Related papers: Short time scale variability at gamma rays in FSRQ…

Challenging the high-energy emission zone in FSRQs

The blazar zone in quasars is commonly assumed to be located inside the broad-line region at some hundreds of Schwartzschild radii from the central black hole. Now, the simultaneous Fermi/LAT and MAGIC observations of a strong flare in the…

High Energy Astrophysical Phenomena · Physics 2019-08-15 A. Stamerra , J. Becerra , G. Bonnoli , L. Maraschi , F. Tavecchio , D. Mazin , K. Saito , Y. Tanaka , D. Wood

Temporal and spectral gamma-ray properties of Mkn421 above 250 GeV from CAT observations between 1996 and 2000

The gamma-ray emission above 250 GeV from the BL Lac object Markarian 421 was observed by the CAT Cherenkov imaging telescope between December, 1996, and June, 2000. In 1998, the source produced a series of small flares, making it the…

Astrophysics · Physics 2014-10-13 F. Piron , A. Djannati-Atai , M. Punch , J. -P. Tavernet , A. Barrau , R. Bazer-Bachi , L. -M. Chounet , G. Debiais , B. Degrange , J. -P. Dezalay , P. Espigat , B. Fabre , P. Fleury , G. Fontaine , P. Goret , C. Gouiffes , B. Khelifi , I. Malet , C. Masterson , G. Mohanty , E. Nuss , C. Renault , M. Rivoal , L. Rob , S. Vorobiov

Constraining the location of the emitting region in Fermi blazars through rapid gamma-ray variability

We consider the 1.5 years Fermi Large Area Telescope light curves (E > 100 MeV) of the flat spectrum radio quasars 3C 454.3 and PKS 1510-089, which show high activity in this period of time. We characterise the duty cycle of the source by…

Cosmology and Nongalactic Astrophysics · Physics 2015-05-18 F. Tavecchio , G. Ghisellini , G. Bonnoli , G. Ghirlanda

H.E.S.S. detection of very-high-energy gamma-ray emission from the quasar PKS 0736+017

Flat-spectrum radio-quasars (FSRQs) are rarely detected at very-high-energies (VHE; E>100 GeV) due to their low-frequency-peaked SEDs. At present, only 6 FSRQs are known to emit VHE photons, representing only 7% of the VHE extragalactic…

High Energy Astrophysical Phenomena · Physics 2020-01-29 H. E. S. S. Collaboration , : , H. Abdalla , R. Adam , F. Aharonian , F. Ait Benkhali , E. O. Angüner , M. Arakawa , C. Arcaro , C. Armand , H. Ashkar , M. Backes , V. Barbosa Martins , M. Barnard , Y. Becherini , D. Berge , K. Bernlöhr , R. Blackwell , M. Böttcher , C. Boisson , J. Bolmont , S. Bonnefoy , J. Bregeon , M. Breuhaus , F. Brun , P. Brun , M. Bryan , M. Büchele , T. Bulik , T. Bylund , M. Capasso , S. Caroff , A. Carosi , S. Casanova , M. Cerruti , T. Chand , S. Chandra , A. Chen , S. Colafrancesco , M. Curylo , I. D. Davids , C. Deil , J. Devin , P. deWilt , L. Dirson , A. Djannati-Ataï , A. Dmytriiev , A. Donath , V. Doroshenko , L. O'C. Drury , J. Dyks , K. Egberts , G. Emery , J. -P. Ernenwein , S. Eschbach , K. Feijen , S. Fegan , A. Fiasson , G. Fontaine , S. Funk , M. Füßling , S. Gabici , Y. A. Gallant , F. Gaté , G. Giavitto , D. Glawion , J. F. Glicenstein , D. Gottschall , M. -H. Grondin , J. Hahn , M. Haupt , G. Heinzelmann , G. Henri , G. Hermann , J. A. Hinton , W. Hofmann , C. Hoischen , T. L. Holch , M. Holler , D. Horns , D. Huber , H. Iwasaki , M. Jamrozy , D. Jankowsky , F. Jankowsky , A. Jardin-Blicq , I. Jung-Richardt , M. A. Kastendieck , K. Katarzynski , M. Katsuragawa , U. Katz , D. Khangulyan , B. Khélifi , J. King , S. Klepser , W. Kluzniak , Nu. Komin , K. Kosack , D. Kostunin , M. Kraus , G. Lamanna , J. Lau , A. Lemière , M. Lemoine-Goumard , J. -P. Lenain , E. Leser , C. Levy , T. Lohse , I. Lypova , J. Mackey , J. Majumdar , D. Malyshev , V. Marandon , A. Marcowith , A. Mares , C. Mariaud , G. Martí-Devesa , R. Marx , G. Maurin , P. J. Meintjes , A. M. W. Mitchell , R. Moderski , M. Mohamed , L. Mohrmann , J. Muller , C. Moore , E. Moulin , T. Murach , S. Nakashima , M. de Naurois , H. Ndiyavala , F. Niederwanger , J. Niemiec , L. Oakes , P. O'Brien , H. Odaka , S. Ohm , E. de Oña Wilhelmi , M. Ostrowski , I. Oya , M. Panter , R. D. Parsons , C. Perennes , P. -O. Petrucci , B. Peyaud , Q. Piel , S. Pita , V. Poireau , A. Priyana Noel , D. A. Prokhorov , H. Prokoph , G. Pühlhofer , M. Punch , A. Quirrenbach , S. Raab , R. Rauth , A. Reimer , O. Reimer , Q. Remy , M. Renaud , F. Rieger , L. Rinchiuso , C. Romoli , G. Rowell , B. Rudak , E. Ruiz-Velasco , V. Sahakian , S. Saito , D. A. Sanchez , A. Santangelo , M. Sasaki , R. Schlickeiser , F. Schüssler , A. Schulz , H. Schutte , U. Schwanke , S. Schwemmer , M. Seglar-Arroyo , M. Senniappan , A. S. Seyffert , N. Shafi , K. Shiningayamwe , R. Simoni , A. Sinha , H. Sol , A. Specovius , M. Spir-Jacob , L. Stawarz , R. Steenkamp , C. Stegmann , C. Steppa , T. Takahashi , T. Tavernier , A. M. Taylor , R. Terrier , D. Tiziani , M. Tluczykont , C. Trichard , M. Tsirou , N. Tsuji , R. Tuffs , Y. Uchiyama , D. J. van der Walt , C. van Eldik , C. van Rensburg , B. van Soelen , G. Vasileiadis , J. Veh , C. Venter , P. Vincent , J. Vink , F. Voisin , H. J. Völk , T. Vuillaume , Z. Wadiasingh , S. J. Wagner , R. White , A. Wierzcholska , R. Yang , H. Yoneda , M. Zacharias , R. Zanin , A. A. Zdziarski , A. Zech , A. Ziegler , J. Zorn , N. Zywucka , P. S. Smith

Detection of persistent VHE gamma-ray emission from PKS 1510-089 by the MAGIC telescopes during low states between 2012 and 2017

PKS 1510-089 is a flat spectrum radio quasar strongly variable in the optical and GeV range. We search for low-state VHE gamma-ray emission from PKS 1510-089. We aim to characterize and model the source in a broad-band context, which would…

Astrophysics of Galaxies · Physics 2018-11-21 MAGIC Collaboration , V. A. Acciari , S. Ansoldi , L. A. Antonelli , A. Arbet Engels , C. Arcaro , D. Baack , A. Babić , B. Banerjee , P. Bangale , U. Barres de Almeida , J. A. Barrio , J. Becerra González , W. Bednarek , E. Bernardini , A. Berti , J. Besenrieder , W. Bhattacharyya , C. Bigongiari , A. Biland , O. Blanch , G. Bonnoli , R. Carosi , G. Ceribella , A. Chatterjee , S. M. Colak , P. Colin , E. Colombo , J. L. Contreras , J. Cortina , S. Covino , P. Cumani , V. D'Elia , P. Da Vela , F. Dazzi , A. De Angelis , B. De Lotto , M. Delfino , J. Delgado , F. Di Pierro , A. Domínguez , D. Dominis Prester , D. Dorner , M. Doro , S. Einecke , D. Elsaesser , V. Fallah Ramazani , A. Fattorini , A. Fernández-Barral , G. Ferrara , D. Fidalgo , L. Foffano , M. V. Fonseca , L. Font , C. Fruck , S. Gallozzi , R. J. García López , M. Garczarczyk , M. Gaug , P. Giammaria , N. Godinović , D. Guberman , D. Hadasch , A. Hahn , T. Hassan , J. Herrera , J. Hoang , D. Hrupec , S. Inoue , K. Ishio , Y. Iwamura , H. Kubo , J. Kushida , D. Kuveždić , A. Lamastra , D. Lelas , F. Leone , E. Lindfors , S. Lombardi , F. Longo , M. López , A. López-Oramas , C. Maggio , P. Majumdar , M. Makariev , G. Maneva , M. Manganaro , K. Mannheim , L. Maraschi , M. Mariotti , M. Martínez , S. Masuda , D. Mazin , M. Minev , J. M. Miranda , R. Mirzoyan , E. Molina , A. Moralejo , V. Moreno , E. Moretti , V. Neustroev , A. Niedzwiecki , M. Nievas Rosillo , C. Nigro , K. Nilsson , D. Ninci , K. Nishijima , K. Noda , L. Nogués , S. Paiano , J. Palacio , D. Paneque , R. Paoletti , J. M. Paredes , G. Pedaletti , P. Peñil , M. Peresano , M. Persic , P. G. Prada Moroni , E. Prandini , I. Puljak , J. R. Garcia , W. Rhode , M. Ribó , J. Rico , C. Righi , A. Rugliancich , L. Saha , T. Saito , K. Satalecka , T. Schweizer , J. Sitarek , I. Šnidarić , D. Sobczynska , A. Somero , A. Stamerra , M. Strzys , T. Surić , F. Tavecchio , P. Temnikov , T. Terzić , M. Teshima , N. Torres-Albà , S. Tsujimoto , G. Vanzo , M. Vazquez Acosta , I. Vovk , J. E. Ward , M. Will , D. Zarić , C. M. Raiteri , A. Sandrinelli , T. Hovatta , S. Kiehlmann , W. Max-Moerbeck , M. Tornikoski , A. Lähteenmäki , J. Tammi , V. Ramakrishnan , C. Thum , I. Agudo , S. N. Molina , J. L. Gómez , A. Fuentes , C. Casadio , E. Traianou , I. Myserlis , J. -Y. Kim

Gamma-ray flaring activity of NGC 1275 in 2016-2017 measured by MAGIC

We report on the detection of flaring activity from the Fanaroff-Riley I radio galaxy NGC 1275 in very-high-energy (VHE, E $>$ 100 GeV) gamma rays with the MAGIC telescopes. Observations were performed between 2016 September and 2017…

High Energy Astrophysical Phenomena · Physics 2018-09-26 MAGIC collaboration , S. Ansoldi , L. A. Antonelli , C. Arcaro , D. Baack , A. Babić , B. Banerjee , P. Bangale , U. Barres de Almeida , J. A. Barrio , J. Becerra González , W. Bednarek , E. Bernardini , R. Ch. Berse , A. Berti , W. Bhattacharyya , C. Bigongiari , A. Biland , O. Blanch , G. Bonnoli , R. Carosi , G. Ceribella , A. Chatterjee , S. M. Colak , P. Colin , E. Colombo , J. L. Contreras , J. Cortina , S. Covino , P. Cumani , V. D'Elia , P. Da Vela , F. Dazzi , A. De Angelis , B. De Lotto , M. Delfino , J. Delgado , F. Di Pierro , A. Domínguez , D. Dominis Prester , D. Dorner , M. Doro , S. Einecke , D. Elsaesser , V. Fallah Ramazani , A. Fattorini , A. Fernández-Barral , G. Ferrara , D. Fidalgo , L. Foffano , M. V. Fonseca , L. Font , C. Fruck , D. Galindo , S. Gallozzi , R. J. García López , M. Garczarczyk , M. Gaug , P. Giammaria , N. Godinović , D. Gora , D. Guberman , D. Hadasch , A. Hahn , T. Hassan , M. Hayashida , J. Herrera , J. Hoang , J. Hose , D. Hrupec , K. Ishio , Y. Konno , H. Kubo , J. Kushida , A. Lamastra , D. Lelas , F. Leone , E. Lindfors , S. Lombardi , F. Longo , M. López , C. Maggio , P. Majumdar , M. Makariev , G. Maneva , M. Manganaro , K. Mannheim , L. Maraschi , M. Mariotti , M. Martínez , S. Masuda , D. Mazin , K. Mielke , M. Minev , J. M. Miranda , R. Mirzoyan , A. Moralejo , V. Moreno , E. Moretti , T. Nagayoshi , V. Neustroev , A. Niedzwiecki , M. Nievas Rosillo , C. Nigro , K. Nilsson , D. Ninci , K. Nishijima , K. Noda , L. Nogués , S. Paiano , J. Palacio , D. Paneque , R. Paoletti , J. M. Paredes , G. Pedaletti , P. Peñil , M. Peresano , M. Persic , K. Pfrang , P. G. Prada Moroni , E. Prandini , I. Puljak , J. R. Garcia , I. Reichardt , W. Rhode , M. Ribó , J. Rico , C. Righi , A. Rugliancich , L. Saha , T. Saito , K. Satalecka , T. Schweizer , J. Sitarek , I. Šnidarić , D. Sobczynska , A. Stamerra , M. Strzys , T. Surić , M. Takahashi , F. Tavecchio , P. Temnikov , T. Terzić , M. Teshima , N. Torres-Albà , S. Tsujimoto , G. Vanzo , M. Vazquez Acosta , I. Vovk , J. E. Ward , M. Will , D. Zarić , D. Glawion , L. O. Takalo , J. Jormanainen

The Statistic Analysis of GeV Spectral Breaks in Bright Gamma-Ray FSRQs

We present the statistic results of GeV spectral breaks of bright gamma-ray flat-spectrum radio quasars (FSRQs) in the energy range of 0.1-10 GeV based on New Pass 8 date of the Large Area Telescope abroad on Fermi Gamma-ray Space…

High Energy Astrophysical Phenomena · Physics 2024-12-04 K. R. Zhu , J. M. Chen , L. Zhang

On the Nature of MeV-blazars

Broad-band spectra of the FSRQ (flat-spectrum-radio quasars) detected in the high energy gamma-ray band imply that there may be two types of such objects: those with steep gamma-ray spectra, hereafter called MeV-blazars, and those with flat…

Astrophysics · Physics 2009-11-07 Marek Sikora , Michal Blazejowski , Rafal Moderski , Greg Madejski

Characteristic X-ray Variability of TeV Blazars: Probing the Link between the Jet and the Central Engine

We have studied the rapid X-ray variability of three extragalactic TeV gamma-ray sources; Mrk 421, Mrk 501 and PKS 2155-304. Analyzing the X-ray light curves obtained from ASCA and/or RXTE observations between 1993 and 1998, we have…

Astrophysics · Physics 2009-11-06 Jun Kataoka , Tadayuki Takahashi , Stefan J. Wagner , Naoko Iyomoto , Philip G. Edwards , Kiyoshi Hayashida , Susumu Inoue , Greg M. Madejski , Fumio Takahara , Chiharu Tanihata , Nobuyuki Kawai

Simultaneous multiwavelength observations of the second exceptional gamma-ray flare of PKS 2155-304 in July 2006

Simultaneous HESS/CHANDRA/optical observations were performed on the BL Lac object PKS 2155-304 in the night of July 29-30 2006, when the source underwent its second major gamma-ray outburst in Summer 2006. This event took place about 44…

Cosmology and Nongalactic Astrophysics · Physics 2015-05-13 HESS Collaboration , F. Aharonian

The first GeV flare of the radio-loud narrow-line Seyfert 1 galaxy PKS 2004-447

On 2019 October 25, the Fermi-Large Area Telescope observed the first gamma-ray flare from the radio-loud narrow-line Seyfert 1 (NLSy 1) galaxy PKS 2004$-$447 ($z=0.24$). We report on follow-up observations in the radio, optical-UV, and…

High Energy Astrophysical Phenomena · Physics 2021-05-19 Andrea Gokus , Vaidehi S. Paliya , Sarah M. Wagner , Sara Buson , Filippo D'Ammando , Philip G. Edwards , Matthias Kadler , Manuel Meyer , Roopesh Ojha , Jamie Stevens , Jörn Wilms

MAGIC observations of variable very-high-energy gamma-ray emission from PKS1510-089 during May 2015 outburst

PKS1510-089 is a flat spectrum radio quasar located at a redshift of 0.36. It is one of only a few such sources detected in very-high-energy (VHE, >100 GeV) gamma rays. Though PKS1510-089 is highly variable at GeV energies, until recently…

High Energy Astrophysical Phenomena · Physics 2017-08-22 Julian Sitarek , Josefa Becerra González , Vandad Fallah Ramazani Elina Lindfors , Giovanna Pedaletti , Fabrizio Tavecchio , Monica Vazquez Acosta , Stefan Larsson , the MAGIC Collaboration , the Fermi-LAT Collaboration , Kiran Baliyan , Navpreet Kaur , Sameer , Svetlana Jorstad , Claudia Raiteri

Fermi Large Area Telescope observations of GRB 110625A

Gamma-ray bursts (GRBs) that emit photons at GeV energies form a small but significant population of GRBs. However, the number of GRBs whose GeV-emitting period is simultaneously observed in X-rays remains small. We report gamma-ray…

High Energy Astrophysical Phenomena · Physics 2012-08-01 P. H. Thomas Tam , Albert K. H. Kong , Yi-Zhong Fan

Localising the gamma-ray emission region during the June 2014 outburst of 3C 454.3

In May - July 2014, the flat spectrum radio quasar 3C 454.3 exhibited strong flaring behaviour. Observations with the Large Area Telescope detector on-board the Fermi Gamma-ray Space Telescope captured the $\gamma$-ray flux at energies 0.1…

High Energy Astrophysical Phenomena · Physics 2016-03-23 Rosemary T. Coogan , Anthony M. Brown , Paula M. Chadwick

Multiwavelength observations of a VHE gamma-ray flare from PKS 1510-089 in 2015

Context. PKS 1510-089 is one of only a few flat spectrum radio quasars detected in the VHE (very-high-energy, > 100 GeV) gamma-ray band. Aims. We study the broadband spectral and temporal properties of the PKS 1510-089 emission during a…

High Energy Astrophysical Phenomena · Physics 2020-01-09 MAGIC Collaboration , M. L. Ahnen , S. Ansoldi , L. A. Antonelli , C. Arcaro , A. Babić , B. Banerjee , P. Bangale , U. Barres de Almeida , J. A. Barrio , J. Becerra González , W. Bednarek , E. Bernardini , A. Berti , B. Biasuzzi , A. Biland , O. Blanch , S. Bonnefoy , G. Bonnoli , F. Borracci , T. Bretz , R. Carosi , A. Carosi , A. Chatterjee , P. Colin , E. Colombo , J. L. Contreras , J. Cortina , S. Covino , P. Cumani , P. Da Vela , F. Dazzi , A. De Angelis , B. De Lotto , E. de Oña Wilhelmi , F. Di Pierro , M. Doert , A. Domínguez , D. Dominis Prester , D. Dorner , M. Doro , S. Einecke , D. Eisenacher Glawion , D. Elsaesser , M. Engelkemeier , V. Fallah Ramazani , A. Fernández-Barral , D. Fidalgo , M. V. Fonseca , L. Font , C. Fruck , D. Galindo , R. J. García López , M. Garczarczyk , M. Gaug , P. Giammaria , N. Godinović , D. Gora , D. Guberman , D. Hadasch , A. Hahn , T. Hassan , M. Hayashida , J. Herrera , J. Hose , D. Hrupec , G. Hughes , K. Ishio , Y. Konno , H. Kubo , J. Kushida , D. Kuveždić , D. Lelas , E. Lindfors , S. Lombardi , F. Longo , M. López , P. Majumdar , M. Makariev , G. Maneva , M. Manganaro , K. Mannheim , L. Maraschi , M. Mariotti , M. Martínez , D. Mazin , U. Menzel , R. Mirzoyan , A. Moralejo , E. Moretti , D. Nakajima , V. Neustroev , A. Niedzwiecki , M. Nievas Rosillo , K. Nilsson , K. Nishijima , K. Noda , L. Nogués , S. Paiano , J. Palacio , M. Palatiello , D. Paneque , R. Paoletti , J. M. Paredes , X. Paredes-Fortuny , G. Pedaletti , M. Peresano , L. Perri , M. Persic , J. Poutanen , P. G. Prada Moroni , E. Prandini , I. Puljak , J. R. Garcia , I. Reichardt , W. Rhode , M. Ribó , J. Rico , T. Saito , K. Satalecka , S. Schroeder , T. Schweizer , S. N. Shore , A. Sillanpää , J. Sitarek , I. Šnidarić , D. Sobczynska , A. Stamerra , M. Strzys , T. Surić , L. Takalo , F. Tavecchio , P. Temnikov , T. Terzić , D. Tescaro , M. Teshima , D. F. Torres , N. Torres-Albà , T. Toyama , A. Treves , G. Vanzo , M. Vazquez Acosta , I. Vovk , J. E. Ward , M. Will , M. H. Wu , D. Zarić , R. Desiante , J. Becerra González , F. D'Ammando , S. Larsson , C. M. Raiteri , R. Reinthal , A. Lähteenmäki , E. Järvelä , M. Tornikoski , V. Ramakrishnan , S. G. Jorstad , A. P. Marscher , V. Bala , N. R. MacDonald , N. Kaur , Sameer , K. Baliyan , J. A. Acosta-Pulido , C. Lazaro , C. Martínez-Lombilla , A. B. Grinon-Marin , A. Pastor Yabar , C. Protasio , M. I. Carnerero , H. Jermak , I. A. Steele , V. M. Larionov , G. A. Borman , T. S. Grishina

Exploring the blazar zone in High Energy flares of FSRQs

The gamma-ray emission offers a powerful diagnostic tool to probe jets and their surroundings in flat spectrum radio quasars (FSRQ). In particular, sources emitting at high energies (>10 GeV) give us the strongest constraints. This…

High Energy Astrophysical Phenomena · Physics 2015-06-18 L. Pacciani , F. Tavecchio , I. Donnarumma , A. Stamerra , L. Carrasco , E. Recillas , A. Porras , M. Uemura

Long-term monitoring of PKS 0537-441 with Fermi-LAT and multiwavelength observations

We report on multiwavelength observations of the blazar PKS 0537-441 (z = 0.896) obtained from microwaves through gamma rays by SMA, REM, ATOM, Swift and Fermi during 2008 August-2010 April. Strong variability has been observed in gamma…

High Energy Astrophysical Phenomena · Physics 2015-06-15 F. D'Ammando , E. Antolini , G. Tosti , J. Finke , S. Ciprini , S. Larsson , M. Ajello , S. Covino , D. Gasparrini , M. Gurwell , M. Hauser , P. Romano , F. Schinzel , S. J. Wagner , D. Impiombato , M. Perri , M. Persic , E. Pian , G. Polenta , B. Sbarufatti , A. Treves , S. Vercellone , A. Wehrle , A. Zook

Multi-wavelength Observations of PKS 2142-758 during an Active Gamma-Ray State

PKS 2142-758 is a flat spectrum radio quasar which emits few, weak but significant gamma-ray flares in the MeV through GeV energy range. The first flare occured on April 4th, 2010, when the source reached a daily flux of (1.1 \pm 0.3) *…

High Energy Astrophysical Phenomena · Physics 2012-06-11 Michael Dutka , Roopesh Ojha , Katja Pottschmidt , Justin Finke , Jamie Stevens , Jay Blanchard , Roberto Nesci , Philip Edwards , Jim Lovell , Matthias Kadler , Joern Wilms , Gino Tosti , Tapio Pursimo

Observation of the extremely bright flare of the FSRQ 3C279 with H.E.S.S. II

In June 2015, the Flat Spectrum Radio Quasar 3C 279 underwent an extremely bright gamma-ray flare, with an increase of the flux above 100 MeV by a factor 10 in less than 1 day, revealing an intrinsic variability timescale of 2 minutes as…

High Energy Astrophysical Phenomena · Physics 2019-08-14 C. Romoli , M. Zacharias , M. Meyer , F. Ait Benkhali , A. Jacholkowska , A. Wierzcholska , F. Jankowsky , J. P. Lenain

Gamma-ray Flare of PKS 1222+216 in 2010: Effect of Jet Dynamics at the Recollimation Zone

The $\gamma$-ray flare of PKS 1222+216, observed in June 2010, is interpreted as an outcome of jet dynamics at recollimation zone. We obtained the $\gamma$-ray light-curves in three different energy bands, namely, 100--300 MeV, 300 MeV--1…

High Energy Astrophysical Phenomena · Physics 2014-06-06 Pankaj Kushwaha , S. Sahayanathan , L. Resmi , K. P. Singh , S. Bhattacharyya , D. Bhattacharya