373 research outputs found
First person – Poonam Sehgal
No full textABSTRACT
First Person is a series of interviews with the first authors of a selection of papers published in Journal of Cell Science, helping early-career researchers promote themselves alongside their papers. Poonam Sehgal is the first author on ‘Epidermal growth factor receptor and integrins control force-dependent vinculin recruitment to E-cadherin junctions’, published in Journal of Cell Science. The work in this article was carried out while Poonam was a postdoc in the lab of Dr Deborah Leckband at University of Illinois, Urbana-Champaign, USA, investigating the mechanism of E-cadherin-mediated force-transduction signaling in epithelial cells.</jats:p
Targeting two 2-year old Type IIn supernovae with Chandra
No full textType IIn supernovae (SNe IIn) are a heterogeneous class of supernovae (SNe) believed to be powered by vigorous circumstellar (CS) interaction. Their evolutionary status remains an open question. X-rays are the product of CS interaction and hence offer unique constraints on the shock energetics and CS medium characteristics. X-rays are a direct probe of the column density and abundances, and therefore also of the total mass loss of the progenitor star during the last phases before explosion. In SNe IIn late time optical and IR emission is also powered by the CS interaction, providing a good indicator for the X-ray brightness. In this proposal we request 80 ks of Chandra ACIS time to observe optically bright (m_v~14) SNe IIn SN 2017hcc and 2017gas
Floristic diversity in Cold Desert regions of Uttarakhand Himalaya, India
No full textSekar, Chandra, Pandey, Aseesh, Giri, Lalit, Joshi, Bhaskar Chandra, Bhatt, Deepika, Bhojak, Puja, Dey, Dipti, Thapliyal, Neha, Bisht, Kapil, Bisht, Monika, Negi, Vikram Singh, Mehta, Poonam (2022): Floristic diversity in Cold Desert regions of Uttarakhand Himalaya, India. Phytotaxa 537 (1): 1-62, DOI: 10.11646/phytotaxa.537.1.
"There are millions of young Indians who feel frustrated with their lives.... I really want the Indian government to take these frustrations seriously." - Snigdha Poonam
No full textLSE South Asia Centre recently invited Snigdha Poonam, journalist at Hindustan Times and author of Dreamers: How Young Indians are Changing the World, for a panel discussion entitled 'Who are the Middle Class in South Asia?' as part of the South Asia Summit 2018. In conversation with Anirbaan Banerjee, she talks about aggressive Indian nationalism, the political and economic frustrations of the middle-class young Indian, women's aspirations and a growing social-political crisis in India
FIGURE. Map of the study area (Source: http://srtm.csi.cgiar.org) in Floristic diversity in Cold Desert regions of Uttarakhand Himalaya, India
No full textFIGURE. Map of the study area (Source: http://srtm.csi.cgiar.org)Published as part of Sekar, Chandra, Pandey, Aseesh, Giri, Lalit, Joshi, Bhaskar Chandra, Bhatt, Deepika, Bhojak, Puja, Dey, Dipti, Thapliyal, Neha, Bisht, Kapil, Bisht, Monika, Negi, Vikram Singh & Mehta, Poonam, 2022, Floristic diversity in Cold Desert regions of Uttarakhand Himalaya, India, pp. 1-62 in Phytotaxa 537 (1) on page 4, DOI: 10.11646/phytotaxa.537.1.1, http://zenodo.org/record/633195
FIGURE. Dominant families of cold desert of Uttarakhand. in Floristic diversity in Cold Desert regions of Uttarakhand Himalaya, India
No full textFIGURE. Dominant families of cold desert of Uttarakhand.Published as part of Sekar, Chandra, Pandey, Aseesh, Giri, Lalit, Joshi, Bhaskar Chandra, Bhatt, Deepika, Bhojak, Puja, Dey, Dipti, Thapliyal, Neha, Bisht, Kapil, Bisht, Monika, Negi, Vikram Singh & Mehta, Poonam, 2022, Floristic diversity in Cold Desert regions of Uttarakhand Himalaya, India, pp. 1-62 in Phytotaxa 537 (1) on page 51, DOI: 10.11646/phytotaxa.537.1.1, http://zenodo.org/record/633195
Circumstellar Interaction Of Young Supernovae : With inputs From Radio And X-ray Wavebands
No full textThis thesis deals with the radiative emission arising out of the interaction of several core collapse supernovae (SNe) with their dense circumstellar medium (CSM) and uses the radiative properties as diagnostics of the ionized plasma in and around the interaction region. These supernovae include: SN 1993J, SN 1995N, SN 2002ap & SN 2003bg. In a SN explosion, the outer layers of the star are set in motion with high velocities and the collision of the ejecta with the CSM leads to a less dense and hot blast wave forward shock with velocities ~ 20,000 km/s and T ~ 109K. When the external layers of the expanding ejecta decelerate upon interaction with the CSM, a reverse shock develops that starts propagating into the stellar envelope, with velocity few times 1000 km/s relative to the expanding stellar ejecta, heating it to T ~ 107K. Forward shock velocities are typically 1000 times the speed of the wind that was being lost from the progenitor prior to the SN explosion. Consequently, evolution of the shock and the radiative properties of the SN few years after the explosion probes the history of the environment of the progenitor star thousands of years before the explosion. Interaction of the shocked ejecta with the CSM gives rise to emission in radio and X-ray bands. The emission in various wavebands arising due to this interaction usually has a slower decay rate than the initial photospheric emission arising from ionic recombination and radioactive decay. Hence, one is usually able to track the supernovae for longer time.
Radio emission is generated from the forward shocked shell due to the synchrotron emission by relativistic electrons in the presence of the strong magnetic fields. The strong magnetic field in the shocked shell is believed to be generated by Rayleigh Taylor instability, which enhances any seed magnetic field present initially. Relativistic electrons are produced, most likely, by shock mediated acceleration processes. The early rapid rise in the radio flux density results from the shock overtaking progressively further into the progenitor’s stellar wind and therefore in the regions of decreasing optical depth. Since optical depth is larger at lower frequencies lower frequencies turn on later in time. The emission from the shocked region decreases slowly with time as the shock expands, so even when the radio absorption has become negligible, the radio light curve would show this decline. Radio emission is absorbed initially by different mechanisms depending upon the mass loss rate in the progenitor wind, shock velocity, electron temperature etc. If the emission is absorbed by an electron moving in the field of an ion (free-free absorption), then one can infer the mass loss of the progenitor. If on the other hand, the radio emission is absorbed by synchrotron self absorption in which the photon interacts with an electron in a magnetic field, then it gives information about the size of the emitting region.
In contrast, X-ray emission initially comes from the forward shock and is non-thermal in nature. The X-rays could be either due to synchrotron emission or due to inverse Compton scattering in which the photospheric optical photons can be boosted to X-ray energies due to multiple scattering with the electrons. Late time X-rays, which are thermal in nature, arise from the reverse shock and probe the CSM interaction of the SN ejecta and provide information of the plasma and surroundings. However, in an alternate model due to Chugai (1993, Astron. Rep., 41, 672), X-rays can also emerge from the radiative cooling of the shocked, dense clumps (clouds) embedded in the circumstellar wind overtaken by the blast-wave shock and crushed by the pressure of the strongly shocked wind. Line-widths, elemental yields and luminosity curves are the observational signatures to distinguish between the two models. X-ray spectra of SNe can be used to determine what elements are there in the shock heated ejecta, the supernova’s nucleosynthetic yield and thence the (helium) core mass of the progenitor at the explosion stage. Nucleosynthetic studies of SNe and constraints on their progenitor masses are of vital interest to the origin and distribution of elements in the galaxy and its chemical evolution.
Chapter 1 gives a general overview of supernova types, explosion scenarios and essentials of shock dynamics in the CSM. Chapter 2 gives an overview of the radiative processes relevant to radio and X-ray emission. I describe the radio and X-ray data analysis procedures in Chapter 3.
I discuss synchrotron aging in young supernovae in Chapter 4. Synchrotron aging has been seen in many old sources, such as radio galaxies, Compact Steep Spectrum sources etc., where the age of the source is not known. Synchrotron aging was used to determine the age of such sources using magnetic field under equipartition (between magnetic energy density and relativistic energy density) as an input parameter. However in young supernovae (whose ages are known), the magnetic fields are generated due to the instabilities (Rayleigh Taylor) created in the plasma and hence it is difficult to estimate the field correctly. Here synchrotron aging can be used to derive the magnetic field independent of any assumption of equipartition. I discuss the synchrotron aging in detail and derive a significant conclusion about the plasma energetics from the combined GMRT and Very Large Array (VLA) spectrum of a 10 year old type IIb SN 1993J around day 3200 after explosion. I found a steepening of its spectrum caused by synchrotron aging. After taking into account the adiabatic losses and Fermi acceleration of electrons, I estimate from the synchrotron break, the magnetic field in the plasma and derive that the magnetic energy density is 10,000 times larger than the relativistic energy density. In Chapter 4, I also underscore the importance of wide band radio spectrum in dealing with issues of the physics of shocked plasma.
In Chapter 5, I describe further studies of SN 1993J with the GMRT at frequencies 1420, 610, 325 and 235 MHz, from 7.5 years to 10 years since explosion. SN 1993J is a unique supernova for which magnetic field and sizes are determined from model independent measurements; the former from the synchrotron cooling break and the latter from VLBI measurements. Using GMRT spectra and earlier published spectra of SN 1993J, I compare the VLBI sizes of the SN 1993J with that of obtained from the peak of the spectra using synchrotron self absorption (SSA) model. I find that the SSA sizes are roughly equal to the VLBI sizes of the SN. This suggests that the synchrotron self absorption is responsible for the turn over in the spectra of SN 1993J at all the epochs. The size evolution shows that the ejecta expands freely initially and then show a small deceleration in the later epochs. I also plot the magnetic field evolution, which goes as Spectral index initially lies between 0.8 - 1.0 and later seems to flatten with time and lies within the range of 0.5 - 0.7. The mass loss rate roughly remains constant ( ~ 5 x 10−5M yr−1) in two years of GMRT observations, i.e., 8000-10,000 years before explosion. Light curves based on high frequency existing models extrapolated to low frequencies overpredict the flux densities at low frequencies. Some extra opacity is needed to incorporate the difference. This suggests that the low frequency opacity in SN 1993J is not a simple extrapolation of high frequency opacity and a hitherto unaccounted for absorption may be at work at low frequencies.
I describe the Chandra X-ray observatory work on SN 1995N, which we observed on March 28, 2004, in Chapter 6. I detected the X-ray emission from the SN with most of the emission found to be below 2 keV. SN 1995N had also been observed by ROSAT and ASCA earlier on three occasions. Our reanalysis of ASCA 1998 spectra revealed certain line features which were not reported in the published work of Fox et al. (2000, MNRAS, 319, 1154). I detect a Ne X line in both ASCA and Chandra observations, and while I detect a Ne IX line in the Chandra observation this was absent in the ASCA one. At the same time I detect a 1.3 keV line in the ASCA observation, absent in the Chandra spectrum of SN 1995N. No Fe line was detected in either spectrum. The light curves of SN 1995N suggested a non-linear profile due to high ASCA flux. We re-analyzed the ASCA data in view of the high-resolution imaging data obtained by Chandra and found at least ten more sources contributing to the SN flux due to the large ASCA PSF. After taking out the contribution from the contaminating sources, the light curve appears to be consistent with a linear decline. This indicates that the X-ray emission is due to the reverse shock going through a shallow ejecta profile. I also find that the absorption column density is at least 2.5 times more than that calculated from the galactic extinction maps. This suggests that the moderate, extra absorption is likely to be due to the formation of a thin cool ejecta-shell between reverse-shock and the contact discontinuity. About 0.01 M of Ne is estimated to be present in SN 1995N from the Chandra line detection. This, most likely, arises in the partially burnt He core at velocities > 5000 km s−1 . I also observed SN 1995N with the GMRT in radio bands. I describe these results in Chapter 7. The spectrum is seen to be peaking towards lower frequencies with the time. The radio light curve suggests that the SN is already in the optically thin part of the light curve.
Some but not all type Ic supernovae have shown association with Gamma Ray Burst (GRB) sources. This seems to divide the type Ic SNe in two subclasses -the ones associated with GRBs and the ones without the GRB connection. The observations of these two classes of type Ic SNe and their comparison (the ”afterglows”) are likely to be useful in determining the physical conditions inside the progenitor star which leads some of the type Ic supernovae to have GRB associations. Since these SNe are bare core SNe with no hydrogen and little or no helium envelope, their prompt emission in the radio and high energy bands provide the most promising probes of their interior at early times. In Chapter 8, I investigate the origin of prompt X-ray emission in a type Ic supernova SN 2002ap, a non-GRB supernova. An analysis of SN 2002ap, observed with XMM-Newton on Feb 3, 2002 as a Target Of Opportunity is presented and spectral model fits to the prompt X-ray emission are obtained. I model the early X-ray emission with inputs from optical photometry and light curve and find that multiple inverse Compton scattering of optical photons from the supernova photosphere by electrons in the medium can account for the observed early X-ray flux and its spectrum for modest electron temperatures and optical depths. I compare the X-ray image with the GMRT 610 MHz radio image obtained three days apart. While I find no radio counterpart of the SN at such low frequencies, several sources in the field have radio and X-ray counterparts. I compare the radio data obtained from three different supernovae in their early phases and model these using the synchrotron self absorption model. GRB associated SN 1998bw was found to be most rapidly expanding with fastest transition from optically thick to optically thin part in the spectrum.
Radio studies of SN 2003bg, another type Ic supernova is discussed in Chapter 9. I observed SN 2003bg with the GMRT from day 43 till day 600 since explosion. On one occasion (day ~ 350), I combined the GMRT data with the VLA data to get a composite spectrum. From the optically thick part of the spectrum, I find that the dominant absorption mechanism in the SN is synchrotron self absorption. I deduced magnetic field and size of the supernova under the assumption of equipartition.
I discuss the overall results in Chapter 10. In this thesis, I have investigated four supernovae in detail and few more have been observed with lesser sampling frequency (see Chapter 10 and P. Chandra et al 2002, BASI 30, 755). Although they all belong to the ejecta dominated free expansion phase, the core-collapse supernovae are of widely different subclasses and I have observed them at very young ages (few days) to more than 10 years of age, with multiple probes, going through a variety of emission mechanisms and absorption processes. I provide comparison between different supernovae observed by us and others. In cases, where I am able to obtain X-ray spectra, nucleosynthesis arguments lead me to constrain the mass of the progenitor star and the composition of its layers
1000 Days of the Lowest-frequency Emission from the Low-luminosity GRB 171205A
No full textWe report the lowest-frequency measurements of gamma-ray burst (GRB) 171205A with the upgraded Giant Metrewave Radio Telescope (uGMRT) covering a frequency range of 250–1450 MHz and a period of 4–937 days. It is the first GRB afterglow detected in the 250–500 MHz frequency range and the second brightest GRB detected with the uGMRT. Even though the GRB was observed for nearly 1000 days, there is no evidence of a transition to a nonrelativistic regime. We also analyzed the archival Chandra X-ray data on day ∼70 and day ∼200. We also found no evidence of a jet break from the analysis of combined data. We fit synchrotron afterglow emission arising from a relativistic, isotropic, self-similar deceleration as well as from a shock breakout of a wide-angle cocoon. Our data also allowed us to discern the nature and the density of the circumburst medium. We found that the density profile deviates from a standard constant density medium and suggests that the GRB exploded in a stratified wind-like medium. Our analysis shows that the lowest-frequency measurements covering the absorbed part of the light curves are critical to unraveling the GRB environment. Our data combined with other published measurements indicate that the radio afterglow has a contribution from two components: a weak, possibly slightly off-axis jet and a surrounding wider cocoon, consistent with the results of Izzo et al. The cocoon emission likely dominates at early epochs, whereas the jet starts to dominate at later epochs, resulting in flatter radio light curves
Circumstellar Interaction in Supernovae in Dense Environments—An Observational Perspective
No full textIn a supernova explosion, the ejecta interacting with the surrounding circumstellar medium ( CSM) give rise to variety of radiation. Since CSM is created from the mass loss from the progenitor, it carries footprints of the late time evolution of the star. This is one of the unique ways to get a handle on the nature of the progenitor system. Here, I will focus mainly on the supernovae ( SNe) exploding in dense environments, a.k.a. Type IIn SNe. Radio and X-ray emission from this class of SNe have revealed important modifications in their radiation properties, due to the presence of high density CSM. Forward shock dominance in the X-ray emission, internal free-free absorption of the radio emission, episodic or non-steady mass loss rate, and asymmetry in the explosion seem to be common properties of this class of SNe.</p
Radio and X-ray observations of supernovae in dense environments
No full textAbstractHere we discuss the observational properties of supernovae exploding in extremely dense environments, namely Type IIn supernovae (SNe IIn). In SNe IIn, the surrounding environments play significant role in the supernovae energetics and evolution. Thus they are different than other classes of core collapse supernovae, whose energetics are not significantly altered by their environments. Though high density of medium is a prerequisite for radio and X-ray emission, less than 10% on SNe IIn are bright in these bands. This has important implications for their progenitor models. I will discuss the radio and X-ray observations of SNe IIn, which are crucial to unravel their complex environments. We also discuss some individual supernovae belonging to this class and discuss as to how they have refined our understanding of SNe IIn. Finally the importance of well sampled long term light curves in radio and X-ray bands cannot be stressed enough.</jats:p
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