IGO Publications
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Item THE DISCOVERY AND NATURE OF THE OPTICAL TRANSIENT CSS100217:102913+404220(The Astrophysical Journal, 2011-07-10) Drake, A.J; Djorgovski, S.G; Mahabal, A; et.alWe report on the discovery and observations of the extremely luminous optical transient CSS100217:102913+404220 (CSS100217 hereafter). Spectroscopic observations showed that this transient was coincident with a galaxy at redshift z = 0.147 and reached an apparent magnitude of V ∼ 16.3. After correcting for foreground Galactic extinction we determine the absolute magnitude to be MV = −22.7 approximately 45 days after maximum light. Over a period of 287 rest-frame days, this event had an integrated bolometric luminosity of 1.3 × 1052 erg based on time-averaged bolometric corrections of ∼15 from V- and R-band observations. Analysis of the pre-outburst Sloan Digital Sky Survey (SDSS) spectrum of the source shows features consistent with a narrow-line Seyfert 1 galaxy. High-resolution Hubble Space Telescope and Keck follow-up observations show that the event occurred within 150 pc of the nucleus of the galaxy, suggesting a possible link to the active nuclear region. However, the rapid outburst along with photometric and spectroscopic evolution are much more consistent with a luminous supernova. Line diagnostics suggest that the host galaxy is undergoing significant star formation. We use extensive follow-up of the event along with archival Catalina Sky Survey NEO search and SDSS data to investigate the three most likely sources of such an event: (1) an extremely luminous supernova, (2) the tidal disruption of a star by the massive nuclear black hole, and (3) variability of the central active galactic nucleus (AGN). We find that CSS100217 was likely an extremely luminous Type IIn supernova and occurred within the range of the narrow-line region of an AGN. We discuss how similar events may have been missed in past supernova surveys because of confusion with AGN activity.Item Optical variability of radio-intermediate quasars(2009-10-08) Goyal, Arti; Krishna, Gopal; Joshi, S; etWe report the results of our intensive intranight optical monitoring of 8 opti- cally bright ‘radio-intermediate quasars’ (RIQs) having flat or inverted radio spectra. The monitoring was carried out in R-band on 25 nights during 2005- 09. On each night only one RIQ was monitored for a minimum duration of ∼ 4 hours (the average being 5.2 hours per night). Using the CCD as an N-star photometer, an intranight optical variability (INOV) detection threshold of ∼ 1–2% was achieved for the densely sampled differential light curves (DLCs) de- rived from our data. These observations amount to a large increase over those reported hitherto for this rare and sparsely studied class of quasars which can, however, play an important role in understanding the link between the dominant varieties of powerful AGN, namely the radio-quiet quasars (RQQs), radio-loud quasars (RLQs) and blazars. Despite the probable presence of rela- tivistically boosted nuclear jets, inferred from their flat/inverted radio spectra, clear evidence for INOV in our extensive observations was detected only on one night. Also, flux variation between two consecutive nights was clearly seen for one of the RIQs. These results demonstrate that as a class, RIQs are much less extreme in nuclear activity compared to blazars. The availability in the literature of INOV data for another 2 RIQs conforming to our selection crite- ria allowed us to enlarge the sample to 10 RIQs (monitored on a total of 42 nights for a minimum duration of ∼ 4 hours per night). The absence of large 2 Goyal et al. amplitude INOV (ψ > 3%) persists in this enlarged sample. This extensive database has enabled us to arrive at the first estimate for the INOV Duty Cy- cle (DC) of RIQs. The DC is found to be small (∼ 9%), increasing to ∼ 14% if the two cases of ‘probable’ INOV are included. The corresponding value is known to be ∼ 60% for BL Lacs and ≈ 15% for both RLQs and RQQs, if they too are monitored for & 4 − 6 hours in each session. Our observations also provide information about the long-term optical variability (LTOV) of RIQs, which is found to be fairly common and reaches typical amplitudes of ≈ 0.1- mag. The light curves of these RIQs are briefly discussed in the context of a theoretical framework proposed earlier for linking this rare kind of quasars to the much better studied dominant classes of quasars.Item SDSS J092712.64+294344.0: recoiling black hole or merging galaxies?(2009-08-31) Vivek, M; Srianand, R.; Noterdaeme, P; et al.We report long-slit spectroscopic observations of SDSS J092712+294344 carried-out at the recently commissioned 2m telescope in IUCAA Girawali Observatory, India. This AGN-like source is known to feature three sets of emission lines at zem = 0.6972, 0.7020 and 0.7128. Different scenarios such as a recoiling black hole after asymmetric emission of gravitational waves, binary black holes and possible merging systems are proposed for this object. We test these scenarios by comparing our spectra with that fromthe Sloan Digital Sky Survey (SDSS), obtained 4 years prior to our observations. Comparing the redshifts of [Oiii]λλ4960,5008 we put a 3σ limit on the relative acceleration to be less than 32 km s−1 yr−1 between different emitting regions. Using the 2D spectra obtained at different position angles we show that the [Oiii]λ5008 line from the zem = 0.7128 component is extended beyond the spectral point spread function.We infer the linear extent of this line emitting region is ∼ 8 kpc.We also find a tentative evidence for an offset between the centroid of the [Oiii]λ5008 line at zem = 0.7128 and the QSO trace when the slit is aligned at a position angle of 299◦. This corresponds to the zem = 0.7128 system being at an impact parameter of ∼1 kpc with respect to the zem = 0.6972 in the north west direction. Based on our observations we conclude that the binary black hole model is most unlikely. The spatial extent and the sizes are consistent with both black hole recoil and merging scenarios.Item 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.