Welcome to

2008 Nanjing GRB Conference

Welcome

T Lu

It is a great pleasure for me to be honored to give this address and talk about some remarks concerning this conference. As a local colleague, I would like to warmly welcome all of you participating this meeting. I am particularly happy that so many distinguished astronomers and astrophysicists in the field of GRBs all over the world are gathering here to give their interesting talks and posters, and participate two evening discussions on focused questions.

This grand meeting reminds me that 22 years ago, also in Nanjing, I participated in organizing an IAU Symposium No.125 on “Origin and Evolution of Neutron Stars”. I quoted a sentence:“lots of worthies are all coming and bothyounger and elder people are gathering here”from the famous essay “Preface of Anthology of Orchid Pavilion” written by Wang Xizhi (321-379), a great Chinese calligrapher and writer of Jin Dynasty, to describe that symposium. I think, this sentence is also OK to describe our present conference.

王羲之

兰亭集序

5 periods in research history of GRBs

GRB is a rapidly developing field among the astrophysics. There have been 5 periods in its research history, including:

• Discovery era (1967~1978)

• Perplexity era (1979~1991)

• Dawn era (1992~1996)

• BeppoSAX era (1997~2004)

• Swift era (2005 up to now)

Discovery era

• 1) Discovery era (1967~1978): In this period, based on the data of Vela satellites, Klebesadel et al discovered for the first time some events of gamma-ray bursts. Many more GRBs were subsequently observed by various astronomical satellites. GRB light curves are highly variable. Their rise time could be much shorter than one second, even than one milli-second. This indicated, the scales of GRBs should be smaller than hundred kilometers, thus they should be stellar phenomena related with compact stars.

Perplexity era

• 2) Perplexity era (1979~1991): Suspicious discoveries of cyclotron absorption lines and gravitational red-shifted annihilation emission lines in this era both lead to the misunderstanding that the GRBs came from the local processes of magnetic instabilities on galactic neutron stars. This situation was reflected in the book "Gamma-Ray Bursts: Observations, Analyses and Theories", eds. Ho, Epstein and Fenimore, 1992. However, B. Paczynski and J. Goodman held pioneering different ideas, they predicted GRBs being cosmological events.

Dawn era - 1• 3) Dawn era (1992~1996): So

on after the launch of the Compton GRO, the spatial distribution of GRBs detected by BATSE, appeared to be highly isotropic and have a paucity of weak bursts. This observation statistically supports the cosmological origin of GRBs. Based on cosmological origin, Meszaros and Rees even proposed a fire ball model to describe the GRBs which later is known as the standard model, or known as the internal-external shock model. The existence of afterglows is also predicted.

Compton GRO

Dawn era - 2

• However, though this support is strong, but not yet final, as the GRB distances have not yet been directly measured, and the isotropic spatial distribution of GRBs could be also approximately explained as galactic dark halo sources. In 1995, a great debate on cosmological-galactic distances for GRBs happened.

• Also very important, in this era, GRB numbers and BATSE data had been accumulated greatly, and there appear clearly to be two types of GRBs with boundary at about duration of 2 seconds, long GRB and short GRB seem to be quite different.

BeppoSAX era - 1

• 4) BeppoSAX era (1997~2004): In this era, the most important thing is that BeppoSAX, launched in 1996, measured the precise positions of GRBs, which led to the discoveries of various afterglows with much longer durations, X-ray afterglow might be several weeks, optical might be several months and radio might also be several months. These features provide valuable information on the nature of GRBs, especially the red-shifts of afterglows can easily be measured which directly lead to the conclusion that GRBs are cosmological origin.

• They also open a “New Era of Afterglow Physics”, during which great achievements were obtained.

BeppoSAX era - 2

• Also in this era, the standard fireball model has been proved to be basically but not exactly correct. The problem lies in that the assumptions for the model are too simple. This provides a new chance to study post-standard effects. These studies led to the environments of some GRBs to be “wind-like”, indicating their progenitors should be massive stars. The discoveries of GRB/SN associations (also by HETE II) support finally this idea that GRB originates from massive star collapsing, known as hypernova. Observations indicate the supernova associated with GRBs are Type Ib/c. However, BeppoSAX could only observe afterglows of long GRBs, the origin of short GRBs remains still to be explored.

J.Hjorth, Nature, Vol.423, 847

Swift era - 1

• 5) Swift era (2005 up to now): Different from others, Swift launched late 2004 is itself a multi-wavelength observatory, with good qualities to observe short bursts and early afterglows. It brings new surprises and challenges to us, and quickly discovered afterglows of several short GRBs, and showed new features in the light curve of early afterglow. Among others, it gives evidence that the short GRBs indeed are different from the long ones, they seem to be originated from compact star mergers.

Swift era - 2

• It also gives rather detailed multi-component features in some X-ray afterglows, indicating that the standard fireball model needs to be improved or even modified. Besides these ， Swift also discovered an up-to-now highest red-shift GRB050904 with z=6.29; a nearby long-duration GRB without a supernova associated; and recently a GRB080319B even with naked-eye optical flash in early afterglow, and others.

Present conference at a new key point

• We are now here having such a conference again at a new key point, to summarize the wealth of Swift era, and welcome for the coming of the new Glast era. Indeed, the Glast was just launched successfully on 11 June 2008. It will collect rich high energy data for GRBs.

• Indeed, we already have many review papers year by year especially within recent 10 years, they cover BeppoSAX era, Swift era, even also take care of Glast era. GRB is a very active and rather complete field people have ever seen from its discovery to the present status of richness within only about 40 years.

Applications of GRBs

• How to understand the origin and various properties of GRBs is the main goal for research in GRBs, but, as a peculiar signal or a special tool the GRB is also very important in fundamental physics. As the duration of GRBs is very small, all gamma-ray photons reach detector almost simultaneously, the GRB then could be regarded as a very far gamma-ray source to test such as the principle of light speed invariance with respect to different energies or so. GRBs could also be used to test cosmological questions. In fact, some tentative but valuable works have been done to measure the dark energy and dark matter in the Universe. And neutrinos, cosmic rays and gravitational radiations have already been studied widely.

Nanjing GRB Group

• May I now briefly introduce our Nanjing Group on GRBs. I myself was originally working in the field of particle physics. As I turned my interest to astrophysics in 1978, the first input field was naturally compact star physics and high energy astrophysics. By May or June 1997, after getting the information about the discoveries of afterglows of GRB970228 and GRB970508 from the Elba meeting, our group (composed of my students and myself) decided to focus almost our full attention on topics of GRBs.

Some Scientists Visiting Nanjing

Nanjing GRB Group - continued

• Thanks for the GCN and astro-ph (arXiv), we caught the chance to study the post-standard effects, such as environments of GRBs, extention to non-relativistic or even deep-Newtonian phase, some continuing energy injection into the fireball or additional out-put from central engine, Inverse Compton Scattering problems, and so on. Since then, GRBs has been the main subject of our Nanjing group, for example, constraining dark energy based on GRB observations, a model for X-ray flares of short GRBs based on magnetic reconnection-driven events in strongly magnetized millisecond pulsars, late internal shock model for X-ray flares, hyperaccretion disks around neutron stars and others. This conference will be very helpful to us, and I will say welcome again also on behalf of our Nanjing Group.

Observational projects in preparation

• Sure, there are also other groups on GRBs, especially in Beijing and Yunnan. Most efforts are given on the theoretical side. However, in recent years, some big observational apparatuses or projects are in preparation in China, there will be some detailed talks at the present conference.

Wishing the confenerce be successful!

Wishing all participants happy, healthy, successful and lucky!

Thanks!