Sabyasachi Pal | The University of Western Australia (original) (raw)

Papers by Sabyasachi Pal

Research paper thumbnail of wift/XRT localization of the newly discovered transient IGR J17533-2928

The astronomer's telegram, Mar 1, 2021

Research paper thumbnail of Detection of softening of X-ray flare from GX 339-4 using MAXI/GSC

The astronomer's telegram, Feb 1, 2021

Research paper thumbnail of Effect of 2021 Assembly Election in India on Covid-19 Transmission

medRxiv (Cold Spring Harbor Laboratory), Jun 1, 2021

India is one of the countries in the world which is badly affected by the Covid-19 second wave. A... more India is one of the countries in the world which is badly affected by the Covid-19 second wave. Assembly election in four states and a union territory of India was taken place during March-May 2021 when the Covid-19 second wave was close to its peak and affected a huge number of people. We studied the impact of assembly election on the effective contact rate and the effective reproduction number of Covid-19 using different epidemiological models like SIR, SIRD, and SEIR. We also modeled the effective reproduction number for all election-bound states using different mathematical functions. We separately studied the case of all election-bound states and found all the states shown a distinct increase in the effective contact rate and the effective reproduction number during the election-bound time and just after that compared to pre-election time. States, where elections were conducted in single-phase, showed less increase in the effective contact rate and the reproduction number. The election commission imposed extra measures from the first week of April 2021 to restrict big campaign rallies, meetings, and different political activities. The effective contact rate and the reproduction number showed a trend to decrease for few states due to the imposition of the restrictions. We also compared the effective contact rate, and the effective reproduction number of all election-bound states and the rest of India and found all the parameters related to the spread of virus for election-bound states are distinctly high compared to the rest of India.

Research paper thumbnail of Discovery of a transient source close to a micro-quasar

40th COSPAR Scientific Assembly, 2014

Research paper thumbnail of Multi-frequency study of jet in HH 80-81

41st COSPAR Scientific Assembly, Jul 1, 2016

Research paper thumbnail of Broadband radio properties of Galactic micro-quasars during hard and soft states

42nd COSPAR Scientific Assembly, Jul 1, 2018

Research paper thumbnail of Search for transient radio sources near Galactic center region

Research paper thumbnail of Discovery of a young supernova remnant G354.4+0.0

Astronomical Society of India Conference Series, 2014

We have discovered a shell like structure G354.4+0.0 of size 1.6 which shows the morphology of a ... more We have discovered a shell like structure G354.4+0.0 of size 1.6 which shows the morphology of a shell supernova remnant. Part of the structure show polarized emission in NRAO VLA sky survey (NVSS) map. Based on 330 MHz, 1.4 GHz Giant Metrewave Radio Telescope (GMRT) observations and existing observations at higher frequencies, we conclude the partial shell structure showing synchrotron emission is embedded in an extended HII region of size ∼ 4. We find the spectrum of the diffuse HII region to turn over between 1.4 GHz and 330 MHz. HI absorption spectrum puts a lower limit of 5 kpc on its distance from Sun. Based on morphology, non-thermal polarized emission and size, this object is found to be one of the youngest supernova remnants discovered in the Galaxy with an estimated age of ∼ 100 − 500 years.

Research paper thumbnail of Discovery of one of the youngest supernova remnant G354.4 + 0.0

40th COSPAR Scientific Assembly, 2014

Research paper thumbnail of Multiwavelength observations of giant radio galaxy 3C 35 and 3C 284

41st COSPAR Scientific Assembly, Jul 1, 2016

Research paper thumbnail of Probing spectral and timing properties of the X-ray pulsar RX J0440.9+4431 in the giant outburst of 2022-2023

arXiv (Cornell University), May 31, 2023

The X-ray pulsar RX J0440.9+4431 went through a giant outburst in 2022 and reached a record-high ... more The X-ray pulsar RX J0440.9+4431 went through a giant outburst in 2022 and reached a record-high flux of 2.3 Crab, as observed by Swift/BAT. We study the evolution of different spectral and timing properties of the source using NICER observations. The pulse period is found to decrease from 208 s to 205 s, and the pulse profile evolves significantly with energy and luminosity. The hardness ratio and hardness intensity diagram (HID) show remarkable evolution during the outburst. The HID turns towards the diagonal branch from the horizontal branch above a transition (critical) luminosity, suggesting the presence of two accretion modes. Each NICER spectrum can be described using a cutoff power law with a blackbody component and a Gaussian at 6.4 keV. At higher luminosities, an additional Gaussian at 6.67 keV is used. The observed photon index shows negative and positive correlations with X-ray flux below and above the critical luminosity, respectively. The evolution of spectral and timing parameters suggests a possible change in the emission mechanism and beaming pattern of the pulsar depending on the spectral transition to sub-and super-critical accretion regimes. Based on the critical luminosity, the magnetic field of the neutron star can be estimated in the order of 10 12 or 10 13 G, assuming different theoretical models. Moreover, the observed iron emission line evolves from a narrow to a broad feature with luminosity. Two emission lines originating from neutral and highly ionized Fe atoms were evident in the spectra around 6.4 keV and 6.67 keV (higher luminosities).

Research paper thumbnail of Enhanced activity from Centaurus X-3 as detected by Fermi/GBM

The astronomer's telegram, Mar 1, 2021

Research paper thumbnail of Spectral ageing properties of giant radio galaxy Pictor A

41st COSPAR Scientific Assembly, Jul 1, 2016

Research paper thumbnail of X-ray pulsar A 0535+262 reached 3 Crab: update with Neil Gehrels Swift Observatory

The astronomer's telegram, Nov 1, 2020

Research paper thumbnail of Multi-frequency properties of an narrow angle tail radio galaxy J 0037+18

Research paper thumbnail of RAD@home citizen science discovery of an active galactic nucleus spewing a large unipolar radio bubble on to its merging companion galaxy

Monthly Notices of the Royal Astronomical Society: Letters

Active galactic nucleus (AGN) feedback during galaxy merger has been the most favoured model to e... more Active galactic nucleus (AGN) feedback during galaxy merger has been the most favoured model to explain black hole–galaxy co-evolution. However, how the AGN-driven jet/wind/radiation is coupled with the gas of the merging galaxies, which leads to positive feedback, momentarily enhanced star formation, and subsequently negative feedback, a decline in star formation, is poorly understood. Only a few cases are known where the jet and companion galaxy interaction leads to minor off-axis distortions in the jets and enhanced star formation in the gas-rich minor companions. Here, we briefly report one extraordinary case, RAD12, discovered by RAD@home citizen science collaboratory, where for the first time a radio jet–driven bubble (∼ 137 kpc) is showing a symmetric reflection after hitting the incoming galaxy which is not a gas-rich minor but a gas-poor early-type galaxy in a major merger. Surprisingly, neither positive feedback nor any radio lobe on the counter jet side, if any, is detect...

Research paper thumbnail of Swift/MAXI follow-up of recent outburst from X-ray pulsar 2S 1553-542

The astronomer's telegram, 2021

Research paper thumbnail of Radio Identifications in the Nep Deep Field

Publications of The Korean Astronomical Society, Mar 31, 2017

For guidance on citations see FAQs.

Research paper thumbnail of Long term X-ray/radio monitoring of Galactic micro-quasar Cygnus X-1

42nd COSPAR Scientific Assembly, Jul 1, 2018

Research paper thumbnail of Low Frequency GMRT Observations of Microquasar V4641 Sgr

arXiv (Cornell University), Jan 15, 2007

We have observed the micro-quasar V4641 Sgr (SAX J1819.3-2525) at the time of flare during May 20... more We have observed the micro-quasar V4641 Sgr (SAX J1819.3-2525) at the time of flare during May 2002, using the Giant Metrewave Radio Telescope at low radio frequencies (244 and 610 MHz). This is the lowest frequency radio detection of the source. The source showed clear signature of spectral evolution from optically thick to thin state. We model the spectral evolution of the source to obtain some of the source parameters. Assuming the value of magnetic field strength to be in the range of 0.1 to 1 Gauss, we obtain the initial velocity of expansion of the jet to be in the range ∼ 0.45c to 0.25c. This is consistent with the known expansion velocities of the jet with other microquasars.

Research paper thumbnail of wift/XRT localization of the newly discovered transient IGR J17533-2928

The astronomer's telegram, Mar 1, 2021

Research paper thumbnail of Detection of softening of X-ray flare from GX 339-4 using MAXI/GSC

The astronomer's telegram, Feb 1, 2021

Research paper thumbnail of Effect of 2021 Assembly Election in India on Covid-19 Transmission

medRxiv (Cold Spring Harbor Laboratory), Jun 1, 2021

India is one of the countries in the world which is badly affected by the Covid-19 second wave. A... more India is one of the countries in the world which is badly affected by the Covid-19 second wave. Assembly election in four states and a union territory of India was taken place during March-May 2021 when the Covid-19 second wave was close to its peak and affected a huge number of people. We studied the impact of assembly election on the effective contact rate and the effective reproduction number of Covid-19 using different epidemiological models like SIR, SIRD, and SEIR. We also modeled the effective reproduction number for all election-bound states using different mathematical functions. We separately studied the case of all election-bound states and found all the states shown a distinct increase in the effective contact rate and the effective reproduction number during the election-bound time and just after that compared to pre-election time. States, where elections were conducted in single-phase, showed less increase in the effective contact rate and the reproduction number. The election commission imposed extra measures from the first week of April 2021 to restrict big campaign rallies, meetings, and different political activities. The effective contact rate and the reproduction number showed a trend to decrease for few states due to the imposition of the restrictions. We also compared the effective contact rate, and the effective reproduction number of all election-bound states and the rest of India and found all the parameters related to the spread of virus for election-bound states are distinctly high compared to the rest of India.

Research paper thumbnail of Discovery of a transient source close to a micro-quasar

40th COSPAR Scientific Assembly, 2014

Research paper thumbnail of Multi-frequency study of jet in HH 80-81

41st COSPAR Scientific Assembly, Jul 1, 2016

Research paper thumbnail of Broadband radio properties of Galactic micro-quasars during hard and soft states

42nd COSPAR Scientific Assembly, Jul 1, 2018

Research paper thumbnail of Search for transient radio sources near Galactic center region

Research paper thumbnail of Discovery of a young supernova remnant G354.4+0.0

Astronomical Society of India Conference Series, 2014

We have discovered a shell like structure G354.4+0.0 of size 1.6 which shows the morphology of a ... more We have discovered a shell like structure G354.4+0.0 of size 1.6 which shows the morphology of a shell supernova remnant. Part of the structure show polarized emission in NRAO VLA sky survey (NVSS) map. Based on 330 MHz, 1.4 GHz Giant Metrewave Radio Telescope (GMRT) observations and existing observations at higher frequencies, we conclude the partial shell structure showing synchrotron emission is embedded in an extended HII region of size ∼ 4. We find the spectrum of the diffuse HII region to turn over between 1.4 GHz and 330 MHz. HI absorption spectrum puts a lower limit of 5 kpc on its distance from Sun. Based on morphology, non-thermal polarized emission and size, this object is found to be one of the youngest supernova remnants discovered in the Galaxy with an estimated age of ∼ 100 − 500 years.

Research paper thumbnail of Discovery of one of the youngest supernova remnant G354.4 + 0.0

40th COSPAR Scientific Assembly, 2014

Research paper thumbnail of Multiwavelength observations of giant radio galaxy 3C 35 and 3C 284

41st COSPAR Scientific Assembly, Jul 1, 2016

Research paper thumbnail of Probing spectral and timing properties of the X-ray pulsar RX J0440.9+4431 in the giant outburst of 2022-2023

arXiv (Cornell University), May 31, 2023

The X-ray pulsar RX J0440.9+4431 went through a giant outburst in 2022 and reached a record-high ... more The X-ray pulsar RX J0440.9+4431 went through a giant outburst in 2022 and reached a record-high flux of 2.3 Crab, as observed by Swift/BAT. We study the evolution of different spectral and timing properties of the source using NICER observations. The pulse period is found to decrease from 208 s to 205 s, and the pulse profile evolves significantly with energy and luminosity. The hardness ratio and hardness intensity diagram (HID) show remarkable evolution during the outburst. The HID turns towards the diagonal branch from the horizontal branch above a transition (critical) luminosity, suggesting the presence of two accretion modes. Each NICER spectrum can be described using a cutoff power law with a blackbody component and a Gaussian at 6.4 keV. At higher luminosities, an additional Gaussian at 6.67 keV is used. The observed photon index shows negative and positive correlations with X-ray flux below and above the critical luminosity, respectively. The evolution of spectral and timing parameters suggests a possible change in the emission mechanism and beaming pattern of the pulsar depending on the spectral transition to sub-and super-critical accretion regimes. Based on the critical luminosity, the magnetic field of the neutron star can be estimated in the order of 10 12 or 10 13 G, assuming different theoretical models. Moreover, the observed iron emission line evolves from a narrow to a broad feature with luminosity. Two emission lines originating from neutral and highly ionized Fe atoms were evident in the spectra around 6.4 keV and 6.67 keV (higher luminosities).

Research paper thumbnail of Enhanced activity from Centaurus X-3 as detected by Fermi/GBM

The astronomer's telegram, Mar 1, 2021

Research paper thumbnail of Spectral ageing properties of giant radio galaxy Pictor A

41st COSPAR Scientific Assembly, Jul 1, 2016

Research paper thumbnail of X-ray pulsar A 0535+262 reached 3 Crab: update with Neil Gehrels Swift Observatory

The astronomer's telegram, Nov 1, 2020

Research paper thumbnail of Multi-frequency properties of an narrow angle tail radio galaxy J 0037+18

Research paper thumbnail of RAD@home citizen science discovery of an active galactic nucleus spewing a large unipolar radio bubble on to its merging companion galaxy

Monthly Notices of the Royal Astronomical Society: Letters

Active galactic nucleus (AGN) feedback during galaxy merger has been the most favoured model to e... more Active galactic nucleus (AGN) feedback during galaxy merger has been the most favoured model to explain black hole–galaxy co-evolution. However, how the AGN-driven jet/wind/radiation is coupled with the gas of the merging galaxies, which leads to positive feedback, momentarily enhanced star formation, and subsequently negative feedback, a decline in star formation, is poorly understood. Only a few cases are known where the jet and companion galaxy interaction leads to minor off-axis distortions in the jets and enhanced star formation in the gas-rich minor companions. Here, we briefly report one extraordinary case, RAD12, discovered by RAD@home citizen science collaboratory, where for the first time a radio jet–driven bubble (∼ 137 kpc) is showing a symmetric reflection after hitting the incoming galaxy which is not a gas-rich minor but a gas-poor early-type galaxy in a major merger. Surprisingly, neither positive feedback nor any radio lobe on the counter jet side, if any, is detect...

Research paper thumbnail of Swift/MAXI follow-up of recent outburst from X-ray pulsar 2S 1553-542

The astronomer's telegram, 2021

Research paper thumbnail of Radio Identifications in the Nep Deep Field

Publications of The Korean Astronomical Society, Mar 31, 2017

For guidance on citations see FAQs.

Research paper thumbnail of Long term X-ray/radio monitoring of Galactic micro-quasar Cygnus X-1

42nd COSPAR Scientific Assembly, Jul 1, 2018

Research paper thumbnail of Low Frequency GMRT Observations of Microquasar V4641 Sgr

arXiv (Cornell University), Jan 15, 2007

We have observed the micro-quasar V4641 Sgr (SAX J1819.3-2525) at the time of flare during May 20... more We have observed the micro-quasar V4641 Sgr (SAX J1819.3-2525) at the time of flare during May 2002, using the Giant Metrewave Radio Telescope at low radio frequencies (244 and 610 MHz). This is the lowest frequency radio detection of the source. The source showed clear signature of spectral evolution from optically thick to thin state. We model the spectral evolution of the source to obtain some of the source parameters. Assuming the value of magnetic field strength to be in the range of 0.1 to 1 Gauss, we obtain the initial velocity of expansion of the jet to be in the range ∼ 0.45c to 0.25c. This is consistent with the known expansion velocities of the jet with other microquasars.