (SN) may have the main role in the enrich- ment of the ICM. A two-temperature ICM model was ﬁt- ted in the central region yielding a main component with roughly the mean cluster temperature and a cooler com- ponent with temperature less than 0.1 keV.
Diffuse bubble-like radio-halo emission in MRC 0116+111: Imprint of AGN feedback in a distant cluster of galaxies
(2009-02-01) Bagchi, Joydeep
We report the discovery of a luminous, mini radio halo of ∼240 kpc dimension at the center of a distant cluster of galaxies at redshift z = 0.131. Our optical and multi-wavelength GMRT and VLA observations reveal a highly unusual structure showing a twin bubble-like diﬀuse radio halo surrounding a cluster of bright elliptical galaxies; very similar to the large-scale radio structure of M87, the dominant galaxy in Virgo cluster. It presents an excellent opportunity to understand the energetics and the dynamical evolution of such radio jet inﬂated plasma bubbles in the hot cluster atmosphere.
A diffuse bubble-like radio-halo source MRC 0116+111: imprint of AGN feedback in a low-mass cluster of galaxies
(2009-07-09) Bagchi, Joydeep; Jacob, Joe; Gopal, Krishna; et al.
We present detailed observations of MRC 0116+111, revealing a luminous, mini radiohalo of 240 kpc diameter located at the centre of a cluster of galaxies at redshift z = 0.131. Our optical and multi-wavelength GMRT and VLA radio observations reveal a highly unusual radio source: showing a pair of giant ( 100 kpc diameter) bubble-like diffuse structures, that are about three times larger than the analogous extended radio emission observed in M87 - the dominant central radio galaxy in the Virgo Cluster. However, in MRC 0116+111 we do not detect any ongoing Active Galactic Nucleus (AGN) activity, such as a compact core or active radio jets feeding the plasma bubbles. The radio emitting relativistic particles and magnetic fields were probably seeded in the past by a pair of radio-jets originating in the AGN of the central cD galaxy. The extremely steep high-frequency radio spectrum of the north-western bubble, located 100 kpc from cluster centre, indicates radiation losses, possibly because having detached, it is rising buoyantly and moving away into the putative hot intra-cluster medium. The other bubble, closer to the cluster centre, shows signs of ongoing particle re-acceleration.We estimate that the radio jets which inflated these two bubbles might have also fed enough energy into the intra-cluster medium to create an enormous system of cavities and shock fronts, and to drive a massive outflow from the AGN, which could counter-balance and even quench a cooling flow. Therefore, this source presents an excellent opportunity to understand the energetics and the dynamical evolution of radio-jet inflated plasma bubbles in the hot cluster atmosphere.
Discovery of giant 'radio arcs' in cluster Abell 3376 : evidence for shock accleration in a violent cluster merger?
(2002-10-24) Bagchi, Joydeep
New multi-wavelength (radio, optical & X-rays) observational evidences are presented which show that the nearby (z = 0.046), rich cluster of galaxies Abell 3376 is experiencing a major event of binary subcluster merger. The key evidence is the discovery of a pair of large, optically unidentiﬁed diﬀuse radio sources (‘arcs’), symmetrically located about 2.6 h−1 50 Mpc apart at the opposite ends of the hot intra-cluster gas mapped by ROSAT in X-rays. It is argued that the gas-dynamical shock-waves, which occur naturally during cluster formation, are accelerating charged particles (cosmic rays) to relativis- tic energies, leading to synchrotron emission from the megaparsec scale radio arcs. If this is so, cluster Abell 3376 would also be a potential source capa- ble of accelerating cosmic ray particles upto ultra-high energies (UHECR) of Emax ∼ 1018−19 eV. Thus this cluster is an excellent test-bed for understand- ing the physics of merger shocks and origin of enigmatic UHECR particles in structure formation process. Hence, Abell 3376 provides unique oppurtu- nities for further multi-wavelength observations with ground and space-borne observatories
Evidence for shock acceleration and intergalactic magnetic fields in a large-scale filament of galaxies ZwC1 2341.1+0000
(2002-06-19) Bagchi, Joydeep; Enßlin, Torsten A.; Miniati, Francesco; et al.
We report the discovery of large-scale diﬀuse radio emission from what appears to be a large-scale ﬁlamentary network of galaxies in the region of cluster ZwCl 2341.1+0000, and stretching over an area of at least 6 h−1 50 Mpc in diameter. Mul- ticolour CCD observations yield photometric redshifts indicating that a signiﬁcant fraction of the optical galaxies in this region is at a redshift of z=0.3. This is sup- ported by spectroscopic measurements of 4 galaxies in the Sloan Digital Sky Survey (SDSS) at a mean z =0.27. We present VLA images at λ =20 cm (NVSS) and 90 cm, showing the detailed radio structure of the ﬁlaments. Comparison with the high resolution FIRST radio survey shows that the diﬀuse emission is not due to known individual point sources. The diﬀuse radio-emission has a spectral index α . −0.5, and is most likely synchrotron emission from relativistic charged particles in an inter-galactic magnetic ﬁeld. Furthermore, this optical/radio structure is detected in X-rays by the ROSAT all-sky survey. It has a 0.1–2.4 keV luminosity of about 1044 erg s−1 and shows an extended highly non-relaxed morphology. These observa- tions suggest that ZwCl 2341.1+0000 is possibly a proto-cluster of galaxies in which we are witnessing the process of structure formation. We show that the energetics of accretion shocks generated in forming large-scale structures are suﬃcient to pro- duce enough high energy cosmic-ray (CR) electrons required to explain the observed radio emission, provided a magnetic ﬁeld of strength B & 0.3µG is present there. The latter is only a lower limit and the actual magnetic ﬁeld is likely to be higher depending on the morphology of the emitting region. Finally, we show results from Preprint submitted to Elsevier Science 1 February 2008a numerical simulation of large-scale structure formation including acceleration of CR electrons at cosmological shocks and magnetic ﬁeld evolution. Our results are in accord with the observed radio synchrotron and X-ray thermal bremsstrahlung ﬂuxes. Thus we conclude that the reported radio detection is the ﬁrst evidence of cosmic-ray particle acceleration taking place at cosmic shocks in a magnetized inter-galactic medium over scales of & 5 h−1 50 Mpc.
Giant Radio Jet Ejected by An Ultramassive Black Hole in a Single-Lobed Radio Galaxy
(2008-01) Bagchi, Joydeep
We report the discovery of a very unusual, highly asymmetric radio galaxy whose radio jet, the largest yet detected, emits strongly polarized synchrotron radiation and can be traced all the way from the galactic nucleus to the hot spot located ∼ 440 kpc away. This jet emanates from an extremely massive black-hole (> 109 M⊙) and forms a strikingly compact radio lobe. No radio lobe is detected on the side of the counter-jet, even though it is similar to the main jet in brightness up to a scale of tens of kilo parsecs. Thus, contrary to the nearly universal trend, the brightness asymmetry in this radio galaxy increases with distance from the nucleus. With several unusual properties, including a predominantly toroidal magnetic ﬁeld, this Fanaroﬀ-Riley type II mega-jet is an exceptionally useful laboratory for testing the role of magnetic ﬁeld in jet stabilization and radio lobe formation.
A giant radio jet ejected by an ultramassive black hole in a single-lobed radio galaxy
(2004-12-04) Bagchi, Joydeep; Krishna, Gopal; Krause, Marita; Santosh Joshi
We report the discovery of a very unusual, highly asymmetric radio galaxy whose radio jet, the largest yet detected, emits strongly polarized synchrotron radiation and can be traced all the way from the galactic nucleus to the hot spot located 440 kpc away. This jet emanates from an extremely massive black-hole (> 109M⊙) and forms a strikingly compact radio lobe. No radio lobe is detected on the side of the counter-jet, even though it is similar to the main jet in brightness up to a scale of tens of kilo parsecs. Thus, contrary to the nearly universal trend, the brightness asymmetry in this radio galaxy increases with distance from the nucleus. With several unusual properties, including a predominantly toroidal magnetic field, this Fanaroff-Riley type II mega-jet is an exceptionally useful laboratory for testing the role of magnetic field in jet stabilization and radio lobe formation
Giant radio jet of very unusual polarization in a single-lobed radio galaxy
(2009-05-01) Bagchi, Joydeep; Gopal, Krishna; Krause, Marita
We report the discovery of a very unusual, highly asymmetric radio galaxy whose radio jet, the largest yet detected, emits strongly polarized synchrotron radiation and can be traced all the way from the galactic nucleus to the hot spot located ∼ 440 kpc away. This jet emanates from an extremely massive black-hole (> 109M ) and forms a strikingly compact radio lobe. To a surface brightness contrast of at least 15 no radio lobe is detected on the side of the counter-jet, which is similar to the main jet in brightness upto the scale of tens of kpc. Thus, contrary to the nearly universal trend, the brightness asymmetry in this radio galaxy increases with distance from the nucleus. With several unusual properties, including a predominantly toroidal magnetic ﬁeld, this Fanaroﬀ-Riley type II mega-jet is an exceptionally useful laboratory for testing the role of magnetic ﬁeld in jet stabilization and radio lobe formation.
Giant Ringlike Radio Structures Around Galaxy Cluster Abell 3376
(2007-11-09) Bagchi, Joydeep; Durret, Florence; Paul, Surajit; et al.
In the current paradigm of cold dark matter cosmology, large-scale structures are assembling through hierarchical clustering of matter. In this process, an important role is played by megaparsec (Mpc)-scale cosmic shock waves, arising in gravity-driven supersonic ﬂows of intergalactic matter onto dark matter-dominated collapsing structures such as pancakes, ﬁlaments and clusters of galaxies. Here we report Very Large Array telescope observations of giant (∼ 2 Mpc ×1.6 Mpc), ring-shaped non-thermal radio emitting structures, found at the outskirts of the rich cluster of galaxies Abell 3376. These structures may trace the elusive shock waves of cosmological large scale matter ﬂows, which are energetic enough to power them. These radio sources may also be the acceleration sites where magnetic shocks are possibly boosting 1cosmic-ray particles with energies of up to 1018 to 1019 electron volts.
High & ultra-high energy cosmic ray acceleration in structure-formation shocks in Abell 3376 galaxy cluster
(2005-09-01) Bagchi, Joydeep; Paul, Surajit; Durret, Florence; et al.
We describemultiwavelength optical, X-ray and radio evidences of energeticmergers in southern galaxy cluster Abell 3376 (A3376, redshift z=0.046, X-ray luminosity "#$&'($' ( ). Here we report the rare discovery of gigantic, Mpc-scale non-thermal radio structures (‘radio-arcs’), shaped like a pair of bow-shock fronts on the merger axis. We argue for the shock acceleration origin of the radio emission and discuss the possibilty of acceleration of very high energy cosmic ray particles in this and similar clusters
Mega parsec relativistic jets launched from an accreting supermassive blackhole in an extreme spiral galaxy
(IUCAA, 2015-02) Bagchi, Joydeep; Vivek, M.; Vikram, Vinu
Radio Astronomy
(2013-07-24) Bagchi, Joydeep
Radio observations of ZwCl 2341.1+0000: a double radio relic cluster
(2009-10-01) Weeren, R.; Miniati, F.; Bagchi, Joydeep; et al.
infall and mergers of smaller subclusters and galaxy groups. Diﬀuse radio emission, in the form of radio halos and relics, is found in clusters undergoing a merger, indicating that shocks or turbulence associated with the merger are capable of accelerating electrons to highly relativistic energies. Double relics are a rare class of radio sources found in the periphery of clusters, with the two components located symmetrically on the opposite sides of the cluster center. These relics are important probes of the cluster periphery as (i) they provide an estimate of the magnetic ﬁeld strength, and (ii) together with detailed modeling can be used to derive information about the merger geometry, mass, and timescale. Observations of these double relics can thus be used to test the framework of LSS formation. Here we report on radio observations of ZwCl 2341.1+0000, a complex merging structure of galaxies located at z = 0.27, using Giant Metrewave Radio Telescope (GMRT) observations. Aims. The main aim of the observations is to study the nature of the diﬀuse radio emission in the galaxy cluster ZwCl 2341.1+0000. Methods. We carried out GMRT 610, 241, and 157 MHz continuum observations of ZwCl 2341.1+0000. The radio observations are combined with X-ray and optical data of the cluster. Results. The GMRT observations show a double peripheral radio relic in the cluster ZwCl 2341.1+0000. The spectral index is −0.49 ± 0.18 for the northern relic and −0.76 ± 0.17 for the southern relic. We have derived values of 0.48 − 0.93 µGauss for the equipartition magnetic ﬁeld strength. The relics are probably associated with outward traveling merger shock waves.
Spectral and polarization study of the double relics in Abell 3376 using the GMRT and the VLA
(2012-06-15) Kale, Ruta; Dwarakanath, K. S; Bagchi, Joydeep; et.al